Synthesis of the Azaphilones (+)-Sclerotiorin and (+)-8-O-Methylsclerotiorinamine Utilizing (+)-Sparteine Surrogates in Copper-Mediated Oxidative Dearomatization

Abstract

Enantioselective syntheses of the azaphilone natural products (+)-sclerotiorin and (+)-8-O-methylsclerotiorinamine which possess the natural R-configuration at the quaternary center are reported. The syntheses were accomplished using copper-mediated asymmetric dearomatization employing bis-μ-oxo copper complexes prepared from readily available (+)-sparteine surrogates and feature site selective O-methylation of a vinylogous pyridone to access to the isoquinolin-6(7H)-one core of (+)-8-O-methylsclerotiorinamine.

Introduction

The azaphilones¹ are a structurally diverse family of natural products containing a highly oxygenated bicyclic core and a quaternary center. Our laboratory has previously reported the enantioselective synthesis of the azaphilone natural product S-15183a 1 (Figure 1) bearing a (R)-quaternary center.² The synthesis was achieved utilizing enantioselective oxidative dearomatization³ mediated by a [(−)-sparteine]₂Cu₂O₂ complex.⁴ A number of azaphilone natural products including (+)-sclerotiorin 2 and 8-O-methylsclerotiorinamine 3 possess the opposite stereochemistry at C7 compared to 1 (Figure 1).⁵ Since (+)-sparteine is not readily available or readily synthesized,⁶ it was necessary to identify an appropriate (+)-sparteine surrogate for use in asymmetric oxidative dearomatization. In this Article, we report the first asymmetric syntheses of (+)-2 and (+)-3 employing readily available (+)-sparteine surrogates ⁷ in copper-mediated oxidative dearomatization.

Figure 1.

Azaphilone Natural Products.

Results and Dicussion

In our initial studies, we investigated the utility of (+)-sparteine surrogates 5–8 developed by O’Brien and co-workers⁷ for formation of L₂Cu₂O₂ complexes 9 to mediate oxidative dearomatization of alkynylbenzaldehyde 10² in the presence of N,N-diisopropylethylamine (DIEA) as base and 4-dimethylaminopyridine (DMAP) as additive (Table 1). Gratifyingly, (+)-sparteine surrogates 5–7 were found to be suitable for this transformation and provided the desired (S)-enantiomer 11 in good to excellent enantiomeric excess (Table 1).⁸ N-ethyl-(+)-sparteine mimic 6 was found to be the optimal ligand to afford azaphilone core 11 in 70% yield and 95% e.e, (entry 3, Table 1) and was therefore employed in syntheses of (+)-sclerotiorin 2 and (+)-8-O-methylsclerotiorinamine 3 (vide infra). Use of (+)-sparteine surrogate 7 bearing an N-isopropyl substituent decreased both the yield and ee of azaphilone 11 (entry 4, Table 1). Likewise, ligand 8 bearing an N-neopentyl group was not effective, likely due to steric hindrance in the formation of the putative bis-μ-oxo complexes (entry 5, Table 1). (+)-Sparteine surrogates were also found to be more susceptible to oxidative modification^8,9 under the reaction conditions than the alkaloid (−)-sparteine which may explain the slightly lower yields observed relative to (−)-sparteine. Through mass spectrometry analysis, we determined that the (+)-sparteine surrogates are oxidized preferentially on the interior ring system and not on the N-alkyl side chain.⁸

Table 1.

Oxidative dearomatization employing sparteine surrogates.^a

entry	ligand	product	yieldb (ee)
1	4	(R)-11	84% (98%)2
2	5	(S)-11	63% (92%)
3	6	(S)-11	70% (95%)
4	7	(S)-11	54% (74%)
5	8	(R)-11	33% (11%)

^aConditions: (a) 2.2 equiv. Cu(CH₃CN)₄PF₆, 2.4 equiv. ligand, 1.6 equiv. DIEA, 2.4 equiv. DMAP, O₂, −78°C to −10°C; (b) aq. H₂PO₄/K₂HPO₄ buffer (pH 7.2), CH₃CN, RT [b] Isolated yield after silica gel chromatography. DIEA = N,N-diisopropylethylamine, DMAP = 4-(dimethylamino)pyridine.

