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. Author manuscript; available in PMC: 2021 Dec 18.
Published in final edited form as: J Org Chem. 2020 Sep 23;85(24):16035–16042. doi: 10.1021/acs.joc.0c01838

Synthesis of 3-Deoxy-D-manno-oct-2-ulosonic Acid (KDO) and Pseudaminic Acid C-Glycosides

Emmanuel Onobun 1,2,3, David Crich 1,2,3,*
PMCID: PMC7749074  NIHMSID: NIHMS1632322  PMID: 32897074

Abstract

The preparation of glycosyl dibutyl phosphates in the 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) and pseudaminic acid series and their application to the formation of C-glycosides is described. Both donors were obtained from the corresponding thioglycosides by treatment with dibutylphosphoric acid and N-iodosuccinimide. As with the thioglycosides, both donors adopted very predominantly the strongly electron-withdrawing tg conformation of their side chains, which is reflected in the excellent equatorial selectivity of both donors in the formation of exemplary O-glycosides. With respect to C-glycoside formation on the other hand contrasting results were observed: the KDO donor was either relatively unselective or selective for formation of the axial C-glycoside, while the pseudaminic acid donor was selective for the formation of the equatorial C-glycoside. These observations are rationalized in terms of the greater electron-withdrawing ability of the azides in the pseudaminic acid donor compared to the corresponding acetoxy groups in the KDO series, resulting in reaction through tighter ion pairs even at the SN1 end of the general glycosylation mechanism. The contrast in axial vs equatorial selectivity between C- and O-glycosylation cautions against the extrapolation of models for SN1 type glycosylation with weak nucleophiles to the explanation of O-glycosylation.

Graphical Abstract

graphic file with name nihms-1632322-f0001.jpg

Introduction

Among sialic and octulosonic acid glycosyl donors the KDO donor 1 and its pseudoenantiomeric counterpart, pseudaminic acid donor 2 display greater selectivity for the formation of equatorial O-glycosides on activation at low temperature with N-iodosuccinamide (NIS) and trifluoromethanesulfonic acid (TfOH) than the legionaminic acid donor 3, and the neuraminic acid donors 4 and 5.1, 2 This difference in selectivity is ascribed to the different relative configurations spanning C5-C7 (Figure 1), which directly influences the side chain conformation. Thus, 1 and 2 have the arabino-configuration from C5-C7, which enforces the most electron-withdrawing tg conformation of the exocyclic C6-C7 bond, whereas 3-5, all have the lyxo configuration spanning C5-C7 and the gg conformation about the exocyclic bond.3 A further neuraminic acid donor 6 retains the C5-C7 lyxo configuration and gg conformation of the side chain but is highly equatorially selective in O-glycosylation because of the presence of the fused N-acetyl oxazolidinone ring.4–6

Figure 1.

Figure 1.

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Side chain conformations of donors 1-6 and their relative configurations from C5-C7 as depicted on Fischer projection formulas of the parent sugars.

The relationship between side chain conformation and O-glycosylation selectivity arises from the stereoelectronic relationship between the vicinal C6-O6 and C7-O7 bonds and the manner in which this influences the build-up of positive charge in the transition state for glycosylation. In the tg conformation (1 and 2) the C7-O7 bond is antiperiplanar to the C6-O6 bond and its electron-withdrawing influence is maximized, which in turn maximizes destabilization of transition states carrying high degrees of positive charge on the anomeric center and the ring oxygen. In the gg conformation on the other hand (3-5) the C7-O7 bond is gauche to the C6-O6 bond and is considered to stabilize positive build up on the ring during glycosylation.7, 8 In other words, 1 and 2 are considered to react through mechanisms closer to the SN2-like end of the general glycosylation mechanism9 whereas 3-5, while still exhibiting high degrees of SN2-like character consistent with predominant formation of equatorial O-glycosides, are viewed as having looser transition states. In donor 6 the presence of the N-acetyloxazolidinone group compensates for the gg conformation of the side chain mostly resulting in excellent equatorial selectivity in O-glycoside formation.

C-Glycosylation,10 with weaker nucleophiles11, 12 generally considered insufficiently potent to take part in SN2-reactions,13–16 represents the more SN1-like extreme of the general glycosylation reaction as is clear from the limited kinetic evidence available.17 As such the selectivities of C-glycosylation reactions are usually interpreted in terms of competing diastereoselective transition states for attack on the two faces of a glycosyl oxocarbenium ions in the product forming step of an SN1 reaction.13–15 Earlier we demonstrated that potent C-nucleophiles such as allyltributylstannane and various silyl enol ethers were excellent nucleophiles for the selective formation of equatorial C-glycosides on activation of the N-acetyloxazolidinone-protected neuraminic acid dibutylphosphate 7 (Figure 2), derived from 6, with TMSOTf at low temperature in dichloromethane/acetonitrile mixtures.18 On the other hand, recently we showed that the neuraminic dibutylphosphates 8 and 9, derived from 4 and 5, exhibited poor selectivity on coupling to allyltributylstananne,19 thereby demonstrating the importance of protecting groups on selectivity in a series of donors all with the same gg conformation of the side chain. We now turn our attention to C-glycosylation by the reaction of C-nucleophiles with the dibutylphosphates 10 and 11, derived from 1 and 2, with a view to gauging the influence of side chain conformation on C-glycosylation. KDO and related C-glycosides have been previously synthesized, with axial or equatorial selectivity, by alkylation of anomeric lithium or samarium enolates,20–23 and with axial selectivity from anomeric radicals.24, 25 We are not aware of any pseudaminic acid C-glycosides.

