Silylpyrrole Oxidation En Route to Saxitoxin Congeners Including 11-Saxitoxinethanoic Acid

Abstract

Saxitoxin (STX) is the archetype of a large family (>50) of architecturally distinct, bisguanidinium natural products. Among this collection of isolates, two members, 11-saxitoxinethanoic acid (11-SEA) and zetekitoxin AB (ZTX), are unique, bearing carbon substitution at C11. A desire to efficiently access these compounds has motivated the development of new tactical approaches to a late-stage C11-ketone intermediate 26, designed to enable C–C bond formation using any one of a number of possible reaction technologies. Highlights of the synthesis of 26 include a metal-free, silylpyrrole oxidative dearomatization reaction and a vinylsilane epoxidation–rearrangement cascade to generate the requisite ketone. Nucleophilic addition to 26 makes possible the preparation of unnatural C11-substituted STXs. Olefination of this ketone is also demonstrated and, when followed by a redox-neutral isomerization reaction, affords 11-SEA.

Graphical Abstract

1. INTRODUCTION

Bisguanidinium toxins, as exemplified by saxitoxin (STX), act as acute poisons by inhibiting the flow of ions through voltage-gated sodium ion channels (Na_vs).¹ These unique metabolites are the products of an unusual biosynthetic pathway and comprise >50 discrete congeners.^2,3 Among this family of natural products, 11-saxitoxinethanoic acid (11-SEA) and zetekitoxin AB (ZTX) are distinct as the only bisguanidinium toxins having C11 alkyl substitution (Figure 1).^4,5 With an interest in preparing 11-SEA, ZTX, and derivatives thereof through a divergent synthesis, we have investigated different strategies for generating functionally disparate C11-structures. This work has culminated in the identification of a novel silyl epoxide rearrangement for the preparation of a C11-ketone intermediate, which enables access to 11-SEA. The chemistry outlined in this report makes available a number of unique bisguanidinium derivatives and is driving subsequent efforts to explore Na_v physiology.^1b,2,6

Figure 1.

STX and naturally occurring C11-substituted derivatives.

2. RESULTS AND DISCUSSION

In our previously reported synthesis of 11-SEA, a Stille cross-coupling reaction with an intermediate iodoenaminone formed the requisite C11 carbon–carbon bond (Scheme 1).^7,8 Difficulties associated with preparing the stannane reagent and the capriciousness of the Pd-catalyzed coupling reaction reduced the overall efficiency of this route. Alternative cross-coupling technologies to prepare C11 derivatives were similarly ineffective, and a synthesis of ZTX using such chemistry was deemed unworkable. These findings motivated our efforts to develop an alternative set of tactics for assembling 11-SEA and other C11-substituted structures.

Scheme 1.

Cross-Coupling Step to Install C11 Substitution in First-Generation Synthesis of 11-SEA^a

^aR = Si^tBuPh₂

Two strategies for introducing a carbon-substituent group at the C11 position on the tricyclic core of STX were envisioned: (1) cross-coupling with an appropriate vinyl halide 1 and (2) nucleophilic addition to ketone 2 (Figure 2). These approaches present a complementary set of reaction conditions for C–C bond formation. Implementing both plans would thus facilitate access to a large number of STX derivatives to support toxin structure–activity relationship studies.

Figure 2.

Two strategies for C11-C–C bond formation.

In considering alternative means for assembling C11-STX derivatives including 11-SEA, we were drawn initially to vinyl bromide 1 as a cross-coupling partner. In principle, this compound could be derived through a multistep route from 3-bromo-N-triisopropylsilyl pyrrole 4 (Scheme 2). Pyrrole, a π-electron-rich heterocycle, is intrinsically nucleophilic at the 2- and 5-positions; however, bromination at the 3-position is favored on the N-SiⁱPr₃ derivative.⁹ Desilylation of 4 was accomplished using ⁿBu₄NF, revealing the corresponding N–H pyrrole (Scheme 2). This material was used immediately due to its susceptibility to oxidation. Deprotonation of the N–H pyrrole with potassium tert-butoxide and addition to CO₂ generated pyrrole carboxylic acid 5. As this compound was also prone to decomposition, a subsequent condensation reaction with l-serine methyl ester was telescoped to afford 6 (53% yield over four steps from 4). Low-temperature reduction of methyl ester 6 to the corresponding aldehyde using ⁱBu₂A1H occurred without incident. Allyl imine formation and Pictet–Spengler cyclization were then effected in a one-flask procedure, which generated a single isomeric product 7 in a respectable 59% yield.¹⁰ Unfortunately, ¹H NMR of this material clearly indicated that the Br substituent was incorrectly placed at C12 (SEA numbering). This result is perhaps unsurprising given the ortho/para-directing nature of the Br group.

Scheme 2.

Pictet–Spengler Cyclization of Bromopyrrole 6 Yields the Undesired Regioisomer (Carbon Numbering is Based on 11-SEA)^a

^aReagents and conditions: (a) ⁿBu₄NF, tetrahydrofuran (THF); (b) KO^lBu, THF/Et₂O; then CO₂; (c) (COCl)₂, cat. N,N-dimethylformamide (DMF), THF, 0 °C; then l-serine methyl ester, aqueous NaHCO₃ (d) ^tBuPh₂SiCl, imidazole, DMF, 53% (4 steps); (e) (1) ⁱBu₂AlH, CH₂Cl₂, −90 °C, (2) allylamine, then BF₃·OEt₂ 59%, >20:1 dr (2 steps).

Unable to access the desired C11-brominated bicycle 8, an analogous R₃Si-pyrrole intermediate was targeted with the expectation that the silyl substituent would sterically and electronically bias formation of the C11-isomer in the cyclization event.¹¹ To assess whether the choice of silyl group would influence regioselectivity in the Pictet–Spengler reaction, different C3-silyl pyrroles were generated through lithium–halogen exchange of bromopyrrole 4 followed by trapping with an appropriate silyl chloride (Scheme 3). Formation of pyrrole N-carboxylic acid 10 was achieved in two steps through N-SiⁱPr₃ deprotection with KF in MeOH–conditions that did not cleave the C-silyl group–followed by carboxylation with CO₂. As with 3-bromo-1H-pyrrole and the carboxylated intermediate 5, the analogous silyl derivatives were processed immediately to urea 11.

Scheme 3.

Synthesis of Urea 11^a

^aReagents and conditions: (a) ⁿBu₄NF, THF, −78 °C; then R₃SiCl; (b) KF, MeOH; (c) KO^tBu, THF/Et₂O; then CO₂; (d) (COCl)₂ cat. DMF, THF, 0 °C; then l-serine methyl ester, aqueous NaHCO₃, 43% (4 steps); (e) ^tBuPh₂SiCl, imidazole, DMF, 94%. R = Me₃Si-, ^tBuMe₂Si-, PhMe₂Si-, ⁱPr₃Si-.

Conversion of each of the four silylated intermediates (11) to the corresponding bicyclic product was accomplished through ester reduction and BF₃·OEt₂-promoted Pictet–Spengler reaction (Table l). The desired C11-isomer 12 was favored irrespective of the choice of silyl group, though the best results were obtained with the sterically largest silyl derivatives (entries 2 and 4). A small amount of the protodesilylated material (i.e., R = H) was obtained in most cases as well. From these results, it appears that both steric and hyperconjugative (i.e., β-silyl cation) effects bias production of the C11-isomer 12. As in the synthesis of Br-pyrrole 7, the Pictet–Spengler reaction furnished bicyclic urea 12 as the trans stereoisomer with >20:1 diastereoselectivity.

Table 1.

Influence of the Silyl Group on Pictet–Spengler Regioselectivity

Entry	R	12(%)a	13(%)	R = H (%)
1	Me3Si	32	11	10
2	tBuMe2Si	53	3	13
3	PhMe2Si	43	11	–
4	iPr3Si	50	–	3

^aReported yields are combined for the two-step process and based on isolated material.

Although the ⁱPr₃Si derivative (Table 1, entry 4) was the most selective for generating 12 over the two-step sequence, we opted to advance the ^tBuMe₂Si and PhMe₂Si products (entries 2 and 3, respectively), uncertain as to which silyl intermediate would perform best in subsequent transformations. Deprotection of the allylamine in 12 was accomplished using catalytic Pd(PPh₃)₄ and 1,3-dimethylbarbituric acid (Scheme 4), and the resulting primary amine was smoothly converted to an isothiourea (95% yield, 2 steps).¹² Subsequent O-ethylation of the cyclic urea furnished 14. Both isourea moieties in 14 were displaced with ammonia to give the bisguanidine, and finally, acylation of the 6-membered ring guanidine yielded Troc guanidine 16. The choice of C11-silyl substituent was inconsequential to the performance of this chemistry.^7,13

Scheme 4.

Synthesis of Troc Guanidine 16^a

^aReagents and conditions: (a) Pd(PPh₃)₄, 1,3-dimethylbarbituric acid, CH₂Cl₂; then TcesN=C(SMe)Cl, Na₂CO₃, 95%; (b) EtOTf, 2,4,6-tri-t-butylpyrimidine, CH₂Cl₂, 38 °C, 84%; (c) NH₃, NH₄OAc, MeOH, 75 °C, 68%; (d) Troc benzimidazolium triflate 15, 1,2-dichloroethane, 50 °C, 74%. Similar yields were obtained for C11 ^tBuMe₂Si substrates. Tces = −S(O)₂OCH₂CCl₃; Troc = −C(O)-OCH₂CCl₃.

Assembling the tricyclic core of the bisguanidinium toxins from intermediate 16 requires forging a C–N bond between N9 and C4 (SEA numbering, Scheme 5). In our prior synthesis of gonyautoxin 2/3,¹³ this step (18 → 19) was accomplished through a novel [Rh₂(esp)₂]-catalyzed oxidative dearomatization reaction. The modest performance of this reaction, however, did not translate to the silylated material 16, as <20% of desired product 17 was obtained with no recovered starting material. Upon screening alternative conditions to promote this reaction, we noted a marked improvement in the reaction outcome when the Rh catalyst was omitted. By simply combining PhI(OAc)₂, MgO, and 16, the tricyclic bisguanidine 17 was obtained in 55% yield (Table 2, entry 1).¹⁴

Scheme 5.

Cyclic Guanidine Formation through Pyrrole Oxidation

Table 2.

Metal-free Pyrrole Oxidative Dearomatization

entry	R	R′	conditions	% yielda
1	SiMe2Ph	SitBuPh2	PhI(OAc)2, MgO	55
2	H	SitBuMc2	PhI(OAc)2, MgO	43
3	H	SitBuMe2	PhI(OAc)2, Na2CO3	37
4	H	SitBuMe2	PhI(OPiv)2, MgO	43
5	H	SitBuMe2	PhI(OPiv)2, Na2CO3	47
6	H	SitBuMe2	DMP, Na2CO3	89
7	SiMe 2 Ph	Si t BuPh 2	DMP, Na 2 CO 3	74

^a% yield is based on isolated material. DMP = Dess–Martin periodinane.

In light of our discovery of a Rh-free pyrrole dearomatization reaction, further optimization of this critical step was pursued (Table 2). These experiments were performed using pyrrole 18 (i.e., R = H) given the ready availability of this compound. Different combinations of hypervalent iodine(III) oxidants and inorganic bases were tested to little effect (entries 2–5). Switching to Dess–Martin periodinane, however, resulted in a substantial improvement in reaction performance, affording the desired tricycle in 89% yield (entry 6). Applying these same conditions to silylpyrrole 16 generated the product 17 in 74% yield (entry 7). This finding offers a convenient solution to a critical reaction step and enables multigram production of the tricyclic core of SEA and related toxins.

Relying on prior work from our lab, transforming N,O-acetal 17 to the corresponding allylic alcohol 21 proved straightforward (Scheme 6).^7,15 An unusual Mislow–Evans [2,3]-rearrangement to transpose the C11, C12-alkene highlights this sequence.¹⁶ Isolation of 21 accomplishes the intended goal of introducing a C11-substituent that can enable C–C bond formation at this site. In principle, silyl–halogen exchange could deliver a vinyl halide suitable for cross-coupling. Alternatively, oxidation of the vinylsilane would afford the C11-ketone. The potential versatility of intermediate 21 is a defining feature of this approach to the bisguanidinium natural products.