Following development of an efficient route to the S-azaphilone core, we initiated studies towards the synthesis of (+)-sclerotiorin 2 and 8-O-methylsclerotiorinamine (3, Figure 1). Azaphilone 2 was the first azaphilone reported in the literature^5a and has been shown to be an inhibitor of several important protein targets.¹⁰ Azaphilone 3 has been shown to significantly inhibit binding between the Grb2-SH2 domain and the phosphopeptide derived from the Shc protein.^5d According to our retrosynthetic analysis (Figure 2), we anticipated that 8-O-methylsclerotiorinamine 3 could be obtained by amination and methylation of sclerotiorin 2 which itself could be derived from acylation and chlorination of 12. Azaphilone core structure 12 may be obtained via diastereoselective oxidative dearomatization³ of alkynylbenzaldehyde 13 followed by cycloisomerization.²

Figure 2.

Retrosynthetic analysis for 8-O-methylsclerotiorinamine 3.

Synthesis of the requisite alkynyl benzaldehyde substrate 13 was initiated with preparation of an iodo-diene side-chain fragment (Scheme 1). Swern oxidation of the commercially available chiral alcohol 14, followed by treatment of the intermediate aldehyde with (carbethoxyethylidene)triphenylphosphorane, afforded allylic ester 15 (85 % yield, >20:1 E:Z selectivity).¹¹ DIBAL-H reduction of 15 provided alcohol 16 (55 %) which was cleanly oxidized with manganese oxide to produce enal 17.¹² Chlorous chromium-mediated Takai olefination¹³ of 17 produced iodo-diene 18 (65 % yield over two steps, 10:1 E:Z selectivity).

Scheme 1.

Synthesis of iodo diene 18.

a) (COCl)₂, DMSO, Et₃N, CH₂Cl₂ 0°C, 3 h; b) (carboethoxyethylidene)triphenylphosphorane, reflux, 12 h, 85%, 20:1 E:Z; c) DIBAl-H, CH₂Cl₂,rt, 2 h, 55%; d) MnO₂, THF, 24h e) CrCl₂, CHI₃, dioxane:THF 6:1, 2 h, 0°C, 65% 2 steps, 10:1 E:Z.

For expedient access to the Sonogashira coupling partner for 18, we developed an optimized route to iodobenzaldehyde 19 (Scheme 2).¹⁴ We have previously reported a six step route to the corresponding bromobenzaldehyde involving installation of a nitro group to 3-methyl-2,4-dimethoxybenzaldehyde 20 to allow for ortho-halogenation of the aldehyde.¹⁵ A more concise route was developed utilizing the directed lithiation conditions originally reported by Comins and coworkers.¹⁶ Treatment of 20 with N,N,N′-trimethylethylenediamine, followed by ortho-lithiation of the amino-alkoxide intermediate with n-butyl lithium and subsequent treatment with 1,2-diiodoethane, produced the desired protected iodobenzaldehyde. Deprotection using boron tribromide proceeded smoothly to yield iodobenzaldehyde 19 (33% yield, three steps) using a single purification step. Sonogashira coupling of iodobenzaldehyde 19 with trimethylsilylacetylene, followed by silyl deprotection, afforded alkynylbenzaldehyde 21 (80 % yield, Scheme 3).¹⁷ Alkynylbenzaldehyde 21 was subjected to Sonogashira conditions with vinyl iodide 18 providing alkynylbenzaldehyde 13 in 55% yield.

Scheme 2.

Synthesis of alkynylbenzaldehyde 13.

a) N,N,N′-trimethylethylenediamine, n-BuLi, THF, 0°C 30 min.; n-BuLi, THF, −20°C, 16 h, then 1,2-diiodoethane; BBr₃, CH₂Cl₂, 18 h (33% yield three steps b) CuI, trimethylsilylacetylene, (t-Bu)₃P-HBF₄, PdCl₂(CH₃CN)₄, diisopropylamine, rt, 12 h, then K₂CO₃, MeOH, rt, 1 h 80%, two steps; c) Pd(PPh₃)₄, CuI, Et₃N, 12 h, rt, 55%.