Figure 2.

Figure 2.

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Side chain conformations of donors 7-11

Results and Discussion

The KDO dibutyl phosphate 10 was obtained from the thioglycoside 1 by treatment with dibutylphosphoric acid, NIS and triflic acid in dichloromethane at 0 °C as a single (α) anomer following the method used earlier for the preparation of 7-9 from 4-6,18, 19 and initially applied for the formation of 7 from the phenylthioglycoside analog of 6 by Wong and coworkers.26 The pseudaminic acid donor 2 was similarly converted to 11 in 78% yield as a 1:3 α:β mixture of anomers. Both 10 and the two anomers of 11, retain the predominant tg conformation of the side chain as evidence by their 3JH6,H7 coupling constants of ~ 7.5 Hz. The anomeric configuration of 10 and the major anomer of 11 was confirmed by their 3JC1,H3ax heteronuclear coupling constants27–30 of < 1 and 6.7 Hz, respectively.

Before turning to C-glycosylation we verified that 10 and 11 function as effective and selective O-glycosyl donors like their thioglycoside counterparts. Thus, activation of 10 in 2:1 dichloromethane/acetonitrile at -78 °C in the presence of the methyl 2-O-acetyl-4-O-benzyl-α-L-rhamnopyranoside by the addition of TMSOTf gave glycoside 12 in 81% yield as a single equatorial or β-anomer (Scheme 1), whose spectral data were consistent with those of a sample obtained previously from thioglycoside 1.1 Comparable activation of 11 in the presence of methyl 2,4,6-tri-O-benzyl-β-D-galactopyranoside gave the glycoside 13 in 72% yield as a β anomer, fully consistent with the analogous coupling employing thioglycoside 2.2 When the coupling of 10 with methyl 2,4,6-tri-O-benzyl-β-D-galactopyranoside was conducted in pure dichloromethane 12 was obtained in 81% yield as a single anomer indicating that the addition of acetonitrile is not necessary for the O-glycosylation.

Scheme 1.

Scheme 1.

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Selective O-glycoside synthesis with donors 10 and 11

We surveyed C-glycosylations of donor 10 with a panel of nucleophiles with Mayr nucleophilicities (N)11, 31 ranging from 3.44–6.22 (Table 1) with activation by TMSOTf at -78 °C. Reactions were conducted in pure dichloromethane and in 2:1 dichloromethane/acetonitrile. While good to excellent yields were obtained in each case (Table 1), selectivities were modest and favored the formation of the axial or α-anomer over the equatorial or β-anomer, with the exception of the use of the silyl enol ether derived from t-butyl methyl ketone which was highly selective for formation of the axial isomer. Only modest differences in selectivity were observed between the two solvents employed. These relatively minor solvent effects, coupled with the lengthy synthesis of the donor,2 prompted us confine our investigations in the pseudaminic acid series to reactions conducted in the 2/1 dichloromethane/acetonitrile mixture. In contrast to KDO donor 10, the pseudaminic acid donor 11 showed exquisite equatorial selectivity on coupling to allyltributylstannane giving C-glycoside 18 in dichloromethane/acetonitrile in 80% yield (Table 1, entry 5). With a second nucleophile, acetophenone trimethylsilyl enol ether, the pseudaminic acid C-glycoside 19 was obtained with 85:15 selectivity for the equatorial over the axial glycoside (Table 1, entry 6).

Table 1.

Formation of C-Glycosidesa

Donor nucleophile N product CH2Cl2 2:1 CH2Cl2:MeCN
yield α:βb yield α:βb
1 graphic file with name nihms-1632322-t0002.jpg graphic file with name nihms-1632322-t0003.jpg 5.46 graphic file with name nihms-1632322-t0004.jpg 82% 75:25 77% 80:20
2 graphic file with name nihms-1632322-t0005.jpg graphic file with name nihms-1632322-t0006.jpg 3.78 graphic file with name nihms-1632322-t0007.jpg 62% 100:0 67% 100:0
3 graphic file with name nihms-1632322-t0008.jpg graphic file with name nihms-1632322-t0009.jpg 3.44c graphic file with name nihms-1632322-t0010.jpg 63% 88:12 59% 70:30
4 graphic file with name nihms-1632322-t0011.jpg graphic file with name nihms-1632322-t0012.jpg 6.22 graphic file with name nihms-1632322-t0013.jpg 82% 35:65 80% 53:47
5 graphic file with name nihms-1632322-t0014.jpg graphic file with name nihms-1632322-t0015.jpg 5.46 graphic file with name nihms-1632322-t0016.jpg - - 80% 0:100
6 graphic file with name nihms-1632322-t0017.jpg graphic file with name nihms-1632322-t0018.jpg 6.22 graphic file with name nihms-1632322-t0019.jpg - - 83% 15:85
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a:

all reactions were carried out at -78 °C with actiation by TMSOTf

b:

anomeric ratios were determined by integration of the 1H NMR spectra of the crude reaction mixtures

c:

the nucleophilicity parameter reported for acetaldehyde TMS enol ether is that for the corresponding TIPS enol ether31

The anomeric configuration of the various C-glycosides was assigned on the basis of the 3JC1,H3ax heteronuclear coupling constant as previously described for KDO and sialic acid C-glycosides: the equatorial C-glycosides displayed 3JC1,H3ax of 8.0–8.4 Hz, while the axial anomers had 0–3.5 Hz.19, 21, 23 These assignments were confirmed by ROESY measurements (see Supporting Information) with the axial isomers showing correlation of H4 and H6 in the pyranose ring with the methylene protons of the new substituents.