Scheme 6.

Mislow–Evans Rearrangement Gives C12-Alcohol 21^a

^aReagents and conditions: (a) PhSH, BF₃·OEt₂, CH₂Cl₂, 75%; (b) m-CPBA, CH₂Cl₂, 94%; (c) NaSPh, Cl₃CCH₂OH, 80 °C, 88%. Similar yields were obtained for R = ^tBuMe₂Si substrates.

Attempts to promote ipso-halogen substitution of either the C11-^tBuMe₂Si or PhMe₂Si group have been unsuccessful thus far.¹⁷ As such, our efforts turned to formation of a C11 hydroxyketone, the expected product of a sequential epoxidation/rearrangement cascade (Figure 3). In the event, treatment of ^tBuMe₂Si-21 with m-CPBA furnished α-silyl ketone 24; this product, which is obtained as a single diastereomer, results from opening of the putative epoxide and [1,2]-silyl migration. In striking contrast, subjecting PhMe₂Si-21 to the same reaction conditions yielded ketone 26 quantitatively. While somewhat unexpected, these results are consistent with Malacria’s observations on the rearrangement of silyl allylic epoxides.^18,19

Figure 3.

C11-Silyl substituent dictates the product outcome from m-CPBA reaction. (A) Silyl-[1,2]-shift affords α-silyl ketone 24. (B,C) Putative mechanisms for C11-ketone 26 formation involving silyl group elimination or Brook rearrangement. R’ = Si^tBuPh₂.

Evident differences in the reactivity of ketones 24 and 26 were revealed upon attempts to isolate each product. The C10-^tBuMe₂Si material 24 was easily obtained following chromatography on silica gel, whereas 26 readily decomposed under such conditions. Addition of nucleophiles (e.g., MeMgBr, Cp₂TiMe₂, and Zn-enolate) to ketone 24 were fruitless, presumably owing to the steric bulk of the ^tBuMe₂Si group on the re-face of the tricycle. All attempts to cleave the silyl substituent using different fluoride sources (ⁿBu₄NF, ⁿBu₄N-(Ph₃SiF₂), and KF) or Brønsted acid conditions (CF₃CO₂H, HBF₄, and PPTS) were equally unsuccessful. Fortunately, despite the instability of ketone 26 to chromatography, subsequent experiments with this material were possible, as its preparation from 21 was quantitative. However, addition of conventional nucleophiles such as MeMgBr to this compound was mired by low product yields and the formation of a byproduct 29 resulting from enolization at C12 and β-elimination of the N9-guanidine (Table 3, entry 1). Attempts to temper the kinetic basicity of the nucleophile using Ce and Zn additives resulted in marginal improvements in the production of diol 28 (entries 2 and 3). Of all the organometallic reagents tested, the reaction of allylindium bromide was distinguished, providing the corresponding allylic alcohol in 47% (entry 4).²⁰ Even nonbasic conditions (i.e., the combination of silyl ketene acetal nucleophiles and Lewis acid catalysts) afforded only modest amounts of the desired products (entry 7).

Table 3.

Nucleophilic Addition to C11-Ketone 26

entry	nucleophile	conditions	% yielda
1	MeMgBr	Et2O	10
2	MeMgBr	CeCl3, THF	20
3	AllylMgBr	ZnCl2, THF	35
4	AllylBr	In, THF	47
5	BrCH2CO2Et	Zn, THF	–
6	MeOC(OSiMe3)=CMe2	Sc(OTf)3, CH2Cl2	–
7	MeOC(OSiMe3)=CMe2	TiCl4, CH2Cl2	20

^a% yield is based on isolated material.

In searching for a means to efficiently install a C11–C14 bond in 26, methods for ketone olefination were also examined. Given the evident sensitivity of 26 to base, it was unsurprising that only decomposition ensued upon treatment of this material with simple Wittig reagents (e.g., Ph₃P=CH₂). Other olefination methods using sulfone reagents (eq 1) and different bases (e.g., LiN(SiMe₃)₂, NaO^tBu, and DBU) were equally unproductive. Organometallic methods for ketone methylenation, including Cp₂TiMe₂ and the Tebbe reagent, also failed to deliver the desired product. A modicum of success was realized when 26 was exposed to the lithium salt of the triethyl phosphonoacetate anion (Table 4, entry 1). In this case, ~30% yield of the α,β-unsaturated ester was obtained as a 2:1 mixture of alkene isomers. Switching to the equivalent potassium phosphonoacetate salt afforded only trace product (entry 2). The use of triethylamine and lithium chloride, conditions that have proven successful with base-sensitive ketone substrates, decomposed the starting ketone (entry 3).²¹ Application of the Still–Gennari modification gave a 2:3 Z/E product ratio and was poor yielding (entry 4).²² A protocol employing NaH with triethyl phosphonoacetate at −40 °C was ultimately selected to advance material through this step (entry 5).²³

(1)

Table 4.

Olefination of C11-Ketone 26

entry	R	conditions	% yielda	Z/E ratio
1	OEt	LiOtBu, THF	29	2:1
2	OEt	KOtBu, THF	<5	–
3	OEt	NEt3, LiCl, MeCN	–	–
4	OCH2CF3	KHMDS, 18-C-6, THF	10	2:3
5	OEt	NaH, THF, −40 °C	37	3:2

^a% yield is based on isolated material.

With access to 30 and 31, it is possible to complete a formal synthesis of 11-SEA by oxidizing the secondary alcohol at C12 and reducing the C11–C14 alkene. In an initial attempt to effect the former reaction, treatment of the Z/E mixture of alcohols with Dess–Martin periodinane proved successful and delivered ketone 32 in 76% yield (Figure 4). Interestingly, the product of this reaction was obtained exclusively as the E-isomer in spite of the fact that the starting alcohol was a near 1:1 mixture of olefins. A putative mechanism to account for this result is depicted in Figure 4. As either the Z- or E-isomer, 30 or 31, can be converted to 11-SEA, the unanticipated isomerization reaction was inconsequential.

Figure 4.

Alcohol oxidation of an isomeric mixture of 30/31 affords a single product.

Selective reduction of the C11–C14 olefin in 32 can be accomplished without cleaving the Troc or Tces guanidine protecting groups (Scheme 7). Using Crabtree’s Ir catalyst with 500 psi of H₂, the saturated product 33 was obtained as a 2.5:1 mixture of C11 isomers.²⁴ In nature, 11-SEA occurs as a 3:1 diastereomeric mix, as the C11 center is intrinsically labile.⁴ The saturated ester 33 intercepts our previously published route to the natural product, necessitating only three additional steps to complete the synthesis.⁷

Scheme 7.

Ir-Catalyzed Alkene Hydrogenation^a

^aReagents and conditions: (a) H₂ (500 psi), 30 mol% [Ir(cod)-(PCy₃)(py)]PF₆, B(OⁱPr)₃, CH₂Cl₂, 35%; (b) ⁿBu₄NF, AcOH, THF, 73%.

Although successful conversion of alcohols 30/31 to 33 was achieved through an oxidation/reduction sequence, a more appealing process would furnish 33 in a single-step, redox isomerization reaction.²⁵ The fortuitous observation that Dess–Martin periodinane promoted oxidation and isomerization of 30/31 led us to consider using a mild base to facilitate production of 33 (Table 5). Following a brief screening of different acetate salts, we found that low-temperature treatment of 30 with ⁿBu₄NOAc afforded approximately equal portions of ketone 33 and allylic alcohol 35 (entry 1). Additional experimentation revealed that redox isomerization of 30 occurred in a THF solution of ⁿBuN₄F buffered with AcOH, conditions that yielded a significantly reduced amount of the undesired allylic alcohol 35. In addition, this protocol also cleaved the C13 ^tBuPh₂Si ether to furnish 34, an added bonus as this transformation represents the antepenultimate step in our synthesis of 11-SEA. Two final maneuvers from 34—(1) installation of the C13-carbamate and (2) hydrogenolytic removal of the Tces and Troc groups with concomitant hydrolysis of the ethyl ester—afforded 11-SEA as a 3:1 mixture of stereoisomers (Scheme 8).^4,7,8

Table 5.

Base-Promoted Isomerization of Z-Alkene 30

entry	Conditions	R′	% yielda	33/35 ratio
1	nBu4NOAc, −78 °C → rt	SitBuPh2	50	1.2:1
2	nBu4NF, AcOH, −78 → 0 °C	H	73	9:1b

^a% yield is based on isolated material.

^bKetone 34 is obtained from this reaction.

Scheme 8.

Completed Synthesis of 11-SEA^a

^aReagents and conditions: (a) 1,1′-carbonyldiimidazole, THF; then 0.5 M NH₃ in THF, 54%; (b) 1 atm H₂, PdCl₂, CF₃CO₂H, MeOH/H₂O; then 1.0 M aqueous HCl.

As an addendum to the discussion above, it appears that only the Z-isomer, 30, is predisposed to the redox isomerization reaction. Treatment of E-isomer 31 with ⁿBuN₄F/AcOH results exclusively in deprotection of the C13-silyl ether. We speculate that hydrogen bonding between the ester carbonyl and the C12 alcohol in 30 predisposes the conformation of the C12-H for facile deprotonation. Given the relative rigidity of the tricyclic bisguanidinium skeleton, the structural subtleties responsible for the success or failure of this transformation are striking.

3. CONCLUSIONS

We have capitalized on the versatility of hydroxyketone 26 to complete the synthesis of 11-SEA in 16 steps from l-serine methyl ester. Importantly, this same intermediate enables introduction of disparate C11-alkyl substituents through nucleophilic addition or olefination. The route to 26 is punctuated by a number of novel transformations including regio- and diastereoselective Pictet–Spengler cyclization, Dess–Martin periodinane-promoted silylpyrrole dearomatization, and silyl olefin epoxidation. The chemistry described herein facilitates access to new STX derivatives and propels efforts to develop high-precision small molecule tools for investigating sodium channel physiology.

4. EXPERIMENTAL SECTION

General.

All reagents were obtained commercially unless otherwise noted. Reactions were performed using oven-dried glassware under an atmosphere of nitrogen. Air- and moisture-sensitive liquids were transferred via syringe or a stainless-steel cannula. Organic solutions were concentrated under reduced pressure (~15 Torr) by rotary evaporation. Dichloromethane (CH₂Cl₂), THF, DMF, and acetonitrile (MeCN) were passed through two columns of activated alumina immediately prior to use. Methanol was distilled over sodium methoxide. Dry ethanol was used from a Sure/Seal bottle. Triethylamine, pyridine, and pyrrolidine were distilled over calcium hydride. (Diacetoxyiodo)benzene was recrystallized from 5 M acetic acid,²⁶ and m-CPBA was purified according to the method of Aggarwal et al.²⁷ Dess–Martin periodinane (1,1,1-tris(acetyloxy)-1,1-dihydro-1,2-benziodoxol-3-(1H)-one) was purchased from Millipore Sigma or Combi-Blocks, Inc. and used as received.

Product purification was accomplished through forced flow chromatography on SiliCycle ultrapure silica gel (40–63 μm). Compounds purified by chromatography were typically applied to the adsorbent bed using the indicated solvent conditions with a minimum amount of added dichloromethane as needed for solubility. Thin-layer chromatography (TLC) was performed on EM Science silica gel 60 F₂₅₄ plates (250 μm). Visualization of the developed chromatogram was accomplished using fluorescence quenching or by staining with ethanolic anisaldehyde, aqueous potassium permanganate (KMnO₄), or ceric ammonium molybdate solution.