Scheme 3.

Synthesis of (+)-sclerotioin 2.

a) 2.2 equiv. Cu(CH₃CN)₄PF₆, 2.4 equiv. N-ethyl-(+)-sparteine mimic 6, 1.6 equiv. DIEA, 2.4 equiv. DMAP, O₂, −78°C to −10°C; b) aq. H₂PO₄/K₂HPO₄ buffer (pH 7.2), CH₃CN, rt 76% over two steps c) acetic anhydride, DMAP, DIEA, 0°C, 30 min d) NCS, CH₃CN, rt, 24 h 65% over two steps; e.) NH₄OAc, MeOH, 30 min, 60%.

(+)-Sclerotiorin 2 was obtained in four steps (49% yield) from alkynylbenzaldehyde substrate 13 (Scheme 4). Copper-mediated, diastereoselective oxidative dearomatization of 13 afforded azaphilone 12 (76% yield, d.r.=12:1) with (S)-stereochemistry at C-7 based on the sign of the optical rotation ([α]_D²²=+163) after conversion to 2 (lit. reported [α]_D²²=+133)^5a utilizing the N-ethyl (+)-sparteine surrogate 6 to access the desired L₂Cu₂O₂ complex.^7b,8 Since the optical rotation of the final product 8-O-methylsclerotiorinamine (+)-3 was consistent with that reported in the literature (vide infra, [α]_D²²=+104, literature reported [α]_D²²=+102),^5d the apparent erosion in stereoselectivity in the formation of (+)-12 in comparison to the formation of (+)-11 may be an artifact of an inseparable impurity (vida infra). Azaphilone 12 was acylated (Ac₂O/DMAP) and chlorinated (N-chlorosuccinimide, NCS) to afford the natural product sclerotiorin 2 (65% yield for two steps). Amination of 2 with ammonium acetate provided the vinylogous pyridone sclerotiorinamine 22 (60% yield).¹⁸

Scheme 4.

Isomerization of the C11-C12 olefin.

a) SiO₂, ethyl acetate, 1 h.

During the synthesis of (+)-sclerotiorin 2, we found that the azaphilone core 12 was formed as a mixture of two compounds in a ~10:1 ratio after silica gel purification. Based on comparison with related azaphilones such as isochromophilone I and II which contain the same dienyl sidechain, we determined that the minor compound was the azaphilone containing a Z-double bond at C11–C12.¹⁹ The minor isomer is generally seen as a minor component during isolation, however, the two olefin isomers have been reported to interconvert.²⁰ Through analyses of the isolation procedures of azaphilones containing the dienyl sidechain, it was determined that the minor E,Z isomer (cf. Scheme 4) is present in compounds which have been purified using silica gel chromatography. Accordingly, we suspected that azaphilones bearing dienyl sidechains were prone to mild acid-catalyzed olefin isomerization. The latter was confirmed by treating a natural sample of (+)-sclerotiorin 2 with silica gel which resulted in a mixture of 2:23 in ~10:1 ratio (Scheme 4). We believe that the facile isomerization results from protonation of 2 to afford the stabilized carbocation 24 which allows for equilibration between 2 an 23.⁸ The minor isomer 23 may affect the optical rotation of (+)-sclerotiorin, limiting comparison of optical rotations of synthetic ([α]_D²²=+163) and natural samples ([α]_D²²=+133).