The high equatorial selectivities of the C-C bonding forming reactions of pseudaminic acid donor 11 (Table 1, entries 5 and 6) are noteworthy in comparison to those of the related donors 8 and 9, which showed modest preferences (~2:1) for the formation of the equatorial over the axial isomers.19 This equatorial selectivity of 11 over 8 and 9 in C-glycosylation is consistent with the trend showed in O-glycosylation by the same donors.2 It is consistent with increased electron-withdrawing character of the side chain conformation of 11, as compared to those of 8 and 9, which in turn is a function of the differing conformations of the exocyclic side bond to the side chains: tg in 11 and gg in 8 and 9. The KDO donor 10, however, bucks the trend as, on the basis of the tg conformation of its side chain and its outstanding equatorial selectivity in O-glycosylation, it should have excellent equatorial selectivity in C-glycosylation as opposed to the modest no equatorial selectivity observed (Table 1, entries 1–4). The difference in C-glycosylation selectivity between 10 and 11, despite their having the same configuration and side chain conformation is presumably due either to the differing ability of C-N3 and C-OAc bonds at the 5- and/or 7-positions to impact inherent positive charge on the ring oxygen and at the anomeric center in transition states closer to the SN1 end of the mechanistic spectrum. In this regard we recall that azido groups have been previously noted to be more disarming than esters in other classes of glycosyl donor.32, 33 O-Glycosylation of 10 and 11, with its more powerful nucleophiles is closer to the SN2-end of the mechanistic spectrum and is less susceptible to oxacarbenium ion-stabilizing effects. In so far as no evidence for trapping a seven-membered bridged dioxapenium ion by any C-nucleophile was observed, in contrast to literature reports of trapping of five-membered cyclic dioxalenium ions by allylsilanes and stannanes in the course of C-glycosylations,34, 35 we see no need to invoke remote participation36 by the axial 5-O-acetate ester in the formation of the axial C-glycosides from donor 10.

Conclusion

Continuing our studies on C-glycosylation of sialyl and ulosonyl donors we report excellent equatorial selectivity in the formation of pseudaminic acid C-glycosides employing either allyltributylstannane or acetophenone trimethylsilyl enol ether as acceptor. In contrast, a pseudoenantiomeric KDO donor displays poor selectivity and even favors axial selectivity in some cases under otherwise the same conditions. This differing C-glycosylation selectivity between the KDO and pseudaminic acid series contrasts with O-glycosylation when both donors are highly equatorially selective. Along with other examples of selectivity changes between O- and C-glycosylations for a common donor,14–16, 37, 38 this suggests that models employing weak nucleophiles to model SN1-like glycosylations39 only be extrapolated to O-glycosylations with caution.

Experimental

General experimental

All reactions were performed in oven dried glassware under an atmosphere of argon. Commercially available starting materials were used without further purification. All solvents were dried according to standard methods. Thin-layer chromatography was performed on 250 μm glass backed silica (XHL) plates employing UV absorption (254 nm) and charring with ceric ammonium molybdate for visualization. Chromatographic purification of crude residues was performed over silica gel (230–400 mesh grade 60 silica) using analytical grade solvents. Specific rotations were measured in chloroform on an automatic polarimeter with a path length of 10 cm. 1H and 13C NMR spectra were recorded on 500 and 600 MHz instruments at 298 K. Chemical shifts (δ) are recorded in ppm and the multiplicities are given as follows: s (singlet), d (doublet), t (triplet), q (quadruplet), dd (doublet of doublet), ddd (doublet of doublet of doublet), dt (doublet of triplet), m (multiplet). The anomeric configuration of the glycosides formed was determined by measurement of the 3JC1−H3axial heteronuclear coupling constant using the HSQMBC experiment using data sets of 4096 × 512 points and confirmed by ROESY measurements. High-resolution (HRMS) mass spectra were recorded in the electrospray mode using an orbitrap mass analyzer (ThermoFisher ESI-Orbitrap). UHPLC analysis was performed with an Acclaim™ 120 C18 5μm column (4.6 × 50) equipped with mass analyzer and a UV detector.

Ethyl (1-adamantanyl 4,5,7,8-tetra-O-acetyl-3-deoxy-2-thio-α-D-manno-oct-2-ulopyranosid)onate (1).

This compound was prepared from D-mannose according to literature procedure1 (2.5 g, 76%) as an off white foam. [α]22D +51 (c 3.7, CHCl3). 1H NMR (600 MHz, CD3CN): δ 5.32 – 5.28 (m, 1H, H-5), 5.23 (ddd, J = 12.1, 5.3, 3.1 Hz, 1H, H-4), 5.15 (ddd, J = 8.6, 5.9, 2.7 Hz, 1H, H-7), 4.65 (dd, J = 8.6, 1.5 Hz, 1H, H-6), 4.54 (dd, J = 12.2, 2.8 Hz, 1H, H-8a), 4.31 (dq, J = 10.8, 7.1 Hz, 1H, OCH2CH3), 4.21 (dq, J = 10.8, 7.1 Hz, 1H, OCH2CH3), 4.01 (dd, J = 12.2, 5.9 Hz, 1H, H-8b), 2.22 – 2.17 (m, 1H), 2.16 – 2.13 (m, 1H), 2.06 – 1.94 (m, 15H), 1.93 – 1.89 (m, 6H), 1.71–1.65 (m, 6H), 1.32 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (151 MHz, CD3CN) δ 170.4, 170.1, 169.6, 169.4, 86.2, 69.3, 68.3, 66.6, 64.8, 62.2, 61.9, 49.9, 43.1, 35.7, 34.1, 29.8, 19.98, 19.96, 19.92, 19.85, 13.3. HRMS (ESI) m/z: [M + Na]+ Calcd for C28H40O11SNa 607.2189; Found 607.2186.