Nuclear magnetic resonance (NMR) spectra were acquired on either a Varian Inova spectrometer operating at 400, 500, or 600 MHz and 100, 125, or 150 MHz for ¹H and ¹³C, respectively, or a Varian Mercury spectrometer operating at 400 and 100 MHz for ¹H and ¹³C, respectively. Spectra were referenced internally according to residual solvent signals and were recorded at room temperature unless otherwise noted. Data for ¹H NMR are recorded as follows: chemical shift (δ, ppm), multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; and br, broad), coupling constant (Hz), and integration. Data for ¹³C NMR are reported in terms of chemical shift (δ, ppm). Infrared (IR) spectra were recorded as thin films using NaCl plates on a Thermo Nicolet iS5 FT-IR spectrometer and are reported in frequency of absorption. High-resolution mass spectra were obtained from the Vincent Coates Foundation Mass Spectrometry Laboratory at Stanford University. Samples were analyzed by LC/electrospray ionization mass spectrometry (ESI-MS) using either positive or negative ionization modes (as indicated) on one of two instruments: (1) a Waters Acquity UPLC and Thermo Fisher Exactive mass spectrometer scanning m/z 50–2000, the LC mobile phase being acetonitrile with 0.1% formic acid, or (2) a Shimadzu 2020 ESI–LCMS instrument.

A solution of 3-bromo-1-triisopropylsilyl pyrrole 4²⁸ (15.4 g, 51 mmol) in 150 mL of THF was cooled to −78 °C, and n-BuLi (60 mL of a 1.66 M solution in hexanes, 100 mmol, 2.0 equiv) was added while maintaining the internal temperature below −72 °C. After stirring at −78 °C for 5 min, PhMe₂SiCl (17 mL, 101 mmol, 2.0 equiv) was added dropwise. The mixture was warmed to room temperature over 2 h. Following this time, the solution was cooled to 0 °C and diluted with 150 mL of H₂O. The contents were transferred to a separatory funnel with 100 mL of Et₂O, and the organic phase was collected. The aqueous phase was extracted with 2 × 150 mL of Et₂O. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to give the desired silylated pyrrole, which was used immediately and without further purification.

To a solution of the unpurified pyrrole (51 mmol) in 200 mL of MeOH was added KF (3.0 g, 52 mmol, 1.0 equiv) in a single portion. The solution was stirred at room temperature for 15 h. Following this time, the reaction mixture was concentrated under reduced pressure and the oily residue was partitioned between 150 mL of H₂O and 100 mL of Et₂O. The mixture was transferred to a separatory funnel and the organic phase was collected. The aqueous phase was extracted with 2 × 100 mL of Et₂O. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure. The unpurified material was dissolved in 20 mL of Et₂O and added to an ice-cold solution of KO^tBu (6.0 g, 53.5 mmol, 1.05 equiv) in 150 mL of a 1:1 mixture of THF and Et₂O. The solution was warmed to room temperature and stirred for 20 min, after which time the mixture was sparged with CO₂ gas for 75 min. The reaction mixture was diluted with 100 mL of H₂O and transferred to a separatory funnel. The aqueous phase was collected and acidified to pH 1 with 1.0 M aqueous HC1 (~60 mL) and then extracted with 3 × 100 mL of Et₂O. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to afford the desired pyrrole carboxylic acid 10 (7.3 g, 58% over three steps). The material was determined to be sufficiently pure by ¹H NMR analysis for use in the subsequent step. ¹H NMR (400 MHz, CDCl₃): δ 7.56−7.52 (m, 2H), 7.38−7.34 (m, 4H), 7.34−7.32 (m, 1H), 6.33 (dd, 1H, J = 3.2, 1.7 Hz), 0.49 (s, 6H) ppm.

To an ice-cold solution of the pyrrole carboxylic acid 10 (7.3 g, 30 mmol) in 50 mL of THF, DMF (0.25 mL, 3.2 mmol, 0.1 equiv) and oxalyl chloride (2.6 mL, 30 mmol) were added sequentially. Vigorous gas evolution was observed upon addition of oxalyl chloride. The reaction mixture was stirred at 0 °C until all bubbling ceased (approximately 30 min), and then, a solution of l-serine methyl ester hydrochloride (4.78 g, 31 mmol) in 100 mL of saturated aqueous NaHCO₃ was added in a single portion. The biphasic mixture was stirred vigorously at 0 °C for 15 min, then warmed to room temperature, and stirred for 15 h. The mixture was transferred to a separatory funnel and the organic phase was collected. The aqueous phase was extracted with 3 × 100 mL of Et₂O. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure. Purification of the isolated material by chromatography on silica gel (gradient elution: 3:1 → 3:2 hexanes/EtOAc) afforded S1 as a clear oil (7.6 g, 74%; 43% from 4). TLC R_f = 0.42 (1:1 hexanes/EtOAc); ¹H NMR (400 MHz, CDCl₃): δ 7.56−7.52 (m, 2H), 7.37−7.29 (m, 5H), 6.65 (d, 1H, J = 7.1 Hz), 6.33 (dd, 1H, J = 3.0, 1.5 Hz), 4.73−4.68 (m, 1H), 4.10−4.03 (m, 1H), 4.00−3.93 (m, 1H), 3.78 (s, 3H), 2.78 (t, 1H, J = 6.0 Hz), 0.49 (s, 6H) ppm; ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 170.9, 150.7, 138.6, 133.9, 129.1, 127.9, 125.0, 120.6, 119.8, 117.2, 63.0, 55.7, 53.1, −1.8 ppm; IR (thin film) ν: 3368, 2954, 1744, 1681, 1542, 1479, 1379, 1342, 1220 cm⁻¹; HRMS (ES⁺) calcd for C₁₇H₂₃N₂O₄Si⁺, 347.1422 [M + H]⁺; found, 347.1420.

To a solution of alcohol S1 (3.53 g, 10.2 mmol) in 14 mL of DMF were added sequentially imidazole (904 mg, 13.3 mmol, 1.3 equiv) and ^tBuPh₂SiCl (2.65 mL, 10.2 mmol). The reaction mixture was stirred for 12 h and then diluted with 150 mL of H₂O and 100 mL of Et₂O. The contents were transferred to a separatory funnel, and the organic phase was collected and washed with 2 × 100 mL of H₂O. The ethereal extract was dried over MgSO₄, filtered, and concentrated under reduced pressure to an oily residue. Purification of this material by chromatography on silica gel (10:1 hexanes/EtOAc) afforded 11 as a clear oil (5.6 g, 94%). TLC R_f = 0.29 (9:1 hexanes/EtOAc); ¹H NMR (400 MHz, CDCl₃): δ 7.62−7.55 (m, 6H), 7.47−7.30 (m, 9H), 7.26−7.25 (m, 1H), 7.23 (dd, 1H, J = 3.0, 2.0 Hz), 6.42 (d, 1H, J = 8.1 Hz), 6.37 (dd, 1H, J = 3.0, 1.5 Hz), 4.71 (dt, 1H, J = 8.0, 2.8 Hz), 4.22 (dd, 1H, J = 10.7, 3.2 Hz), 4.01 (dd, 1H, J = 10.7, 3.2 Hz), 3.81 (s, 3H), 1.06 (s, 9H), 0.52 (s, 6H) ppm; ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 170.7, 150.1, 138.7, 135.5, 135.4, 133.9, 132.6, 132.5, 130.1, 129.1, 127.96, 127.91, 127.87, 124.9, 120.3, 119.6, 117.1, 64.2, 55.4, 52.8, 26.8, 19.3, −1.8 ppm; IR (thin film) ν: 3360, 3070, 2954, 2858, 1749, 1712, 1508, 1475, 1428, 1250, 1113 cm⁻¹.

To a −90 °C solution of 11 (12.1 g, 20.7 mmol) in 210 mL of CH₂Cl₂ was added ⁱBu₂AlH (27.6 mL of a 1.50 M solution in toluene, 41.4 mmol, 2.0 equiv) dropwise over 40 min. The reaction mixture was stirred at −90 °C for an additional 15 min and then quenched at this temperature by the slow addition of 20 mL of EtOAc. The contents were poured into an Erlenmeyer flask containing 210 mL of 1.0 M aqueous sodium potassium tartrate and 180 mL of EtOAc and stirred vigorously for 3 h. Following this time, the contents were transferred to a separatory funnel. The organic phase was collected, and the aqueous layer was extracted with 2 × 100 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a pale yellow oil. This material was used immediately and without additional purification to avoid potential loss of optical purity.

To an ice-cold solution of unpurified aldehyde (20.7 mmol) in 210 mL of CH₂Cl₂ was added allylamine (2.3 mL, 30.7 mmol, 1.5 equiv). The mixture was warmed to room temperature over 20 min and then cooled to −78 °C. Neat BF₃·OEt₂ (9 mL, 71.7 mmol, 3.5 equiv) was added dropwise over 10 min to this solution. The mixture was stirred at −78 °C for 20 min and slowly warmed to room temperature over a 90 min period. The reaction was quenched by the addition of 200 mL of saturated aqueous NaHCO₃, stirred vigorously for 12 h, and transferred to a separatory funnel. The organic phase was collected, and the aqueous layer was extracted with 3 × 100 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a brown oil. Purification of this material by chromatography on silica gel (gradient elution: 2:1 → 1:1 hexanes/EtOAc) afforded 12 as a white foam (5.3 g, 43% over 2 steps). TLC R_f = 0.27 (1:1 hexanes/EtOAc); ¹H NMR (400 MHz, CDCl₃): δ 7.63−7.54 (m, 6H), 7.45−7.30 (m, 10H), 6.16−6.14 (m, 1H), 6.12 (d, 1H, J = 3.1 Hz), 5.89−5.78 (m, 1H), 5.21−5.09 (m, 2H), 3.94 (d, 1H, J = 3.6 Hz), 3.75−3.69 (m, 1H), 3.61−3.57 (m, 2H), 3.32−3.18 (m, 2H), 1.03 (s, 9H), 0.51 (s, 6H) ppm; ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 149.5, 138.7, 136.3, 135.6, 134.0, 132.74, 132.69, 130.3, 130.0, 129.1, 127.89, 127.86, 127.84, 124.7, 118.7, 116.6, 114.9, 64.5, 58.4, 49.7, 49.5, 26.8, 19.2, −1.78, −1.83 ppm; IR (thin film) ν: 3241, 3122, 3070, 2955, 2931, 1718, 1428, 1249, 1113 cm⁻¹; HRMS (ES⁺): calcd for C₃₅H₄₄N₃O₂Si₂⁺, 594.2967 [M + H]⁺; found, 594.2982.

Amine 12 (3.1 g, 5.2 mmol), 1,3-dimethylbarbituric acid (2.4 g, 15.6 mmol, 3.0 equiv), and Pd(PPh₃)₄ (129 mg, 0.11 mmol, 0.02 equiv) were combined in a 250 mL round-bottomed flask, and the flask was flushed with N₂ for several minutes. To the reaction vessel was added 52 mL of degassed CH₂Cl₂ (3× freeze/pump/thaw cycle). The orange solution was stirred for 13 h, following which time 50 mL of a 1 M aqueous solution of Na₂CO₃ and TcesN=C(SMe)Cl²⁹ (1.69 g, 5.3 mmol, 1.02 equiv) was added sequentially. The biphasic mixture was stirred for 20 min, then diluted with 50 mL of EtOAc, and transferred to a separatory funnel. The organic phase was collected, and the aqueous layer was extracted with 2 × 50 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to an orange oil. Purification of this material by chromatography on silica gel (gradient elution: 4:1 → 3:1 hexanes/EtOAc) afforded isothiourea S2 as a yellow foam (4.2 g, 95%). TLC R_f = 0.36 (2:1 hexanes/EtOAc; ¹H NMR (400 MHz, CDCl₃): δ 8.13 (d, 1H, J = 9.3 Hz), 7.62−7.57 (m, 2H), 7.57−7.52 (m, 4H), 7.47−7.30 (m, 10H), 6.33 (dd, 1H, J = 1.6, 0.8 Hz), 5.71 (br s, 1H), 5.25−5.18 (m, 1H), 4.63 (s, 2H), 3.80–3.74 (m, 1H), 3.67 (dd, 1H, J = 10.9, 6.0 Hz), 3.54 (dd, 1H, J = 10.9, 6.9 Hz), 2.48 (s, 3H), 1.04 (s, 9H), 0.50 (s, 6H) ppm; ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 170.5, 148.6, 138.0, 135.5, 134.0, 132.2, 132.1, 130.26, 130.25, 129.3, 128.06, 128.05, 127.96, 125.8, 120.2, 116.9, 93.7, 78.6, 63.3, 58.0, 47.9, 26.8, 19.2, 14.7, −1.86, −1.91 ppm; IR (thin film) ν: 3361, 2955, 2858, 1717, 1559, 1428, 1351, 1351, 1165, 1113 cm⁻¹.