With a sample of sclerotiorinamine 22 in hand, we next investigated conditions for O-methylation²¹ to access the congener 8-O-methylsclerotiorinamine. After screening of various reported methylation procedures, it was found that selective N-methylation could be obtained upon treatment of 22 with trimethyloxonium tetrafluoroborate (Me₃OBF₄) with diisopropylamine as base (0°C) to afford N-methylsclerotiorinamine 25 (Scheme 5) (85% yield). The selectivity for N-methylation was confirmed by alternative synthesis of 25 by treatment of (+)-sclerotiorin 2 with methylamine (>95%).²² Interestingly, methylation of 22 with Me₃OBF₄ and 2,6-di-t-butylpyridine (DTBP) as base led to apparent methylation of the C6-oxygen based on the crude NMR spectra. However, the presumed 7,8-dihydroisoquinoline was found to be unstable to purification and isolation. It is possible that utilization of a weaker base may alter the selectivity from N to O-methylation by not significantly deprotonating the N-H of the vinylogous amide until activation by O-alkylation. Other attempts to employ alternant electrophilic methylating agents (e.g. methyl triflate) and various weak bases did not result in isolable products.

Scheme 5.

Synthesis of N-methylsclerotiorinamine 25.

a) Me₃OBF₄, iPr₂NH, THF, 85%; b) methylamine, THF, >95%

A related method for O-methylation of amides involves treatment with diazomethane (CH₂N₂).²³ These reactions generally proceed without the need for base, likely arising from methylation of the enol tautomer of the amide. Gratifyingly, treatment of vinylogous pyridine 22 with trimethylsilyldiazomethane²⁴ in a mixture of dichloromethane and methanol resulted in quantitative formation of 8-O-methylsclerotiorinamine 3 (Scheme 6) which had spectroscopic data consistent with that reported in the literature including the optical rotation ([α]_D²²=+104, literature reported [α]_D²²=+102) (Scheme 7).^5d In the case at hand, derived enol 26 may be deprotonated by CH₂N₂ to give ion pair 27 which may expel nitrogen to yield 3 (Figure 3). Presumably, in this case selective methylation at C8 was observed⁸ due to steric congestion around the C6 ketone.

Scheme 6.

Synthesis of (+)-8-O-methylsclerotiorinamine 3.

a) TMSCHN₂, CH₂Cl₂:MeOH 9:1, quantitative

Figure 3.

Proposed Mechanism for O-methylation

7-Epi-Sclerotiorin 28, a naturally occurring diasteromer of (+)-sclerotiorin varying only at the C7 stereocenter^5c and the analogous 7-epi-8-O-methylsclerotiorinamine 29 were synthesized using the same synthetic sequence described above⁸ by substituting N-ethyl (+)-sparteine surrogate 6 for (−)-sparteine in the copper-mediated, oxidative dearomatization (Figure 4). Comparison of both diastereomers of the natural products with the rotation of the natural samples thus confirms the stereochemistry of 8-O-methylsclerotiorinamine.⁸

Figure 4.

Epimers of the Azaphilone Natural Products.

Conclusion

We have developed methodology to access azaphilone natural products containing the natural S-configuration at the quaternary center found in various members of this family utilizing (+)-sparteine surrogates. The methodology has been utilized in the asymmetric syntheses of the azaphilone natural products (+)-sclerotiorin 2 and (+)-8-O-methylsclerotiorinamine 3. Further studies towards the synthesis of related natural products and biological evaluation of 2, 3, and related molecules will be reported in due course.