Dibutyl [ethyl (4,5,7,8-tetra-O-acetyl-3-deoxy-α-D-manno-oct-2-ulopyranosyl)onate] phosphate (10).

A mixture of donor 1 (500 mg, 1.2 mmol), dibutyl phosphate (0.42 mL, 2.1 mmol) and activated 4Å acid-washed powdered molecular sieves (2 g.mmol−1) in dry CH2Cl2 (17.1 mL) was stirred for 1 h at room temperature under argon, then cooled to 0 °C. The reaction mixture was then treated with NIS (222 mg, 1.3 mmol) and TfOH (22.7 μL, 0.3 mmol) and stirred at 0 °C for 15 min, then quenched with Hunig’s base (2 equiv) at -10 °C and gradually warmed to rt. The reaction mixture was diluted with CH2Cl2 (20 mL) filtered through Celite and washed with 20% aqueous Na2S2O3 (20 mL). The aqueous layer was extracted with CH2Cl2 (20 mL) twice and the combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The residue was purified on a silica cartridge eluting with hexane/ethyl acetate (70:30) to give the desired product 10 (380 mg, 71%) as a pale-yellow oil. [α]22D +52 (c 0.3, CHCl3). 1H NMR (500 MHz, CDCl3): δ 5.41 – 5.38 (m, 1H, H-5), 5.35 (ddd, J = 12.4, 5.0, 3.0 Hz, 1H, H-4), 5.29 (td, J H6,H7 = 7.0, 3.2 Hz, 1H, H-7), 4.51 – 4.46 (m, 1H, H-8a), 4.46 – 4.44 (m, 1H, H-6), 4.28 (q, J = 7.1 Hz, 2H, OCH2CH3), 4.14 (dd, J = 12.2, 6.5 Hz, 1H, H-8b), 4.12 – 4.02 (m, 4H), 2.31 (dd, J = 13.2, 4.9 Hz, 1H, 3eq), 2.18 (td, J = 12.6, 4.4 Hz, 1H, 3ax), 2.08 (s, 3H), 2.04 (s, 3H), 1.99 (s, 3H), 1.97 (s, 3H), 1.71 – 1.56 (m, 4H), 1.40 (m, 4H), 1.35 – 1.31 (m, 3H), 0.97 – 0.89 (m, 6H). 13C{1H} NMR (126 MHz, CDCl3): δ 170.8 (C), 170.5 (C), 169.8 (C), 165.8 (C, s, 3JC1−H3axial = <1 Hz), 99.8 (C, d, JC-P = 6.8 Hz), 71.0 (CH), 68.5 (CH2, d, JC-P = 6.0 Hz), 68.3 (CH2, d, JC-P = 6.6 Hz), 67.7 (CH), 65.9 (CH), 64.2 (CH), 63.1 (CH2), 62.6 (CH2), 32.3 (CH2, d, JC-P = 2.8 Hz), 32.2 (CH2, d, JC-P = 3.1 Hz), 32.2 (CH2, d, JC-P = 7.2 Hz), 20.8 (CH3), 20.8 (CH3), 20.8 (CH3), 20.7 (CH3), 18.7 (2 CH2), 14.0 (CH3), 13.6 (2 CH3). 31P NMR (202 MHz, CDCl3): δ -5.29. HRMS (ESI) m/z: [M + Na]+ Calcd for C26H43O15PNa 649.2237; Found 649.2231.

Dibutyl [methyl 4,8-di-O-acetyl-5,7-di-azido-3,5,7,9-tetra-deoxy-L-glycero-α-L-manno-non-2-ulopyranosyl)onate] phosphate (11α) and Dibutyl [methyl 4,8-di-O-acetyl-5,7-di-azido-3,5,7,9-tetra-deoxy-L-glycero-β-L-manno-non-2-ulopyranosyl)onate] phosphate (11β).

A mixture of donor2 2 (20 mg, 0.04 mmol), dibutyl phosphate (18 μL, 0.1 mmol) and activated 4Å acid-washed powdered molecular sieves (2 g.mmol−1) in dry CH2Cl2 (0.7 mL) was stirred for 1 h at room temperature under argon, then cooled to -20 °C. The reaction mixture was treated with NIS (9.4 mg, 0.05 mmol), followed by TfOH (1 μL, 0.01 mmol). After stirring at -20 °C for 15 min, the reaction was quenched with Hunig’s base (2 equiv) at -20 °C and gradually warmed to room temperature. The reaction mixture was diluted with CH2Cl2 (1 mL) filtered through Celite and washed with 20% aqueous Na2S2O3 (1 mL). The aqueous layer was extracted with CH2Cl2 (1 mL) twice and the combined organic layer was dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography (hexanes/EtOAc 90:10 gradient to 60:40) to provide 11 (16.8 mg, 78 %) as a light yellow oil with a 1:3 α:β mixture of anomers. Repeated chromatography over silica gel eluting with hexane/ethyl acetate (60:40) gave analytical samples of the pure isomers.