To a solution of S2 (5.56 g, 6.63 mmol) and 2,4,6-tri-t-butylpyrimidine (1.65 g, 6.63 mmol) in 6.5 mL of CH₂Cl₂ was added ethyl trifluoromethanesulfonate (2.2 mL, 17 mmol, 2.6 equiv). The reaction mixture was stirred at 38 °C for 30 h. Following this time, the reaction was cooled to room temperature and quenched by the addition of 100 mL of saturated aqueous NaHCO₃. The biphasic mixture was stirred vigorously for 12 h, diluted with 25 mL of EtOAc, and transferred to a separatory funnel. The organic layer was collected, and the aqueous phase was extracted with 3 × 25 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a brown oil. Purification of this material by chromatography on silica gel (gradient elution: 1:0 → 4:1 hexanes/EtOAc) afforded isourea 14 as a pale yellow foam (4.84 g, 84%). TLC R_f = 0.41 (4:1 hexanes/EtOAc); ¹H NMR (500 MHz, CDCl₃): δ 8.08 (d, 1H, J = 8.8 Hz), 7.67−7.64 (m, 2H), 7.62−7.57 (m, 4H), 7.47−7.36 (m, 7H), 7.31−7.27 (m, 2H), 7.07 (d, 1H, J = 1.5 Hz), 6.40−6.39 (m, 1H), 5.40 (dd, 1H, J = 9.0, 4.1 Hz), 4.64 (s, 2H), 4.37−4.23 (m, 2H), 4.02 (quint, 1H, J = 7.6, 3.7 Hz), 3.76 (dd, 1H, J = 10.3, 3.9 Hz), 3.29 (dd, 1H, J = 10.6, 8.0 Hz), 2.51 (s, 3H), 1.37 (t, 3H, J = 7.1 Hz), 1.04 (s, 9H), 0.53 (d, 6H, 1.6 Hz) ppm; ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 170.6, 146.7, 138.5, 135.61, 135.57, 134.0, 132.9, 132.8, 129.9, 129.8, 129.2, 127.91, 127.85, 127.7, 126.6, 123.2, 118.6, 115.7, 93.8, 78.5, 63.5, 63.0, 61.1, 48.1, 26.8, 19.3, 14.7, 14.2, −1.6, −1.7 ppm; IR (thin film) ν: 3288, 3070, 2956, 2931, 2858, 1668, 1572, 1479, 1335, 1164, 1113 cm⁻¹; HRMS (ES⁺): calcd for C₃₈H₄₈Cl₃N₄O₅S₂Si₂⁺, 865.1665 [M + H]⁺; found, 865.1676.

A 100 mL thick-walled tube containing a magnetic stir bar was charged with 14 (4.84 g, 5.59 mmol), NH₄OAc (2.2 g, 28.5 mmol, 5.1 equiv), and NH₃ (28 mL of a 2.0 M solution in MeOH, 56 mmol, 10 equiv). The vessel was sealed with a Teflon screw cap and the contents heated at 75 °C for 24 h. The reaction mixture was cooled to room temperature, transferred to a collection flask, and concentrated under reduced pressure. Purification of the isolated material by chromatography on silica gel (gradient elution: 1:1:0.01 hexanes/EtOAc/AcOH, then 14:1:0.01 → 12:1:0.01 CH₂Cl₂/MeOH/AcOH) afforded bisguanidine S3·HOAc as a white solid. In a separatory funnel, S3·HOAc was partitioned between 25 mL of Et₂O and 25 mL of 1.0 M NaOH. The organic phase was collected, and the aqueous layer was extracted with 2 × 25 mL of Et₂O. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to yield S3 as a white foam (3.05 g, 68%). TLC R_f = 0.4 (10:1:0.01 CH₂Cl₂/MeOH/AcOH); ¹H NMR (400 MHz, CD₃OD, S3·HOAc): δ 7.60−7.46 (m, 6H), 7.46−7.29 (m, 10H), 6.51−6.49 (m, 1H), 5.43 (br s, 1H), 4.62 (s, 2H), 3.95−3.90 (m, 1H), 3.72 (dd, 1H, J = 11.1, 4.1 Hz), 3.64 (dd, 1H, J = 11.1, 5.3 Hz), 1.92 (s, 3H), 0.90 (s, 9H), 0.49 (s, 6H) ppm; ¹³C{¹H} NMR (100 MHz, CD₃OD, S3·HOAc): δ 179.8, 157.5, 151.8, 138.5, 136.7, 134.9, 133.5, 133.3, 131.14, 131.11, 130.4, 129.6, 129.0, 125.1, 124.8, 118.3, 95.6, 79.3, 65.6, 57.8, 45.2, 27.2, 23.5, 19.8, −1.9, −2.0 ppm; IR (thin film) ν: 3462, 3353, 3212, 2956, 2858, 1700, 1628, 1546, 1428, 1185, 1113 cm⁻¹.

To a solution of S3 (3.05 g, 3.78 mmol) in 38 mL of 1,2-dichloroethane at 50 °C was added Troc-benzimidazolium salt 15¹³ (2.13 g, 3.98 mmol, 1.05 equiv) in a single portion. The reaction mixture was stirred at this temperature for 2 h. Following this time, the reaction was cooled to room temperature and the mixture concentrated under reduced pressure to a brown oil. Purification of this material by chromatography on silica gel (gradient elution: 15:1 → 12:1 CH₂Cl₂/EtOAc) afforded 16 as a white foam (2.73 g, 74%). TLC R_f = 0.43 (2:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.64 (d, 1H, J = 3.9 Hz), 7.58−7.52 (m, 6H), 7.45−7.32 (m, 10H), 6.41 (br s, 1H), 6.39 (dd, 1H, J = 1.6, 0.9 Hz), 6.18 (br s, 2H), 5.32 (br s, 1H), 4.87 (d, 1H, J = 11.9 Hz), 4.82 (d, 1H, J = 11.9 Hz), 4.60 (d, 1H, J = 11.2 Hz), 4.57 (d, 1H, J = 11.2 Hz), 4.03−3.98 (m, 1H), 3.62 (d, 2H, J = 5.7 Hz), 0.91 (s, 9H), 0.47 (s, 6H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 163.0, 156.8, 154.7, 139.0, 136.24, 136.23, 134.7, 133.3, 133.2, 130.89, 130.88, 130.1, 128.79, 128.77, 128.76, 128.20, 125.5, 121.3, 117.3, 96.5, 95.1, 78.7, 75.7, 64.9, 57.2, 44.9, 26.9, 19.5, −2.04, −2.07 ppm; IR (thin film) ν: 3463, 3360, 3264, 2955, 2858, 1617, 1540, 1482, 1390, 1154, 1114 cm⁻¹; HRMS (ES⁺): calcd for C₃₈H₄₅Cl₆N₆O₆SSi₂⁺, 979.0785 [M + H]⁺; found, 979.0783.

A 25 mL oven-dried round-bottom flask was charged with Tces guanidine 16 (410 mg, 0.42 mmol) and Na₂CO₃ (221 mg, 2.09 mmol, 5 equiv). To this mixture was added 20 mL of CH₂Cl₂ followed by Dess–Martin periodinane (354 mg, 0.83 mmol, 2 equiv). The flask was placed in an oil bath heated to 40 °C, and the mixture was stirred at this temperature for 5 h. Following this time, the reaction mixture was cooled to room temperature and ~3 g of Celite and 10 mL of CH₂Cl₂ were added. The suspension was stirred for 10 min and then filtered through a pad of Celite. The flask and filter cake were rinsed with ~20 mL of CH₂Cl₂. The combined filtrates were concentrated under reduced pressure to a yellow foam. Purification of this material by chromatography on silica gel (gradient elution: 4:1 → 3:1 hexanes/EtOAc) afforded the desired tricycle 17 as a yellow solid (321 mg, 74%). TLC R_f = 0.26 (3:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.25 (d, 1H, J = 2.9 Hz), 7.62−7.59 (m, 2H), 7.59−7.56 (m, 2H), 7.49−7.44 (m, 4H), 7.43−7.31 (m, 7H), 7.11 (br s, 1H), 7.08 (br s, 1H), 6.92 (s, 1H), 6.47 (s, 1H), 4.77 (d, 1H, J = 12.2 Hz), 4.71 (d, 1H, J = 12.2 Hz), 4.62 (s, 2H), 4.35−4.33 (m, 1H), 3.82−3.77 (m, 1H), 3.60 (dd, 1H, J = 10.3, 5.6 Hz), 3.42 (dd, 1H, J = 10.7, 8.6 Hz), 1.67 (s, 3H), 0.98 (s, 9H), 0.38 (s, 3H), 0.34 (s, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 170.3, 162.0, 159.3, 158.8, 143.6, 143.2, 136.23, 136.17, 134.9, 133.31, 133.30, 130.99, 130.5, 128.84, 128.82, 128.78, 96.7, 95.0, 88.2, 82.4, 78.7, 76.5, 64.2, 57.7, 56.5, 26.9, 21.1, 19.6, −3.1, −3.9 ppm; IR (thin film) ν: 3393, 2956, 2858, 1744, 1617, 1522, 1389 1268, 1114 cm⁻¹; HRMS (ES⁺): calcd for C₄₀H₄₇Cl₆N₆O₈SSi₂⁺ 1037.0840 [M + H]⁺; found, 1037.0864.

To a solution of N,O-acetal 17 (465 mg, 0.45 mmol) in 17 mL of CH₂Cl₂ was added thiophenol (230 μL, 2.25 mmol, 5.0 equiv) and BF₃·OEt₂ (170 μL, 1.35 mmol, 3.0 equiv). The mixture was stirred for 4 h, following which time the reaction was quenched by the addition of 20 mL of saturated aqueous NaHCO₃. The biphasic mixture was stirred vigorously for 10 min and transferred to a separatory funnel containing 30 mL of EtOAc. The organic layer was collected, and the aqueous portion was extracted with 3 × 15 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to an oily residue. Purification of this material by chromatography on silica gel (gradient elution: 6:1 → 3:1 hexanes/EtOAc) afforded N,S-acetal S4 as a pale yellow foam (366 mg, 75%). TLC R_f = 0.23 (4:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.34 (d, 1H, J = 1.9 Hz), 7.65−7.54 (m, 6H), 7.52−7.35 (m, 9H), 7.35−7.28 M, 5H), 7.08 (br s, 1H), 6.23 (d, 1H, J = 1.0 Hz), 6.02 (br s, 1H), 5.97 (d, 1H, J = 1.0 Hz), 4.63−4.60 (m, 3H), 4.42 (d, 1H, J = 12.9 Hz), 4.04 (d, 1H, J = 4.4 Hz), 3.74−3.67 (m, 2H), 3.42 (dd, 1H, J = 10.3, 8.5 Hz), 1.00 (s, 9H), 0.44 (s, 3H), 0.43 (s, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 162.1, 158.64, 158.61, 146.5, 139.2, 136.6, 136.29, 136.25, 135.4, 134.9, 133.34, 133.29, 133.2, 131.00, 130.98, 130.6, 130.0, 129.8, 128.89, 128.85, 128.8, 96.7, 95.0, 83.1, 78.7, 77.3, 76.0, 64.2, 56.3, 55.2, 27.0, 19.6, −2.0, −2.7 ppm; IR (thin film) ν: 3387, 3071, 2954, 2858, 1597, 1516, 1389, 1264, 1177, 1114 cm⁻¹.