Experimental Section

Azaphilone core 11

Alkynylbenzaldehyde 10 (27 mg, 0.10 mmol) and N,N-diisopropylethylamine (DIEA) (28 μL, 0.16 mmol) in 400 μL CH₂Cl₂ solution were added to the Cu₂L₂O₂ complex prepared from Cu(CH₃CN)₄PF₆ (82 mg, 0.22 mmol) and (−)-sparteine (56 μL, 0.24 mmol) in 1.6 mL anhydrous CH₂Cl₂ under an oxygen atmosphere at −78°C. After 5 min, 4-dimethylaminopyridine (29.2 mg, 0.24 mmol) in 400 μL CH₂Cl₂ was added and the mixture was warmed and stirred at −10°C for 30 h under an oxygen atmosphere. The reaction was quenched at −10°C with 2 mL of 10% sulfuric acid and 2 mL sat. brine. The mixture was extracted three times with EtOAc, and the combined extracts were concentrated in vacuo. The crude residue was dissolved in 2.0 mL of CH₃CN, and 1.0 mL of 1:1.6 aqueous KH₂PO₄/K₂HPO₄ buffer (pH 7.2, prepared by dissolving 1.7 g KH₂PO₄ and 3.5 g K₂HPO₄ in 50 mL of distilled water) was added to the solution. The resulting heterogeneous mixture was stirred at room temperature for 1 h. 10% H₂SO₄ was added dropwise to the reaction mixture to adjust the pH to 2–3. The resulting mixture was extracted three times with EtOAc. The organic extracts were dried over anhydrous Na₂SO₄, concentrated, and purified on silica gel (hexane: EtOAc = 10:1 to 2:1) to afford azaphilone core 11. The ¹H, ¹³C NMR, IR, and CIHRMS were found to be identical to those previously reported.² The enantiomeric excess for 11 was determined using a HPLC (Chiralcel OD, 15% ⁱPrOH in hexane, 1.0 mL/min) using UV detection at 320 nm and are listed in Table 1 corresponding to the ligand used.

2-Iodo-4,6-dihydroxy-5-methylbenzaldehyde 19

N,N,N′-trimethylethylenediamine (4.0 mL, 30.5 mmol) was dissolved in 200 mL of THF. A solution of 1.9M nBuLi (15 mL, 36.7 mmol) was added at 0°C. After stirring for 15 minutes, the reaction was cooled to −20°C and a solution of benzaldehyde 20 (5.0 g, 27.7 mmol) in 50 mL THF was added. The mixture was stirred for 30 minutes and then a solution of 1.9M nBuLi (45 mL, 73.3 mmol) was added dropwise. The reaction was stirred overnight. The solution was cooled to −40°C and a solution of diiodoethane (23.5 g, 83.2 mmol) in 50 mL THF was added dropwise. After 5 minutes, the reaction was warmed to room temperature and the reaction was quenched by addition of saturated ammonium chloride (100 mL), followed by saturated sodium thiosulfate (40 mL), and finally extracted three times with diethyl ether (3 X 120 mL). The organic layer was concentrated in vacuo and purified on silica (ethyl acetate:hexanes 1:10) to yield 19 (2.8 g, 33% yield, three steps) as a faint yellow powder. mp 75–77°C; ¹H NMR (400 MHz, CDCl₃) δ 10.25 (1H, s), 6.91 (1H, s), 3.88 (3H, s), 3.79 (3H, s), 2.08 (3H, s); ¹³C NMR (75.0 MHz, CDCl₃) δ 189.6, 162.7, 161.8, 124.0, 120.68, 120.65, 112.2, 62.3, 55.9, 8.1; IR (thin film) 2944, 1685, 1584, 1559, 1456, 1376, 1283, 1234, 1133, 1019, 816 cm⁻¹; CIHRMS M⁺ calculated for C₈H₇IO₃ 278.9518, found 278.9520.

Vinyl iodide 18

To CrCl₂ (5.4 g, 43.7 mmol) was added anhydrous THF (75 mL) at 0°C. In a separate flask, unsaturated aldehyde 17 (1.0 g, 7.9 mmol), and iodoform (4.3 g, 11.0 mmol) were added to anhydrous THF (45 mL). The aldehyde mixture was added to the reaction mixture via cannula dropwise and the resulting mixture was stirred for 1 h. The reaction was filtered through Celite® and then washed with ether. The filtrate was finally washed with water and brine, the organic layer dried with sodium sulfate, filtered, and concentrated in vacuo. The concentrated material was pass through a plug of neutral alumina (hexanes) to afford vinyl iodide 18 as a light yellow oil 1.28 g (65% yield, 10:1 E:Z by ¹H NMR analysis). Due to stability issues, 18 was used immediately and was not fully characterized. ¹H NMR (400 MHz, CDCl₃) δ 7.05 (1H, d, J=15.3 Hz), 6.12 (1H, d, J=15.3 Hz), 5.24 (1H, d, J=10.2 Hz), 2.35 (1H, m), 1.70 (3H, s), 1.40 (2H, m), 0.95 (3H d), 0.83 (3H,t, J=7.5 Hz).