  • (11α): [α]22D -84 (c 0.2, CH2Cl2). 1H NMR (500 MHz, C6D6): δ 5.56 (qd, J = 6.4, 2.5 Hz, 1H), 5.39 (ddd, J = 12.0, 4.7, 3.2 Hz, 1H), 4.13 – 4.10 (m, 1H), 4.10 – 4.05 (m, 2H), 4.01 (dd, J = 6.5, 1.6 Hz, 1H), 3.99 – 3.96 (m, 2H), 3.85 (dt, J = 3.1, 1.4 Hz, 1H), 3.29 (s, 3H), 2.57 (ddd, J = 13.3, 4.6, 1.2 Hz, 1H), 2.36 (ddt, JH3a,H3b = JH3a,H4 = 12.8 Hz, JH3a,P = 3.5 Hz, 1H), 1.58 (s, 3H), 1.56 (s, 3H), 1.52 – 1.45 (m, 2H), 1.42 (d, J = 6.4 Hz, 5H), 1.22 (m, 4H), 0.81 – 0.69 (m, 6H). 13C{1H} NMR (151 MHz, C6D6): δ 168.7, 168.6, 165.9 (s, 3JC1−H3axial = <1 Hz), 99.8 (d, JC-P = 5.6 Hz), 71.1, 70.4, 68.2 (d, JC-P = 4.1 Hz), 67.9, 67.8, 64.5, 58.7, 52.4, 32.2 (d, JC-P = 4.9 Hz), 32.2, 32.1 (d, JC-P = 6.3 Hz), 20.2, 19.7, 18.6 (d, JC-P = 4.5 Hz), 13.3, 13.2 (d, JC-P = 3.9 Hz). 31P NMR (202 MHz, CDCl3): δ -5.9. HRMS (ESI) m/z: [M + Na]+ Calcd for C22H37N6O11PNa 615.2156; Found 615.2148.

  • (11β): [α]22D -20 (c 0.3, CHCl3); 1H NMR (500 MHz, C6D6): δ 5.66 (qd, J = 6.4, 2.9 Hz, 1H), 4.90 (ddd, J = 12.3, 4.9, 3.2 Hz, 1H), 4.30 – 4.23 (m, 1H), 4.22 – 4.14 (m, 2H), 4.07 – 3.98 (m, 2H), 3.98 – 3.93 (m, 1H), 3.81 – 3.76 (m, 1H), 3.33 (s, 3H), 2.59 (dd, J = 12.5, 4.9 Hz, 1H), 2.54 (t, J = 12.5 Hz, 1H), 1.61 (s, 3H), 1.52 (s, 3H), 1.43 (dt, J = 7.9, 6.4 Hz, 2H), 1.38 (d, J = 6.5 Hz, 3H), 1.36 – 1.30 (m, 3H), 1.20 (m, 3H), 0.79 (t, J = 7.4 Hz, 3H), 0.71 (t, J = 7.4 Hz, 3H); 13C{1H} NMR (151 MHz, C6D6): δ 168.9, 168.6, 167.8 (s, 3JC1−H3axial = 6.7 Hz), 98.2 (d, JC-P = 6.3 Hz), 73.0, 70.6, 68.8, 68.0 (d, JC-P = 6.1 Hz), 67.5 (d, JC-P = 6.0 Hz), 63.7, 58.1, 52.5, 33.2 (d, JC-P = 8.0 Hz), 32.1 (d, JC-P = 7.2 Hz), 32.1 (d, JC-P = 7.2 Hz), 29.9, 20.1, 19.6, 18.7, 18.5, 13.4, 13.3, 12.7; 31P NMR (202 MHz, CDCl3): δ -5.9 (d, J = 15.4 Hz); HRMS (ESI) m/z: [M + Na]+ Calcd for C22H37N6O11PNa 615.2156; Found 615.2150.

General Coupling Protocol with Donor 10.

A mixture of KDO phosphate 10, acceptor (5 equiv) and activated 4Å acid-washed pulverized molecular sieves (2 g/mmol of donor) in dry CH2Cl2 (0.05 M) or CH2Cl2/CH3CN (2:1) (0.05 M) was stirred under argon at room temperature for 1 h before cooling to -78 °C. The reaction mixture was then treated with TMSOTf (1 equiv.) and stirred at -78 °C for 5 h before quenching with triethylamine (2 equiv). The crude mixture was diluted with CH2Cl2, filtered through a pad of Celite, washed with 20% aqueous Na2S2O3, dried over Na2SO4 and concentrated in vacuo. The anomeric ratio of the products was determined by integration of the 1H NMR spectra of crude reaction mixtures, and in the case of 16αβ by integration of the HPLC trace of the crude reaction mixture (see Supporting Information).

Methyl (ethyl 4,5,7,8-tetra-O-acetyl-3-deoxy-β-D-manno-oct-2-ulopyranosyl)onate-(2→3)-2-O-acetyl-4-O-benzyl-α-L-rhamnopyranoside (12).