To a solution of N,S-acetal S4 (366 mg, 0.34 mmol) in 13 mL of CH₂Cl₂ was added m-CPBA (58 mg, 0.34 mmol). The reaction mixture was stirred for 10 min and then quenched by the addition of 10 mL of saturated aqueous Na₂S₂O₃. The biphasic mixture was stirred vigorously for 5 min and transferred to a separatory funnel containing 10 mL of EtOAc. The organic phase was collected and washed with 10 mL of 1 M NaOH. The organic phase was collected, and the aqueous layer was extracted with 3 × 10 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a yellow solid. This material was purified by chromatography on silica gel (3:1 hexanes/EtOAc) to yield sulfoxide 20 as a pale yellow foam (349 mg, 94%). TLC R_f = 0.39 (3:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.50 (br s, 1H), 8.18 (br s, 1H), 7.65−7.58 (m, 6H), 7.56−7.51 (m, 2H), 7.50−7.38 (m, 12H), 6.84 (br s, 1H), 6.60 (d, 1H, J = 1.0 Hz), 6.20 (d, 1H, J = 1.1 Hz), 4.66 (s, 2H), 4.46 (d, 1H, J = 12.4 Hz), 4.25 (d, 1H, J = 12.4 Hz), 3.86 (dd, 1H, J = 10.4, 3.8 Hz), 3.67 (d, 1H, J = 7.1 Hz), 3.62−3.57 (m, 1H), 3.54 (dd, 1H, J = 10.5, 8.4 Hz), 1.00 (s, 9H), 0.60 (s, 3H), 0.55 (s, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 161.7, 158.0, 157.6, 144.2, 141.4, 140.6, 136.3, 136.0, 135.0, 133.3, 132.6, 131.06, 131.03, 130.96, 130.0, 129.1, 128.86, 128.83, 126.0, 96.6, 95.1, 89.6, 85.2, 78.8, 75.8, 64.2, 53.6, 51.5, 27.0, 19.6, −2.7, −2.9 ppm; IR (thin film) ν: 3424, 3071, 2955, 2858, 1654, 1595, 1389, 1330, 1262, 1178, 1114 cm⁻¹; HRMS (ES⁺): calcd for C₄₄H₄₉Cl₆N₆O₇S₂Si₂⁺, 1103.0768 [M + H]⁺; found, 1103.0792.

To a solution of sulfoxide 20 (70 mg, 63 μmol) in 2.5 mL of 2,2,2-trichloroethanol was added sodium thiophenolate (9.5 mg, 72 μmol, 1.1 equiv). The mixture was stirred at 80 °C for 5 h and then concentrated under reduced pressure to an oily residue. Purification of this material by chromatography on silica gel (gradient elution: 5:1 → 3:1 hexanes/EtOAc) yielded allylic alcohol 21 as a white solid (56 mg, 88%). TLC R_f = 0.38 (2:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.01 (d, 1H, J = 4.0 Hz), 7.61−7.56 (m, 4H), 7.55−7.51 (m, 2H), 7.47−7.42 (m, 2H), 7.41−7.33 (m, 7H), 7.18 (br s, 1H), 7.06 (br s, 1H), 6.65 (d, 1H, J = 2.3 Hz), 4.96−4.93 (m, 1H), 4.84 (d, 1H, J = 11.9 Hz), 4.63 (d, 1H, J = 11.9 Hz), 4.60 (d, 1H, J = 11.2 Hz), 4.57 (d, 1H, J = 11.2 Hz), 4.42−4.41 (m, 1H), 3.85 (d, 1H, J = 7.0 Hz), 3.81−3.76 (m, 1H), 3.64−3.57 (m, 2H), 0.95 (s, 9H), 0.41 (s, 3H), 0.39 (s, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 162.3, 159.5, 157.0, 138.0, 136.3, 135.5, 134.8, 133.2, 131.1, 130.2, 128.82, 128.80, 128.75, 120.5, 96.7, 94.9, 84.0, 82.6, 78.7, 75.5, 64.6, 59.7, 57.6, 26.9, 19.5, −2.62, −2.67 ppm; IR (thin film) ν: 3393, 2955, 2859, 1588, 1526, 1381, 1189, 1105 cm⁻¹; HRMS (ES⁺): calcd for C₃₈H₄₅Cl₆N₆O₇SSi₂⁺, 995.0735 [M + H]⁺; found, 995.0745.

To a solution of 21 (50 mg, 50 μmol) in 2.5 mL of CH₂Cl₂ was added m-CPBA (11.8 mg, 68 μmol, 1.3 equiv). The solution was stirred for 3 h and then concentrated under reduced pressure. The off-white solids were dissolved in 2 mL of Et₂O and transferred to a separatory funnel containing 2 mL of saturated aqueous Na₂S₂O₃. The organic phase was collected, and the aqueous layer was extracted with 2 × 2 mL of Et₂O. The combined organic extracts were washed with 3 × 3 mL of pH 7.5 phosphate buffer, dried over MgSO₄, filtered, and concentrated under reduced pressure to a white solid. This material was unstable to purification by chromatography on silica gel and was used in the subsequent reaction without further purification. A sample of 26 suitable for analysis was obtained by purification on a short plug of Davisil (gradient elution: 3:1 → 1:1 hexanes/EtOAc). TLC R_f = 0.53 (3:2 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN): δ 9.19−9.17 (m, 1H), 7.67−7.60 (m, 4H), 7.49−7.38 (m, 6H), 7.15 (br s, 1H), 7.10 (br s, 1H), 4.82 (d, 1H, J = 12.2 Hz), 4.70 (d, 1H, J = 12.2 Hz), 4.60 (d, 1H, J = 11.2 Hz), 4.57 (d, 1H, J = 11.2 Hz), 4.47 (br s, 1H), 4.31 (br s, 1H), 3.93 (d, 1H, J = 18.9 Hz), 3.87−3.74 (m, 4H), 1.02 (s, 9H) ppm; IR (thin film) ν: 3303, 2930, 2860, 1778, 1629, 1588, 1381, 1258, 1166, 1107 cm⁻¹; HRMS (ES⁺): calcd for C₃₀H₃₅Cl₆N₆O₈SSi⁺, 877.0132 [M + H]⁺; found, 877.0113.

Alcohol S5 was prepared in an analogous manner to S1. Purification by chromatography on silica gel (3:2 hexanes/EtOAc) afforded S5 as a clear oil (3.5 g, 36% over 4 steps). TLC R_f = 0.3 (3:2 hexanes/EtOAc); ¹H NMR (500 MHz, CDCl₃): δ 7.29 (dd, 1H, J = 3.0, 2.1 Hz), 7.26−7.25 (m, 1H), 6.62 (d, 1H, J = 6.3 Hz), 6.31 (dd, 1H, J = 3.0, 1.5 Hz), 4.75−4.70 (m, 1H), 4.12−4.06 (m, 1H), 4.04−3.98 (m, 1H), 3.80 (s, 3H), 2.74 (t, 1H, J = 5.5 Hz), 0.88 (s, 9H), 0.17 (s, 6H) ppm; ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 171.0, 150.9, 124.8, 120.0, 119.3, 117.7, 63.0, 55.7, 53.0, 26.4, 16.7, −5.5 ppm; IR (thin film) ν: 3362, 2954, 2855, 1741, 1701, 1543, 1479, 1380, 1250 cm−¹.

To a foil-wrapped flask containing alcohol S5 (14.7 g, 45 mmol) in 90 mL of THF, pyridine (18 mL, 223 mmol, 5.0 equiv) and AgNO₃ (7.9 g, 46 mmol, 1.02 equiv) were added sequentially. The mixture was stirred for 10 min, following which time ^tBuPh₂SiCl (11.5 mL, 45 mmol) was added dropwise over 5 min. The reaction mixture was stirred in the dark at room temperature for 13 h and then filtered through Celite. The flask and filter cake were rinsed with 50 mL of THF. The combined filtrates were transferred to a separatory funnel containing 300 mL of H₂O. The organic phase was collected and washed sequentially with 250 mL of 1 M aqueous HCl and 250 mL of H₂O. The organic layer was dried over MgSO₄, filtered, and concentrated under reduced pressure to an oily residue. Purification of this material by chromatography on silica gel (gradient elution: 12:1 → 10:1 hexanes/EtOAc) afforded S6 as a clear oil (23.7 g, 93%). TLC R_f = 0.26 (9:1 hexanes/EtOAc); ¹H NMR (400 MHz, CDCl₃): δ 7.60−7.56 (m, 4H), 7.46−7.30 (m, 6H), 7.22−7.21 (m, 1H), 7.20 (dd, 1H, J = 2.9, 1.9 Hz), 6.43 (d, 1H, J = 8.2 Hz), 6.34 (dd, 1H, J = 3.1, 1.7 Hz), 4.70 (dt, 1H, J = 8.2, 3.1 Hz), 4.20 (dd, 1H, J = 10.5, 2.9 Hz), 4.00 (dd, 1H, J = 10.2, 3.2 Hz), 3.80 (s, 3H), 1.05 (s, 9H), 0.91 (s, 9H), 0.19 (s, 6H) ppm; ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 170.8, 150.2, 135.5, 135.4, 132.6, 132.5, 130.1, 128.0, 127.9, 124.7, 119.8, 119.2, 117.6, 64.3, 55.4, 52.8, 26.8, 26.5, 19.4, 16.8, −5.47, −5.49 ppm; IR (thin film) ν: 3439, 3361, 2950, 2930, 2857, 1750, 1713. 1506, 1475, 1250, 1113 cm⁻¹.

Amine S7 was prepared in an analogous manner to 12. Purification by chromatography on silica gel (gradient elution: 2:1 → 1:2 CH₂Cl₂/EtOAc) afforded S7 as a white foam (1.86 g, 53%). TLC R_f = 0.32 (3:2 CH₂Cl₂/EtOAc); ¹H NMR (500 MHz, CDCl₃): δ 7.62−7.56 (m, 4H), 7.44−7.31 (m, 7H), 6.13−6.11 (m, 1H), 5.89 (d, 1H, J = 3.0 Hz), 5.87−5.78 (m, 1H), 5.20−5.14 (m, 1H), 5.13−5.08 (m, 1H), 3.92 (d, 1H, J = 3.6 Hz), 3.73−3.68 (m, 1H), 3.62−3.53 (m, 2H), 3.30−3.24 (m, 1H), 3.23−3.16 (m, 1H), 1.02 (s, 9H), 0.89 (s, 9H), 0.18 (s, 6H) ppm; ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 149.5, 136.4, 135.6, 132.76, 132.74, 130.0, 129.7, 127.9, 124.4, 118.2, 116.6, 115.5, 64.6, 58.6, 49.7, 49.5, 26.8, 26.5, 19.2, 16.8, −5.46, −5.54 ppm; IR (thin film) ν: 3242, 3122, 3073, 2953, 2930, 2856, 1717, 1471, 1428, 1249, 1176, 1113 cm⁻¹.

Isothiourea S8 was prepared in an analogous manner to S2. Purification by chromatography on silica gel (gradient elution: 4:1 → 2:1 hexanes/EtOAc) afforded S8 as a yellow foam (2.37 g, 89%). TLC R_f = 0.41 (2:1 hexanes/EtOAc); ¹H NMR (400 MHz, CDCl₃): δ 8.14 (d, 1H, J = 8.7 Hz), 7.63−7.54 (m, 4H), 7.47−7.33 (m, 7H), 6.32 (dd, 1H, J = 1.6, 0.9 Hz), 6.05 (br s, 1H), 5.28−5.21 (m, 1H), 4.64 (s, 2H), 3.84−3.76 (m, 1H), 3.68 (dd, 1H, J = 10.5, 6.0 Hz), 3.54 (dd, 1H, J = 10.5, 6.5 Hz), 2.49 (s, 3H), 1.05 (s, 9H), 0.90 (s, 9H), 0.19 (d, 6H, J = 1.1 Hz) ppm; ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 170.6, 149.0, 135.52, 135.51, 132.3, 132.1, 130.23, 130.21, 128.04, 128.02, 125.5, 125.4, 119.5, 117.3, 93.7, 78.6, 63.3, 57.9, 47.9, 26.8, 26.5, 19.2, 16.7, 14.7, −5.48, −5.51 ppm; IR (thin film) ν: 3283, 3134, 3073, 2953, 2930, 2857, 1717, 1572, 1428, 1352, 1166, 1113 cm⁻¹.