Dienyne-benzaldehyde 13

An oven-dried round-bottom flask containing a mixture of vinyl iodide 18 (639 mg, 2.556 mmol), PdCl₂(PPh₃)₂ (60 mg, 0.084 mmol), and CuI (6.6 mg, 0.033 mmol) was evacuated and refilled with argon. Anhydrous triethylamine (6.0 mL, x mmol) was added dropwise and the reaction was stirred at rt for 10 minutes. Alkynylbenzaldehyde 21 (300 mg, 1.70 mmol) in 3 mL THF was added. The resulting mixture was stirred at room temperature until complete conversion of 21 was observed (TLC analysis). The reaction mixture was diluted with water, carefully neutralized with 0.1 N aqueous HCl, and extracted with ethyl acetate. The combined organic layers were washed with water and brine, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. Purification on silica gel (hexane: EtOAc = 10: 1) provided 279 mg (55%) of enyne benzaldehyde 13 as a yellow solid. mp 149–154°C; ¹H NMR (400 MHz, CDCl₃) δ 12.34 (1H, s), 10.21 (1H, s), 6.72 (1H, d, J=16 Hz), 6.49 (1H, s), 5.69 (1H, d, J=16 Hz), 5.44 (2H, ovrlp), 2.40 (1H, m), 2.10 (3H, s) 1.76 (3H, s), 1.36 (2H, m), 0.97 (3H, d, J=6.8 Hz), 0.84 (3H, t, J=7.6 Hz); ¹³C NMR (75.0 MHz, acetone-d₆) δ 195.3, 163.3, 160.7, 148.8, 144.4, 132.6, 131.9, 128.7, 127.3, 114.3, 112.2, 104.0, 96.3, 85.5, 34.9, 30.4, 20.6, 12.3, 7.4; IR (thin film) 3161 (br), 2922, 2360, 1595, 1457, 1288, 1259cm⁻¹; ESIHRMS [M+H]⁺ calculated for C₁₉H₂₃O₃ 299.1667, found C₁₉H₂₃O₃ 299.1647. [α]_D²² = +14.0 (c = 0.3, CHCl₃).

Azaphilone 12

Alkynylbenzaldehyde 13 (27 mg, 0.10 mmol) and N,N-diisopropylethylamine (DIEA) (28 μL, 0.16 mmol) in 400 μL CH₂Cl₂ solution were added to the Cu₂L₂O₂ complex prepared from Cu(CH₃CN)₄PF₆ (82 mg, 0.22 mmol) and N-ethyl-(+)-sparteine mimic 6 (56 μL, 0.24 mmol) in 1.6 mL anhydrous CH₂Cl₂ under an oxygen atmosphere at −78°C. After 5 min, 4-dimethylaminopyridine (29 mg, 0.24 mmol) in 400 μL was added and the mixture was warmed and stirred at −10°C for 30 h under an oxygen atmosphere. The reaction was quenched at −10°C with 2 mL of 10% sulfuric acid and 2 mL sat. brine. The mixture was extracted three times with EtOAc, and the combined extracts were concentrated in vacuo. The crude residue was dissolved in 2.0 mL of CH₃CN, and 1.0 mL of 1:1.6 aqueous KH₂PO₄/K₂HPO₄ buffer (pH 7.2, prepared by dissolving 1.7 g KH₂PO₄ and 3.5 g K₂HPO₄ in 50 mL of distilled water) was added to the solution. The resulting heterogeneous mixture was stirred at room temperature for 1 h. 10% H₂SO₄ was added dropwise to the reaction mixture to adjust the pH to 2–3. The resulting mixture was extracted three times with EtOAc. The organic extracts were dried over anhydrous Na₂SO₄, concentrated, and purified on silica gel (hexane: EtOAc = 10:1 to 2:1) to afford azaphilone core 12 as a red oil (76% over two steps). ¹H NMR (400 MHz, CDCl₃) δ 7.85 (1H, s), 6.95 (1H, d, J=15.6 Hz), 6.13 (1H, s), 5.90 (1H, d, J=15.6 Hz), 5.60 (1H, d, J=9.6 Hz), 5.51 (1H, s), 3.88 (1H, s), 2.40 (2H, m) 1.75 (3H, s), 1.50 (3H, s), 1.33 (2H, m), 0.93 (3H, d, J=6.4 Hz), 0.79 (3H, s); ¹³C NMR (75.0 MHz, CDCl₃) δ 196.0, 156.9, 152.5, 148.3, 144.2, 142.1, 131.8, 115.9, 108.8, 105.5, 83.4, 35.1, 30.1, 28.7, 12.3, 12.0; IR (thin film) 3431 (br), 2961, 2360, 1616, 1540, 1172, 838cm⁻¹ [α]_D²² = +82.0 (c = 0.1, CHCl₃).