Glycoside 12 was prepared according to the general coupling protocol with donor 10 (25 mg, 0.04 mmol) and methyl 2-O-acetyl-4-O-benzyl-α-L-rhamnopyranoside40 (61.9 mg, 0.2 mmol) in CH2Cl2 (0.8 mL). The crude mixture was purified by column chromatography eluting with hexane/ethyl acetate (60:40) to afford 12 (23.5 mg, 81%) as a colorless syrup with spectra data consistent with those reported in literature1. [α]D23 = +65 (c = 0.8, CHCl3). 1H NMR (500 MHz, C6D6) δ 7.54 (d, J = 6.7 Hz, 2H), 7.21 (t, J = 7.7 Hz, 2H), 6.99 (t, J = 7.5 Hz, 1H), 5.62 (ddd, J = 12.5, 5.0, 3.0 Hz, 1H), 5.48 (ddd, J = 9.6, 6.6, 2.9 Hz, 1H), 5.32 (dt, J = 2.7, 1.2 Hz, 1H), 5.25 (dd, J = 3.3, 1.9 Hz, 1H), 4.80 – 4.75 (m, 2H), 4.74 – 4.67 (m, 2H), 4.67 (d, J = 1.9 Hz, 1H), 4.49 (d, J = 10.7 Hz, 1H), 4.21 – 4.14 (m, 1H), 4.15 – 4.09 (m, 1H), 3.99 (dq, J = 10.9, 7.2 Hz, 1H), 3.89 (dq, J = 9.2, 6.1 Hz, 1H), 3.64 (t, J = 9.7 Hz, 1H), 2.98 (s, 3H), 2.58 (dd, J = 12.5, 4.4 Hz, 1H), 2.30 (t, J = 12.8 Hz, 1H), 1.78 (s, 3H), 1.69 (s, 3H), 1.65 (s, 3H), 1.62 (s, 3H), 1.61 (s, 3H), 1.28 (d, J = 6.3 Hz, 3H), 0.97 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (126 MHz, C6D6): δ 169.9, 169.7, 169.5, 169.2, 169.0, 167.4 (s, 3JC1−H3axial = 6.8 Hz) , 137.5, 128.5, 127.90, 127.85, 127.7, 127.5, 98.2, 96.9, 80.1, 75.8, 70.6, 70.4, 69.0, 68.4, 67.57, 66.63, 64.3, 62.9, 62.2, 54.3, 32.6, 20.3, 20.0, 18.0, 13.3. HRMS (ESI) m/z: [M + Na]+ Calcd for C34H46O17Na 749.2628; Found 749.2620.

Ethyl (2-C-allyl-4,5,7,8-tetra-O-acetyl-3-deoxy-α-D-manno-oct-2-ulopyranitol)onate (14α) and Ethyl (2-C-allyl-4,5,7,8-tetra-O-acetyl-3-deoxy-β-D-manno-oct-2-ulopyranitol)onate (14β).

These compounds were prepared according to the general coupling protocol with donor 10 (25 mg, 0.04 mmol) and allyltributylstannane (61.9 μL, 0.2 mmol) in CH2Cl2 (0.8 mL). The crude residue was purified via silica gel column chromatography eluting with hexane/ethyl acetate (70:30) to give first 14β (11.5 mg, 60%) as a colorless syrup and then 14α (4.2 mg, 22%) as colorless syrup.

  • (14α): [α]22D +55 (c 0.7, CHCl3). 1H NMR (500 MHz, CDCl3): δ 5.63 (ddt, J = 17.2, 10.2, 7.0 Hz, 1H, CH2CH=CH2), 5.30 (dt, J = 2.6, 1.2 Hz, 1H, H-5), 5.22 – 5.19 (m, 1H, H-4), 5.19 – 5.17 (m, 1H, H-7), 5.15 – 5.08 (m, 2H, CH2CH=CH2), 4.49 (dd, J = 12.3, 2.4 Hz, 1H, H-8), 4.24 – 4.17 (m, 2H, OCH2CH3), 4.14 (dd, J = 12.2, 4.2 Hz, 1H, H-8), 3.98 (dd, J = 9.4, 1.4 Hz, 1H, H-6), 2.82 (dd, J = 14.8, 6.8 Hz, 1H, CH2CH=CH2), 2.55 (dd, J = 14.8, 7.2 Hz, 1H, CH2CH=CH2), 2.10 (t, J = 12.8 Hz, 1H, H-3), 2.07 (s, 3H), 2.04 (s, 3H), 2.00 (dd, J = 5.1, 1.0 Hz, 1H, H-3), 1.98 (d, J = 1.6 Hz, 6H), 1.27 (t, J = 7.1 Hz, 3H, OCH2CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 171.0 (C), 170.6 (C), 170.2 (C), 169.8 (C, s, 3JC1−H3axial = 3.5 Hz), 131.1 (C), 119.2 (CH2), 79.4 (C), 68.6 (CH), 68.1 (CH), 66.9 (CH), 64.5 (CH), 62.5 (CH2), 61.6 (CH2), 36.4 (CH2), 30.7 (CH2), 20.9 (CH3), 20.9 (CH3), 20.8 (CH3), 20.8 (CH3), 14.3 (CH3). HRMS (ESI) m/z: [M + Na]+ Calcd for C21H30O11Na 481.1686; Found 481.1683.