Isourea S9 was prepared in an analogous manner to 14. Purification by chromatography on silica gel (gradient elution: 1:0 → 4:1 hexanes/EtOAc) afforded S9 as a pale yellow foam (7.7 g, 79%). TLC R_f = 0.46 (4:1 hexanes/EtOAc); ¹H NMR (400 MHz, CDCl₃): δ 8.06 (d, 1H, J = 8.9 Hz), 7.66−7.63 (m, 2H), 7.62−7.58 (m, 2H), 7.46−7.30 (m, 6H), 7.00 (d, 1H, J = 1.6 Hz), 6.36−6.34 (m, 1H), 5.39 (dd, 1H, J = 9.2, 4.2 Hz), 4.62 (s, 2H), 4.37−4.21 (m, 2H), 3.99 (quint, 1H, J = 8.0, 4.1 Hz), 3.73 (dd, 1H, J = 10.2, 4.1 Hz), 3.26 (dd, 1H, J = 10.2, 7.8 Hz), 2.50 (s, 3H), 1.37 (t, 3H, J = 6.5 Hz), 1.02 (s, 9H), 0.91 (s, 9H), 0.20 (s, 3H), 0.19 (s, 3H) ppm; ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 170.7, 146.9, 135.7, 135.6, 133.0, 132.9, 129.9, 129.8, 127.9, 127.8, 126.2, 123.0, 118.0, 116.2, 93.8, 78.6, 63.5, 63.1, 61.2, 48.2, 26.8, 26.6, 19.3, 16.8, 14.7, 14.2, −5.34, −5.38 ppm; IR (thin film) ν: 3288, 2390, 2954, 2885, 2857, 1668, 1573, 1477, 1427, 1335, 1163, 1113 cm⁻¹.

Bisguanidine S10 was prepared in an analogous manner to S3. Purification by chromatography on silica gel (1:1:0.01 hexanes/EtOAc/AcOH, then gradient elution: 14:1:0.01 → 9:1:0.01 CH₂Cl₂/MeOH/AcOH) afforded S10·HOAc as a white foam (1.15 g, 70%). TLC R_f = 0.4 (10:1:0.01 CH₂Cl₂/MeOH/AcOH); ¹H NMR (400 MHz, CD₃OD, S10·HOAc): δ 7.61−7.56 (m, 4H), 7.51 (d, 1H, J = 1.4 Hz), 7.46−7.37 (m, 6H), 6.52−6.51 (m, 1H), 5.47 (br s, 1H), 4.64 (s, 2H), 3.95−3.92 (m, 1H), 3.71 (dd, 1H, J = 11.5, 4.7 Hz), 3.64 (dd, 1H, J = 10.6, 5.3 Hz), 1.93 (s, 3H), 0.92 (s, 9H), 0.91 (s, 9H), 0.21 (s, 3H), 0.20 (s, 3H) ppm; ¹³C{¹H} NMR (100 MHz, CD₃OD, S10·HOAc): δ 179.7, 157.5, 151.8, 136.66, 136.65, 133.5, 133.3, 131.14, 131.11, 129.2, 129.0, 125.1, 124.0, 118.9, 95.6, 79.3, 65.6, 57.8, 45.1, 27.3, 26.9, 23.4, 19.9, 17.5, −5.36, −5.39 ppm; IR (thin film) ν: 3463, 3359, 3231, 2930, 2858, 1697, 1628, 1546, 1413, 1331, 1185, 1114 cm⁻¹.

Bisguanidine S11 was prepared in an analogous manner to 16. Purification by chromatography on silica gel (gradient elution: 4:1 → 2:1 hexanes/EtOAc) afforded S11 as a white foam (1.23 g, 88%). TLC R_f = 0.31 (3:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.65 (d, 1H, J = 3.3 Hz), 7.59−7.53 (m, 4H), 7.47−7.35 (m, 7H), 6.41−6.35 (m, 2H), 6.19 (br s, 2H), 5.32 (br s, 1H), 4.88 (d, 1H, J = 12.4 Hz), 4.84 (d, 1H, J = 12.4 Hz), 4.61 (d, 1H, J = 11.3 Hz), 4.59 (d, 1H, J = 11.3 Hz), 4.04−3.99 (m, 1H), 3.65−3.59 (m, 2H), 0.93 (s, 9H), 0.88 (s, 9H), 0.19 (s, 3H), 0.18 (s, 3H) ppm; ¹³C{H} NMR (125 MHz, CD₃CN): δ 163.0, 153.9, 154.7, 136.24, 136.21, 133.32, 133.27, 130.90, 128.8, 127.7, 125.5, 125.3, 120.5, 118.0, 96.5, 95.1, 78.7, 75.7, 64.9, 57.2, 44.9, 27.0, 26.7, 19.5, 17.1, −5.43, −5.47 ppm; IR (thin film) ν: 3462, 3358, 2954, 2857, 1617, 1540, 1482, 1389, 1154, 1114 cm⁻¹.

N,O-Acetal S12 was prepared in an analogous manner to 17. Purification by chromatography on silica gel (gradient elution: 4:1 → 3:1 hexanes/EtOAc) afforded S12 as a yellow foam (516 mg, 54%). TLC R_f = 0.30 (3:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.26 (d, 1H, J = 3.1 Hz), 7.63−7.59 (m, 2H), 7.57−7.53 (m, 2H), 7.49−7.37 (m, 6H), 7.09 (br s, 1H), 7.04 (br s, 1H), 6.98 (s, 1H), 6.47 (s, 1H), 4.84 (d, 1H, J = 12.5 Hz), 4.74 (d, 1H, J = 12.5 Hz), 4.63 (d, 1H, J = 11.1 Hz), 4.60 (d, 1H, J = 11.1 Hz), 4.36 (br s, 1H), 3.84−3.79 (m, 1H), 3.57 (dd, 1H, J = 10.8, 5.4 Hz), 3.38 (dd, 1H, J = 10.6, 9.0 Hz), 2.04 (s, 3H), 0.98 (s, 9H), 0.78 (s, 9H), 0.10 (s, 3H), 0.02 (s, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 170.3, 162.0, 159.5, 158.8, 144.2, 142.8, 136.3, 136.1, 133.3, 131.01, 130.99, 128.86, 128.85, 96.8, 95.0, 88.1, 82.1, 78.7, 76.6, 63.9, 58.3, 56.7, 26.9, 26.7, 21.7, 19.5, 17.2, −5.8, −6.0 ppm; IR (thin film) ν: 3403, 2931, 2858, 1749, 1617, 1522, 1389, 1178, 1114 cm⁻¹.

N,S-Acetal S13 was prepared in an analogous manner to S4. Purification by chromatography on silica gel (gradient elution: 6:1 → 4:1 hexanes/EtOAc) afforded S13 as a pale yellow foam (193 mg, 74%). TLC R_f = 0.53 (3:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.33 (d, 1H, J = 2.3 Hz), 7.82−7.78 (m, 2H), 7.64−7.60 (m, 2H), 7.59−7.55 (m, 2H), 7.48−7.34 (m, 9H), 7.06 (br s, 1H), 6.32 (br s, 1H), 6.26 (d, 1H, J = 1.0 Hz), 6.16 (d, 1H, J = 0.8 Hz), 4.68 (d, 1H, J = 12.3 Hz), 4.63 (s, 2H), 4.43 (d, 1H, J = 12.3 Hz), 4.09 (d, 1H, J = 4.1 Hz), 3.77−3.72 (m, 1H), 3.67 (dd, 1H, J = 10.3, 4.7 Hz), 3.41 (dd, 1H, J = 10.3, 8.9 Hz), 0.99 (s, 9H), 0.78 (s, 9H), 0.17 (s, 3H), 0.15 (s, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 161.5, 158.1, 158.0, 145.2, 139.3, 135.8, 135.7, 134.5, 133.0, 132.8, 132.7, 130.43, 130.40, 129.5, 129.1, 128.27, 128.25, 96.2, 94.4, 82.2, 78.1, 76.8, 75.5, 63.3, 56.1, 54.6, 26.4, 26.3, 19.0, 16.8, −5.3, −5.8 ppm; IR (thin film) ν: 3385, 2930, 2858, 1596, 1517, 1390, 1252, 1177, 1114 cm⁻¹.

Sulfoxide S14 was prepared in an analogous manner to 20. Purification by chromatography on silica gel (gradient elution: 4:1 → 7:2 hexanes/EtOAc) afforded S14 as a yellow foam (187 mg, 96%). TLC R_f = 0.34 (3:1 hexanes/EtOAc); ¹H NMR (400 MHz, CD₃CN): δ 9.48 (br s, 1H), 8.24 (br s, 1H), 7.67−7.57 (m, 6H), 7.54−7.36 (m, 9H), 6.86 (br s, 1H), 6.62 (d, 1H, J = 1.1 Hz), 6.36 (d, 1H, J = 0.9 Hz), 4.66 (s, 2H), 4.60 (d, 1H, J = 12.1 Hz), 4.31 (d, 1H, J = 12.1 Hz), 3.85 (dd, 1H, J = 10.2, 3.6 Hz), 3.71 (d, 1H, J = 7.0 Hz), 3.63−3.57 (m, 1H), 3.54 (dd, 1H, J = 10.2, 8.4 Hz), 0.99 (s, 9H), 0.92 (s, 9H), 0.32 (s, 3H), 0.26 (s, 3H) ppm; IR (thin film) ν: 3245, 2956, 2931, 2858, 1653, 1595, 1390, 1263, 1178, 1114 cm⁻¹.

Allylic alcohol S15 was prepared in an analogous manner to 21. Purification by chromatography on silica gel (gradient elution: 5:1 → 3:1 hexanes/EtOAc) afforded S15 as a yellow foam (142 mg, 84%). TLC R_f = 0.45 (2:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 9.03 (d, 1H, J = 4.4 Hz), 7.63−7.57 (m, 4H), 7.49−7.37 (m, 6H), 7.17 (br s, 1H), 7.06 (br s, 1H), 6.79, (d, 1H, J = 2.2 Hz), 4.97 (dd, 1H, J = 7.3, 2.4 Hz), 4.86 (d, 1H, J = 12.5 Hz), 4.65 (d, 1H, J = 12.5 Hz), 4.60 (d, 1H, J = 11.3 Hz), 4.58 (d, 1H, J = 11.3 Hz), 4.42 (br s, 1H), 3.85 (d, 1H, J = 7.3 Hz), 3.84−3.80 (m, 1H), 3.64 (dd, 1H, J = 10.4, 5.4 Hz), 3.59 (dd, 1H, J = 10.5, 7.8 Hz), 0.98 (s, 9H), 0.87 (s, 9H), 0.11 (s, 3H), 0.10 (s, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 162.4, 159.6, 157.0, 136.23, 136.21, 135.3, 133.3, 133.2, 130.96, 130.95, 128.83, 128.82, 119.9, 96.7, 94.9, 84.6, 82.5, 78.7, 75.5, 64.2, 59.5, 57.5, 27.1, 27.0, 19.5, 17.3, −5.0, −5.9 ppm; IR (thin film) ν: 3290, 2953, 2858, 1594, 1524, 1381, 1188, 1105 cm⁻¹.

To a solution of S15 (61 mg, 62 μmol) in 2.5 mL of CH₂Cl₂ was added m-CPBA (10.7 mg, 62 μmol). The solution was stirred for 2.5 h and then quenched by the addition of 3 mL of saturated aqueous Na₂S₂O₃. The biphasic mixture was transferred to a separatory funnel containing 3 mL of EtOAc. The organic phase was collected, and the aqueous layer was extracted with 2 × 3 of mL EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure. Purification of this material by chromatography on silica gel (gradient elution: 7:2 → 3:1 hexanes/EtOAc) afforded 24 as a white solid (47 mg, 76%). Note: if m-CPBA remains following workup and chromatographic purification, 24 can be dissolved in Et₂O and washed with pH 7.5 phosphate buffer. The use of 1 M aqueous NaOH should be avoided, as basic solutions will decompose 24. TLC R_f = 0.38 (2:1 hexanes/EtOAc); ¹H NMR (500 MHz, CD₃CN): δ 8.97 (d, 1H, J = 4.1 Hz), 7.68−7.63 (m, 4H), 7.50−7.38 (m, 6H), 7.16 (br s, 1H), 6.86 (br s, 1H), 4.88 (d, 1H, J = 12.2 Hz), 4.61−4.55 (m, 3H), 4.52 (br s, 1H), 4.26 (br s, 1H), 4.24 (s, 1H), 3.90−3.85 (m, 1H), 3.83−3.79 (m, 1H), 3.74 (dd, 1H, J = 10.0, 6.9 Hz), 1.02 (s, 9H), 1.00 (s, 9H), −0.12 (s, 3H), −0.23 (s, 3H) ppm; ¹³C{¹H} NMR (150 MHz, CD₃CN, determined by HSQC and HMBC): δ 206.8, 161.5, 160.3, 159.2, 136(2), 133.1(2), 130.4(2), 128.3(2), 96.4, 94.1, 80.1, 79.7, 78.0, 74.8, 62.4, 59.3, 56.7, 55.2, 26.6, 26.3, 19.4, 17.6, −6.7(2) ppm; IR (thin film) ν: 3403, 2931, 2859, 1751, 1627, 1521, 1387, 1238, 1178, 1112 cm⁻¹; HRMS (ES⁺): calcd for C₃₆H₄₉Cl₆N₆O₈SSi₂⁺, 991.0997 [M + H]⁺; found, 991.1020.