7-epi-Azaphilone 30

Azaphilone 30 was synthesized following the same procedure as above for (+)-12, except for the replacement of ligand 6 with (−)-sparteine 4. All spectroscopic data was indistinguishable from (+)-12. [α]_D²² = −62.0 (c = 0.1, CHCl₃).

(+)-Sclerotiorin 2

To azaphilone (+)-12 (120 mg, 0.41 mmol) in 3 mL methylene chloride at 0°C was added acetic anhydride (77 μL, 0.82 mmol) and N, N-diisopropylethylamine (80 μL, 0.45 mmol). The resulting mixture was stirred at 0°C for 30 m. The reaction mixture was quenched with water, extracted three times with ethyl acetate, washed with 1 N HCl, washed three times with water, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The residue was dissolved in acetonitrile (3 mL) and N-chlorosuccinimide (129 mg, 0.96 mmol) was added. The reaction vessel was covered with aluminum foil and stirred at room temperature until ¹H NMR analysis indicated completion of the reaction. The reaction mixture was diluted with ethyl acetate, washed with water, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo to yield sclerotiorin 2 as a red oil (65%, two steps). The synthetic sample matched the ¹H, ¹³C, and mass spectrum of the natural product. However due to the presence of a small amount of olefin isomer the optical rotations were not comparable. ¹H NMR (400 MHz, CDCl₃) δ 7.91 (1H, s), 7.05 (1H, d, J=15.6 Hz), 6.62 (1H, s), 6.07 (1H, d, J=15.6 Hz), 5.69 (1H, d, J=9.6 Hz), 2.45 (1H, m), 2.15 (3H, s), 1.82 (3H, s) 1.54 (3H, s), 1.39 (2H, m), 1.33 (2H, m), 0.99 (3H, s), 0.85 (3H, t, J=7.2 Hz); ¹³C NMR (75.0 MHz, CDCl₃) δ 196.0, 186.2, 170.3, 158.3, 152.9, 149.1, 143.1, 138.9, 132.2, 115.9, 114.7, 111.0, 106.6, 84.8, 35.3, 30.3, 22.7, 20.4, 20.3, 12.6, 12.2; IR (thin film) 3438 (br), 2924, 2853, 1631cm⁻¹ CIHRMS M⁺ calculated for C₂₁H₂₄O₅Cl 391.1312, found 391.1333. [α]_D²² = +163.0 (c = 0.1, EtOH) lit. reported [α]_D²² = +133.0° (c = 0.3, EtOH).

7-epi-Sclerotiorin 28

Starting from 30, the same procedure employed to prepare (+)-2 was followed. All spectroscopic data was undistinguishable from (+)-2. [α]_D²² = −103.0 (c = 0.1, EtOH).

Sclerotiorinamine (+)-22

To sclerotiorin (+)-2 (217 mg, 0.59 mmol) in THF (6 mL) was added ammonium acetate (54.8 mg, 0.71 mmol). The reaction mixture was stirred at room temperature until TLC indicated completion of the reaction. The mixture was diluted with water, extracted three times with ethyl acetate, washed with water, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo to yield sclerotiorinamine 22 as an orange oil (229 mg, 100%) which was used directly in the next step without further purification.