  • (14β): [α]22D +62 (c 0.5, CHCl3). 1H NMR (500 MHz, , CDCl3): δ 5.75 (ddt, J = 17.3, 10.2, 7.2 Hz, 1H, CH2CH=CH2), 5.27 (dt, J = 2.6, 1.2 Hz, 1H, H-5), 5.11 (td, J = 4.0, 2.2 Hz, 1H, H-7), 5.10 – 5.03 (m, 2H, CH2CH=CH2), 4.90 (ddd, J = 12.7, 4.8, 3.0 Hz, 1H, H-4), 4.49 (dd, J = 12.2, 2.4 Hz, 1H, OCH2CH3), 4.22 (dd, J = 7.9, 4.2 Hz, 1H, OCH2CH3), 4.20 – 4.15 (m, 2H, H-8), 4.03 (dd, J = 9.6, 1.4 Hz, 1H, H-6), 2.48 (dt, J = 7.2, 1.3 Hz, 2H, CH2CH=CH2), 2.23 (dd, J = 12.7, 4.7 Hz, 1H, H-3eq), 2.07 (s, 6H), 1.98 (s, 3H), 1.96 (s, 3H), 1.91 (t, J = 12.7 Hz, 1H, H-3ax), 1.28 (t, J = 7.1 Hz, 3H, OCH2CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 171.4 (C), 170.8 (C), 170.6 (C), 170.0 (C), 170.0 (C, s, 3JC1−H3axial = 8.3 Hz), 131.2 (CH), 119.3 (CH2), 80.6 (C), 71.1 (CH), 68.2 (CH), 67.6 (CH), 64.6 (CH), 62.9 (CH2), 61.6 (CH2), 44.4 (CH2), 31.6 (CH2), 20.9 (CH3), 20.9 (CH3), 20.8 (CH3), 20.8 (CH3), 14.4 (CH3). HRMS (ESI) m/z: [M + Na]+ Calcd for C21H30O11Na 481.1686; Found 481.1679.

Ethyl [2-C-(3,3-dimethyl-2-oxobutyl)-4,5,7,8-tetra-O-acetyl-3-deoxy-α-D-manno-oct-2-ulopyranitol]onate (15).

Glycoside 15 was prepared according to the general coupling protocol with donor 10 (25 mg, 0.04 mmol) and [(3,3-dimethylbut-1-en-2-yl)oxy]trimethylsilane (43.1 μL, 0.2 mmol) in CH2Cl2 (0.8 mL). The crude mixture was purified via silica gel column chromatography eluting with hexane/ethyl acetate (60:40) to afford 15 (12.8 mg, 62%) as a colorless syrup. [α]22D +48 (c 0.2, CHCl3). 1H NMR (500 MHz, CDCl3): δ 5.29 (dd, J = 2.8, 1.5 Hz, 1H, H-5), 5.14 (ddd, J = 9.3, 4.2, 2.5 Hz, 1H, H-7), 5.08 (ddd, J = 12.9, 4.8, 3.0 Hz, 1H, H-4), 4.48 (dd, J = 12.2, 2.6 Hz, 1H, H-8), 4.20 (qd, J = 7.1, 2.9 Hz, 2H, CH2CHO), 4.08 (dd, J = 12.2, 4.3 Hz, 1H, H-8), 3.85 (dd, J = 9.3, 1.4 Hz, 1H, H-6), 3.29 (d, J = 17.8 Hz, 1H), 3.12 (d, J = 17.8 Hz, 1H), 2.38 (t, J = 13.1 Hz, 1H), 2.08 (s, 3H), 2.04 (s, 3H), 1.98 (s, 3H), 1.96 (s, 4H), 1.25 (t, J = 7.1 Hz, 3H, OCH2CH3), 1.15 (s, 9H). 13C{1H} NMR (126 MHz, CDCl3): δ 211.0, 170.6, 170.5, 170.5, 170.3, 169.7 (s, 3JC1−H3axial = 3.1 Hz), 77.8, 69.7, 68.1, 67.0, 64.4, 62.2, 61.7, 44.4, 38.2, 30.0, 26.5, 20.9, 20.8, 20.8, 20.7, 14.1. HRMS (ESI) m/z: [M + Na]+ Calcd for C24H36O12Na 539.2104; Found 539.2106.

Ethyl [2-C-(2-oxoethyl)-4,5,7,8-tetra-O-acetyl-3-deoxy-α-D-manno-oct-2-ulopyranitol]onate (16α).

Glycoside 16α was prepared according to the general coupling protocol with donor 10 (25 mg, 0.04 mmol) and trimethyl(vinyloxy)silane (29.8 μL, 0.2 mmol) in CH2Cl2 (0.8 mL). The crude mixture was purified via silica gel column chromatography eluting with hexane/ethyl acetate (60:40) to give compound 16α (11.2 mg, 63%) as a colorless syrup. [α]22D +37 (c 0.2, CHCl3). 1H NMR (500 MHz, C6D6) δ 9.37 (dd, J = 2.8, 1.5 Hz, 1H, CH2CHO), 5.45 – 5.37 (m, 1H, H-5), 5.38 (ddd, J = 9.5, 4.9, 2.4 Hz, 1H, H-7), 4.91 (ddd, J = 12.8, 4.9, 3.1 Hz, 1H, H-4), 4.48 (dd, J = 12.2, 2.4 Hz, 1H, H-8), 4.12 (dd, J = 12.2, 5.0 Hz, 1H, H-8), 3.80 (qq, J = 7.1, 3.7 Hz, 2H, OCH2CH3), 3.60 (dd, J = 9.4, 1.4 Hz, 1H, H-6), 2.43 (dd, J = 16.4, 2.7 Hz, 1H, CH2CHO), 2.35 (t, J = 12.9 Hz, 1H, 3ax), 2.21 (dd, J = 16.4, 1.5 Hz, 1H, CH2CHO), 1.79 (m, 1H), 1.70 (d, J = 1.5 Hz, 6H), 1.68 (s, 3H), 1.62 (s, 3H), 0.79 (t, J = 7.1 Hz, 3H, OCH2CH3). 13C{1H} NMR (126 MHz, C6D6): δ 196.5, 170.0, 169.6, 169.1, 169.1(s, 3JC1−H3axial = 3.4 Hz), 77.6, 69.7, 67.8, 66.4, 64.1, 62.3, 61.7, 44.3, 30.7, 20.1, 20.0, 20.0, 13.5. HRMS (ESI) m/z: [M + Na]+ Calcd for C20H28O12Na 483.1478; Found 483.1441. The minor isomer 16β was not obtained in sufficient quantity for characterization: it was identified in the crude reaction mixture by the following diagnostic signal: 1H NMR (500 MHz, CDCl3) δ 9.55 (dd, J = 3.8, 1.7 Hz, 1H, CH2CHO). The UHPLC retention times for 16α and 16β were 4.46 and 4.78 min respectively (flow rate: 1 mL/min; gradient: initial H2O 100/CH3CN 0: 0.5 min; H2O 5/CH3CN 95: 8 min).