A flask containing anhydrous CeCl₃ (45 mg, 0.18 mmol, 6.7 equiv) was heated under vacuum to 160 °C for 2 h. The flask was then cooled to 0 °C, and 0.5 mL of THF was added. The white suspension was stirred at 0 °C for 30 min, and a 2.4 M ethereal solution of MeMgBr (80 μL, 0.19 mmol, 7.1 equiv) was added. The mixture was stirred at 0 °C for 45 min. Following this time, an ice-cold solution of 26 (24 mg, 27 μmol) in 0.3 mL of THF was added dropwise via a cannula. Transfer of 26 was made quantitative with an additional 0.2 mL of THF. The mixture was stirred at 0 °C for 5 min and then quenched by the addition of 3 mL of saturated aqueous NH₄Cl. The contents were stirred vigorously for 10 min while the biphasic mixture warmed to room temperature. The mixture was transferred to a separatory funnel containing 2 mL of EtOAc. The organic phase was collected, and the aqueous portion was extracted with 3 × 2 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure. Purification of this material by chromatography on silica gel (gradient elution: 2:1 → 1:1 hexanes/EtOAc) afforded S16 as a colorless film (4.3 mg, 20%, >20:1 dr). TLC R_f = 0.21 (1:1 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN): δ 9.10−9.08 (m, 1H), 7.68−7.60 (m, 4H), 7.51−7.38 (m, 6H), 6.74 (br s, 1H), 4.79 (d, 1H, J = 12.6 Hz), 4.67 (d, 1H, J = 12.6 Hz), 4.60 (d, 1H, J = 11.2 Hz), 4.57 (d, 1H, J = 11.2 Hz), 4.12 (d, 1H, J = 2.8 Hz), 3.73−3.54 (m, 6H), 3.28 (d, 1H, J = 12.4 Hz), 1.24 (s, 3H), 1.02 (s, 9H) ppm; ¹³C{¹H} NMR (150 MHz, CD₃CN, determined by HSQC and HMBC): δ 162.0, 159.6, 135.8(2), 132.9(2), 130.4(2), 128.3(2), 96.6, 94.7, 80.2, 78.2, 77.1, 74.2, 74.0, 62.7, 55.9, 55.5, 54.9, 27.0, 21.8, 19.2 ppm; IR (thin film) ν: 3300, 2931, 2862, 1617, 1584, 1383, 1174, 1113 cm⁻¹; HRMS (ES⁺): calcd for C₃₁H₃₉Cl₆N₆O₈SSi⁺, 893.0445 [M + H]⁺; found, 893.0452.

To a flame-dried 5 mL Schlenk flask was added 103 mg of indium powder under a positive pressure of argon. The flask was placed under an argon atmosphere, and 2.0 mL of THF was added, followed by 4 μL of 1,2-dibromoethane. To this suspension was added 77 μL of allylbromide. The mixture was stirred at 50 °C for 15 min and then cooled to room temperature. The freshly prepared allylindium solution (140 μL, 60 μmol, 6.0 equiv) was added dropwise via syringe to a −78 °C solution of α-hydroxyketone 26 (8.8 mg, 10 μmol) in 1.0 mL of THF. The reaction mixture was warmed to 15 °C over 4 h and quenched at this temperature by the addition of 1 mL of saturated aqueous NH₄Cl. The mixture was transferred to a separatory funnel containing 1 mL of EtOAc, the organic layer was collected, and the aqueous phase was extracted with 3 × 1 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a clear film. Purification of this material by chromatography on silica gel (3:2 hexanes/EtOAc) afforded diol S17 as a clear film (4.4 mg, 47%). TLC R_f = 0.43 (1:1 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN): δ 9.09 (d, J = 2.6 Hz, 1H), 7.68−7.64 (m, 2H), 7.64−7.60 (m, 2H), 7.52−7.40 (m, 7H), 7.37 (s, 1H), 6.78 (s, 1H), 5.78 (dddd, J = 16.9, 10.2, 8.0, 6.6 Hz, 1H), 5.18−5.08 (m, 2H), 4.80 (d, J = 12.2 Hz, 1H), 4.67 (d, J = 12.3 Hz, 1H), 4.63−4.56 (m, 2H), 4.19−4.13 (m, 1H), 3.99 (s, 1H), 3.74−3.68 (m, 2H), 3.68−3.63 (m, 1H), 3.59−3.56 (m, 1H), 3.55−3.53 (m, 1H), 3.52 (d, J = 12.5 Hz, 1H), 3.36 (d, J = 12.4 Hz, 1H), 2.40 (ddt, J = 14.0, 6.6, 1.3 Hz, 1H), 2.20−2.16 (m, 1H), 1.03 (s, 9H) ppm; ¹³C{¹H} NMR (150 MHz, CD₃CN, determined by HMBC/HSQC): δ 19.7, 27.1, 41.2, 54.8, 55.6, 56.4, 63.4, 75.0, 76.5, 77.8, 77.9, 78.3, 95.0, 96.9, 119.2, 128.9, 131.3, 133.5, 133.5, 136.4, 159.9, 160.5, 162.4 ppm; IR (thin film) ν: 3305, 3073, 2931, 1732, 1614, 1538, 1471, 1428, 1386, 1270, 1170 cm⁻¹; LCMS (ES⁺): calcd for C₃₃H₄₁Cl₆N₆O₈SSi⁺, 919.1 [M + H]⁺; found, 919.0.

To an ice-cold suspension of sodium hydride (12 mg, 0.50 mmol) in 3.0 mL of THF was added triethyl phosphonoacetate (112 μL, 0.55 mmol) dropwise via syringe. Gas evolution was immediately observed and the gray suspension turned into a clear solution. The mixture was warmed to room temperature and stirred for 15 min. To a solution of hydroxyketone 26 (88 mg, 0.1 mmol) in 5 mL of THF at −40 °C was added the triethyl phosphonoacetate solution (790 μL, 0.12 mmol, 1.2 equiv) dropwise via syringe. The reaction mixture was stirred at this temperature for 1 h and then slowly warmed to 0 °C over a 3 h period. Following this time, the reaction mixture was quenched at 0 °C by the addition of 5 mL of saturated aqueous NH₄Cl and then transferred to a separatory funnel containing 5 mL of EtOAc. The organic layer was collected, and the aqueous phase was extracted with 3 × 5 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a yellow oil. Purification of this material by chromatography on silica gel (3:1 hexanes/EtOAc) afforded a mixture of α,β-unsaturated esters 30 and 31 as white foams (36 mg, 37%, Z/E ratio = 3:2). Spectral data for 30: TLC R_f = 0.50 (3:2 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN) : δ 9.03 (d, J = 3.8 Hz, 1H), 7.64−7.62 (m, 2H), 7.61−7.58 (m, 2H), 7.48−7.44 (m, 2H), 7.41 (tdd, J = 8.1, 5.1, 1.0 Hz, 5H), 7.08 (s, 1H), 7.00 (s, 1H), 6.07 (dd, J = 3.5, 2.2 Hz, 2H), 4.81 (d, J = 12.2 Hz, 1H), 4.78 (d, J = 2.5 Hz, 1H), 4.69 (d, J = 12.2 Hz, 1H), 4.58 (d, J = 1.5 Hz, 2H), 4.46 (q, J = 1.3 Hz, 1H), 4.26−4.20 (m, 4H), 3.82 (ddq, J = 7.4, 3.8, 1.9 Hz, 1H), 3.74 (dd, J = 10.5, 5.4 Hz, 1H), 3.70 (dd, J = 10.5, 7.7 Hz, 1H), 1.28 (t, J = 7.1 Hz, 3H), 1.02 (s, 9H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 168.6, 162.3, 159.8, 156.8, 136.4, 136.3, 133.4, 131.0, 131.0, 128.9, 128.8, 117.8, 96.8, 95.0, 80.6, 78.7, 77.6, 75.6, 64.1, 62.6, 59.0, 56.2, 50.4, 27.0, 19.6, 14.2 ppm; IR (thin film) ν: 3313, 2932, 1623, 1586, 1535, 1472, 1428, 1377, 1313, 1233, 1170, 1113 cm⁻¹; HRMS (ES⁺): calcd for C₃₄H₄₁Cl₆N₆O₉SSi⁺, 947.0551 [M + H]⁺; found, 947.0538.

Spectral data for 31: TLC R_f = 0.36 (3:2 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN): δ 9.03 (d, J = 3.6 Hz, 1H), 7.65−7.62 (m, 2H), 7.62−7.60 (m, 2H), 7.49−7.44 (m, 2H), 7.44−7.39 (m, 4H), 7.11 (s, 1H), 6.82 (s, 1H), 5.96 (q, J = 2.6 Hz, 1H), 4.83 (d, J = 12.2 Hz, 1H), 4.71 (d, J = 12.2 Hz, 1H), 4.58 (d, J = 1.0 Hz, 2H), 4.51 (d, J = 2.6 Hz, 2H), 4.48 (t, J = 2.5 Hz, 1H), 4.43 (d, J = 1.9 Hz, 1H), 4.33−4.27 (m, 2H), 4.20 (qd, J = 7.1, 5.3 Hz, 2H), 3.84−3.78 (m, 1H), 3.76 (dd, J = 10.5, 5.2 Hz, 1H), 3.71 (dd, J = 10.5, 7.7 Hz, 1H), 1.28 (t, J = 7.1 Hz, 3H), 1.03 (s, H) ppm; ¹³C NMR (125 MHz, CD₃CN): δ 166.2, 162.3, 160.0, 159.6, 154.6, 136.4, 136.3, 133.8, 133.5, 133.4, 131.0, 131.0, 128.9, 128.8, 116.0, 96.8, 95.0, 78.8, 78.7, 77.6, 75.6, 64.2, 61.3, 58.1, 56.2, 48.9, 27.1, 19.6, 14.5 ppm; IR (thin film) ν: 3297, 3073, 2932, 2859, 1708, 1583, 1472, 1428, 1382, 1350, 1266, 1163 cm⁻¹; HRMS (ES⁺): calcd for C₃₄H₄₁Cl₆N₆O₉SSi⁺, 947.0551 [M + H]⁺; found, 947.0558.

To a solution of α,β-unsaturated esters 30 and 31 (7 mg, 7.3 μmol) in 0.7 mL of CH₂Cl₂ was added Dess–Martin periodinane (4 mg, 8.7 μmol, 1.2 equiv). The suspension was stirred for 3 h, following which time the reaction was quenched by the addition of 1 mL of saturated aqueous NH₄Cl. The mixture was transferred to a separatory funnel containing 2 mL of EtOAc, the organic layer collected, and the aqueous phase extracted with 3 × 2 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a yellow oil. Purification of this material by chromatography on silica gel (3:1 hexanes/EtOAc) afforded ketoester 32 as a white foam (5 mg, 76%, exclusively E-isomer). TLC R_f = 0.43 (3:2 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN): δ 8.99 (d, J = 3.9 Hz, 1H), 7.53 (dq, J = 6.7, 1.5 Hz, 4H), 7.47−7.41 (m, 2H), 7.38 (tdd, J = 8.1, 2.9, 1.0 Hz, 4H), 7.14 (s, 1H), 6.62 (t, J = 2.8 Hz, 1H), 4.87 (d, J = 12.2 Hz, 1H), 4.71 (d, J = 12.2 Hz, 1H), 4.71−4.66 (m, 1H), 4.62−4.59 (m, 3H), 4.49 (dd, J = 18.5, 3.0 Hz, 1H), 4.32−4.24 (m, 2H), 3.88−3.84 (m, 1H), 3.62 (dd, J = 10.7, 5.4 Hz, 1H), 3.59 (dd, J = 10.7, 6.8 Hz, 1H), 1.32 (t, J = 7.1 Hz, 3H), 0.97 (s, 9H) ppm; ¹³C{¹H} NMR (150 MHz, CD₃CN, determined by HMBC and HSQC): δ 194.5, 165.4, 162.3, 160.7, 158.8, 141.6, 136.3, 133.3, 131.0, 128.8, 125.5, 96.6, 95.0, 78.7, 75.6, 75.5, 64.7, 62.6, 58.7, 56.2, 47.7, 27.0, 19.6, 14.5 ppm; IR (thin film) ν: 3302, 3072, 2930, 2858, 1754, 1718, 1587, 1529, 1472, 1428, 1385, 1313, 1113 cm⁻¹; HRMS (ES⁺): calcd for C₃₄H₃₉Cl₆N₆O₉SSi, 945.0394 [M + H]⁺; found, 945.0393.