N-Methylsclerotiorinamine (+)-25

Sclerotiorinamine (+)-22 (4.3 mg, 0.01 mmol) in CH₂Cl₂ (200 μL) was cooled to 0°C. Trimethyloxonium tetrafluoroborate (3.6 mg, 0.24 mmol) was added, followed by the slow addition of diisopropylamine (4.2 μL, 0.02 mmol). The reaction was stirred at 0°C until completion of the reaction was observed (TLC analysis). Purification over silica (EtOAc) provided N-methylsclerotiorinamine (+)-25 as a red oil (3.4 mg, 85%). ¹H NMR (400 MHz, CDCl₃) δ 7.76 (1H, s), 7.04 (1H, s), 6.97 (1H, d, 15.6 Hz), 6.12 (1H, d, 15.6 Hz), 5.72 (1H, d, 9.8 Hz), 3.62 (3H, s), 2.46(1H, m), 2.17 (3H, s), 1.86 (3H, s), 1.54 (3H, s), 1.43 (1H, ovrlp), 1.34 (1H, ovrlp), 1.01 (3H, d, 6.9 Hz), 0.88 (3H, t, 7.5 Hz); ¹³C NMR (75.0 MHz, CDCl₃) δ194.0, 184.6, 170.4, 148.7, 146.3, 145.3, 145.0, 142.1, 131.9, 114.8, 114.7, 111.4, 102.4, 85.0, 42.1, 35.3, 30.3, 23.5, 20.6, 20.5, 12.8, 12.3; IR (thin film) 2961, 2922, 2360, 1734, 1703, 1593, 1500, 1253, 1201cm⁻¹ CIHRMS M⁺ calculated for C₂₂H₂₆NO₄Cl 404.1629, found 404.1625. [α]_D²² = +114 (lit. reported = +112)^5d (c = 0.1, CH₃OH).

(+)-8-O-Methylsclerotiorinamine (+)-3

To sclerotiorinamine 22 (25 mg, 0.064 mmol) in CH₂Cl₂:MeOH 9:1 (5 mL) was added trimethylsilyldiazomethane (2.0M in diethylether) (160 μL, 0.3 mmol). The reaction was stirred at rt for 1 h. Purification using silica gel chromatography (ether:hexanes 2:3) provided (+)-8-O-methylsclerotiorinamine 3 as an orange gum (26 mg, >95%.). ¹H NMR (400 MHz, CDCl₃) δ 9.05 (1H, s), 7.55 (1H, s), 7.5 (1H, d, J = 15.9 Hz), 6.60 (1H, d, 16 Hz), 5.71 (1H, d, 9.6 Hz), 4.01 (3H, s), 2.49 (1H, m), 2.10 (3H, s), 1.84 (3H, s), 1.55 (3H, s), 1.43 (2H, m), 1.34 (1H, m), 0.99 (3H, d, J=9.6 Hz), 0.87 (3H, t, J = 7.5 Hz); ¹³C NMR (75.0 MHz, CDCl₃) δ192.7, 170.1, 162.4, 159.6, 149.5, 146.6, 143.1, 132.5, 130.5, 124.6, 119.1, 115.5, 111.5, 80.7, 61.9, 34.9, 30.2, 23.1, 20.4, 20.4, 12.6, 12.0; IR (thin film) 2961, 2871, 1742, 1696, 1611, 1579, 1279, 1248 cm⁻¹ CIHRMS M⁺ calculated for C₂₂H₂₆NO₄Cl 404.1629, found 404.1635. [α]_D²² = +104 (lit. reported = +102) (c = 0.1, CH₃OH). All spectroscopic data matched those reported in the literature.^5d

7-epi-8-O-Methylsclerotiorinamine 29

Starting from (−)-sclerotiorinamine 30 the same procedure to synthesize (+)-22 and (+)-3 was followed to produce 29. All spectroscopic data was indistinguishable from (+)-3. ([α]_D²² = −80 (c = 0.1, CH₃OH).