A flame-dried 5 mL round-bottom flask containing 32 (31 mg, 33 μmol) and [Ir(cod)(PCy₃)(py)]PF₆ (8.0 mg, 10 μmol, 0.3 equiv) was capped with a rubber septum and flushed with N₂. The solids were dissolved in 300 μL of CH₂Cl₂, and neat B(OⁱPr)₃ (6.2 μL, 33 μmol, 1.0 equiv) was added via syringe. The line feeding N₂ was removed, a 21-gauge disposable needle was inserted into the rubber septum, and the reaction vessel was placed in a high pressure Parr bomb. The bomb was sealed and flushed twice with H₂ gas (500 psi) and then pressurized to 500 psi of H₂. The contents were stirred under this pressure of H₂ for 16 h. Following this time, the Parr bomb was vented, the flask unsealed, and 1 mL of saturated aqueous NH₄Cl was added to the reaction mixture. The reaction mixture was stirred vigorously for 2 min and then transferred to a separatory funnel containing 3 mL of EtOAc. The organic layer was collected, and the aqueous fraction was extracted with 2 × 5 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to an orange film. Purification of this material by preparative TLC on silica gel (1:1 hexanes/EtOAc) afforded ketone 33 as a white solid (11 mg, 35%, 2.5:1 mixture of diastereomers). TLC R_f = 0.4 (3:2 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN, major diastereomer): δ 9.00 (d, J = 4.1 Hz, 1H), 7.62−7.55 (m, 4H), 7.51−7.45 (m, 2H), 7.45−7.39 (m, 4H), 4.84 (d, J = 12.2 Hz, 1H), 4.65 (d, J = 12.2 Hz, 1H), 4.62 (d, J = 11.0 Hz, 1H), 4.59 (d, J = 11.2 Hz, 1H), 4.44 (s, 1H), 4.13 (qd, J = 7.2, 1.2 Hz, 2H), 4.04−4.00 (m, 1H), 3.86−3.82 (m, 1H), 3.65−3.56 (m, 2H), 3.33 (dd, J = 11.3, 9.1 Hz, 1H), 2.89 (dd, J = 18.4, 5.1 Hz, 1H), 2.69 (dd, J = 14.6, 4.0 Hz, 1H), 2.46 (tdd, J = 9.2, 5.0, 3.8 Hz, 1H), 1.22 (t, J = 7.1 Hz, 3H), 1.01 (s, 9H) ppm; ¹³C{¹H} NMR (150 MHz, determined by HSQC/HMBC, CD₃CN, major diastereomer): δ 208.9, 173.1, 162.5, 161.1, 159.8, 136.3, 133.6, 131.2, 128.8, 97.1, 95.1, 79.0, 76.3, 75.8, 63.6, 63.1, 59.8, 59.4, 46.1, 41.7, 32.6, 27.3, 20.2, 14.5 ppm; IR (thin film) ν: 3311, 2930, 1721, 1658, 1639, 1586 cm⁻¹; HRMS (ES⁺): calcd for C₃₄H₄₁Cl₆N₆O₉SSi, 947.0551 [M + H]⁺; found, 947.0556.

To 9.0 mL of THF was added 70 μL of AcOH and 1.0 mL of a 1.0 M solution of n-Bu₄NF in THF. The freshly prepared mixture was cooled to −78 °C, and 400 μL (40 μmol, 1.2 equiv) was added dropwise to a −78 °C solution of α,β-unsaturated ester 30 (31 mg, 33 μmol) in 3.0 mL of THF. The reaction mixture was stirred for 1 h at −78 °C and then warmed to 0 °C over a 2 h period. Following this time, the reaction was quenched by the addition of 1 mL of saturated aqueous NH₄Cl. The mixture was stirred vigorously for 5 min and transferred to a separatory funnel containing 2 mL of EtOAc. The organic layer was separated and the aqueous layer was extracted with 3 × 2 mL of EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a white film. Purification of this material by chromatography on silica gel (gradient elution: 1:1 → 0:1 hexanes/EtOAc) afforded alcohol 34 as a white foam (17 mg, 76%). TLC R_f = 0.25 (1:2 hexanes/EtOAc); ¹H NMR (600 MHz, CD₃CN): δ 8.88 (s, 1H), 4.89 (d, J = 12.2 Hz, 1H), 4.69 (d, J = 12.3 Hz, 1H), 4.64−4.58 (m, 2H), 4.50 (d, J = 1.6 Hz, 1H), 4.34 (dd, J = 11.5, 9.6 Hz, 1H), 4.15 (q, J = 7.1 Hz, 2H), 3.69 (d, J = 4.3Hz, 1H), 3.62−3.52 (m, 2H), 3.41 (dd, J = 11.5, 9.2 Hz, 1H), 3.30 (s, 1H), 2.98 (dd, J = 18.3, 4.9 Hz, 1H), 2.86 (tt, J = 9.2, 4.4 Hz, 1H), 2.77 (dd, J = 18.3, 3.9 Hz, 1H), 1.25−1.20 (m, 3H) ppm; ¹³C{¹H} NMR (125 MHz, CD₃CN): δ 209.2, 173.1, 162.5, 161.4, 159.0, 96.9, 95.0, 78.8, 76.3, 75.6, 64.5, 62.5, 60.0, 55.0, 46.6, 41.6, 33.0, 14.3 ppm; IR (thin film) ν: 3312, 2925, 1772, 1716, 1585, 1380, 1203 cm⁻¹; HRMS (ES⁺): calcd for C₁₈H₂₃Cl₆N₆O₉S⁺, 708.9373 [M + H]⁺; found, 708.9379.

To an ice-cold solution of alcohol 34 (6.5 mg, 9.1 μmol) in 900 μL of THF was added 1,1′-carbonyldiimidazole (7.4 mg, 46 μmol, 5.0 equiv). The reaction mixture was stirred at 0 °C for 3 h, following which time a second portion of carbonyldiimidazole (7.4 mg, 46 μmol, 5.0 equiv) was added. After stirring for an additional 2 h, the reaction was quenched by the addition of 1 mL of saturated aqueous NH₄Cl. The solution was transferred to a separatory funnel containing 2 mL of THF, the aqueous layer was collected, and the organic phase was washed with 1 × 5 mL of saturated aqueous NaCl. The combined organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure to a colorless film. This material was dissolved in 1.0 mL of a saturated solution of NH₃ in THF (generated by bubbling ammonia gas through a solution of THF for 5 min) and the flask sealed with a yellow polyethylene cap and the stopper wrapped in Parafilm. After stirring for 24 h, the flask was unsealed and an additional 1.0 mL of the saturated NH₃ in THF solution was added. The flask was restoppered and stirring continued for an additional 12 h. Following this time, the reaction mixture was concentrated under reduced pressure to a colorless film. Purification of this material by preparative TLC on silica gel (EtOAc) gave carbamate S18 as a colorless film (3.7 mg, 54%). ¹H NMR (600 MHz, CD₃CN): δ 8.86 (d, J = 4.2 Hz, 1H), 4.89 (d, J = 12.2 Hz, 1H), 4.70 (d, J = 12.2 Hz, 1H), 4.60 (d, J = 6.4 Hz, 2H), 4.50 (s, 1H), 4.23 (dd, J = 11.3, 9.0 Hz, 1H), 4.15 (tdd, J = 7.1, 6.4, 1.1 Hz, 2H), 4.06 (d, J = 6.2 Hz, 2H), 3.81 (tdd, J = 7.0, 4.8, 1.6 Hz, 1H), 3.62−3.53 (m, 1H), 3.44−3.38 (m, 1H), 3.05−2.96 (m, 2H), 2.85−2.77 (m, 1H), 1.23 (t, J = 7.2 Hz, 3H) ppm; ¹³C{¹H} NMR (150 MHz, determined by HMBC/HSQC, CD₃CN): δ 209.0, 173.1, 162.5, 161.6, 157.0, 96.9, 94.9, 76.4, 78.5, 75.4, 63.6, 62.3, 58.7, 54.2, 45.9, 41.0, 32.6, 14.2 ppm; IR (thin film) ν: 3310, 1771, 1720, 1587, 1529, 1381, 1331, 1259, 1235, 1206, 1177, 1151, 1109, 1020 cm⁻¹; HRMS (ES⁺): calcd for C₁₉H₂₄Cl₆N₇O₁₀S⁺, 751.9431 [M + H]⁺; found, 751.9441.

To a solution of carbamate S18 (4.8 mg, 6.4 μmol) in 1.9 mL of MeOH was added 640 μL of H₂O and 74 μL of CF₃CO₂H. The mixture was stirred for 1 h, after which time PdCl₂ (0.6 mg, 3.2 μmol, 0.5 equiv) was added. The reaction was sparged with N₂ and then H₂ for 5 min until the solution turned black. A balloon of H₂ was attached to the reaction flask and the mixture was stirred for 3 h. Following this time, the reaction was filtered through a 0.45 μm PTFE filter. The reaction flask and filter were rinsed with 3 × 3 mL of H₂O. The combined filtrates were concentrated under reduced pressure to a white film, which was redissolved in 3.0 mL of 1.0 M aqueous HCl. The solution was stirred for 40 h. The aqueous solution was then lyophilized to remove all volatiles. The isolated material was purified by reverse-phase HPLC (SiliCycle C18, 5 μM, 10 × 250 mm column, eluting with gradient flow over 30 min of 5:95 → 25:75 MeCN/10 mM aqueous C₃F₇CO₂H, 214 nm UV detection). At a flow rate of 4 mL/min, 11-SEA⁷ had a retention time of 10–11 min and was isolated as a white hygroscopic solid following lyophilization (285 μg, 12.4%). ¹H NMR (600 MHz, D₂O, spectrum recorded immediately after dissolving the sample in D₂O): δ 4.80 (s, 1H, by HSQC), 4.28 (dd, J = 11.7, 9.1 Hz, 1H), 4.06 (dd, J = 11.7, 5.3 Hz, 1H), 3.95 (t, J = 9.4 Hz, 1H), 3.83 (ddd, J = 9.1, 5.3, 1.4 Hz, 1H), 3.26−3.19 (m, 1H), 2.80 (ddd, J = 16.6, 9.8, 6.8 Hz, 1H), 2.75−2.66 (m, 1H), 2.54 (dd, J = 16.3, 6.3 Hz, 1H) ppm; ¹H NMR (600 MHz, D₂O, after 15 h in D₂O): δ 4.28 (dd, J = 11.7, 9.3 Hz, 1H), 4.09−4.01 (m, 2H), 3.84 (ddd, J = 9.2, 5.3, 1.3 Hz, 1H), 3.27 (d, J = 10.1 Hz, 1H), 2.84 (d, J = 16.9 Hz, 1H), 2.63 (d, J = 16.9 Hz, 1H) ppm; ¹³C{¹H} NMR (150 MHz, determined by HMBC/HSQC, D₂O): δ 177.3, 159.1, 158.0, 156.1, 99.3, 83.0, 62.1, 56.0, 52.0, 46.8, 38.3, 30.5 ppm; HRMS (ES⁺): calcd for C₁₂H₂₀N₇O₆, 358.1470 [M + H]⁺; found, 358.1471.