Ethyl [2-C-(2-oxo-2-phenylethyl)-4,5,7,8-tetra-O-acetyl-3-deoxy-α-D-manno-oct-2-ulopyranitol]onate (17α) and Ethyl [2-C-(2-oxo-2-phenylethyl)-4,5,7,8-tetra-O-acetyl-3-deoxy-β-D-manno-oct-2-ulopyranitol]onate (17β):

These compounds were prepared according to the general coupling protocol with donor 10 (25 mg, 0.04 mmol) and trimethyl[(1-phenylvinyl)oxy]silane (40.9 μL, 0.2 mmol) in CH2Cl2 (0.8 mL). The crude reaction mixture was purified by column chromatography eluting with hexane/ethyl acetate (50:50) to give first compound 17β (12.2 mg, 57%) as a colorless syrup and then compound 17α (5.4 mg, 25%) as colorless syrup.

  • (17α): [α]22D +57 (c 1.1, CHCl3). 1H NMR (500 MHz, CDCl3): δ 7.90 (d, J = 8.4 Hz, 2H, Ar), 7.58 (t, J = 8.6 Hz, 1H, Ar), 7.47 (t, J = 8.6 Hz, 2H, Ar), 5.35 – 5.30 (m, 1H, H-5), 5.23 (ddt, J = 11.1, 3.4, 1.7 Hz, 1H, H-4), 5.18 – 5.12 (m, 1H, H-7), 4.41 (d, J = 12.2 Hz, 1H, H-8), 4.14 (q, J = 7.1 Hz, 2H, OCH2CH3), 4.05 (dd, J = 12.2, 4.7 Hz, 1H, H-8), 3.97 (d, J = 9.4 Hz, 1H, H-6), 3.83 (d, J = 16.8 Hz, 1H, CH2COPh), 3.51 (d, J = 16.7 Hz, 1H, CH2COPh), 2.31 (t, J = 13.7 Hz, 1H, H-3ax), 2.09 (s, 3H), 2.06 (d, 1H) 1.99 (s, 3H), 1.95 (s, 3H), 1.72 (s, 3H), 1.18 (t, J = 7.1 Hz, 3H, OCH2CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 194.8 (CO), 170.8 (C), 170.6 (C), 170.5 (C), 170.3 (C), 169.7 (C s, 3JC1−H3axial = 3.5 Hz), 136.6 (C), 133.7 (CH), 128.9 (CH), 128.1 (CH), 77.8 (C), 69.4 (CH), 68.0 (CH), 66.9 (CH), 64.4 (CH), 62.3 (CH2), 61.8 (CH2), 39.6 (CH2), 30.9 (CH2), 20.9 (CH3), 20.8 (CH3), 20.7 (CH3), 20.4 (CH3), 14.0 (CH3). HRMS (ESI) m/z: [M + Na]+ Calcd for C26H32O12Na 559.1791; Found 559.1786.

  • (17β): [α]22D +43 (c 0.7, CHCl3). 1H NMR (500 MHz, CDCl3): δ 7.91 (d, J = 8.4 Hz, 2H, Ar), 7.55 (t, J = 7.6 Hz, 1H, Ar), 7.45 (t, J = 7.6 Hz, 2H, Ar), 5.28 (s, 1H, H-5), 5.14 (dd, J = 5.0, 3.0 Hz, 1H, H-4), 4.93 (ddd, J = 9.5, 4.8, 2.1 Hz, 1H, H-7), 4.29 (dd, J = 12.2, 2.3 Hz, 1H, H-8), 4.21 – 4.18 (m, 1H), 4.18 – 4.16 (m, 3H),4.00 (dd, J = 12.2, 4.9 Hz, 1H, H-8), 3.51 (d, J = 15.3 Hz, 1H, CH2COPh), 3.39 (d, J = 15.3 Hz, 1H, CH2COPh), 2.40 (dd, J = 12.9, 4.8 Hz, 1H H-3eq), 2.10 – 2.05 (m, 1H), 2.04 (s, 3H), 2.03 (s, 3H), 1.95 (s, 6H), 1.25 (t, J = 6.7 Hz, 3H). 13C{1H} NMR (126 MHz, CDCl3): δ 196.0 (C), 170.8 (C), 170.7 (C), 170.5 (C), 170.0 (C), 169.9 (C s, 3JC1−H3axial = 8.4 Hz), 137.2 (C), 133.5 (CH), 128.7 (CH), 128.5 (CH), 78.9 (C), 71.0 (CH), 68.0 (CH), 67.1 (CH), 64.6 (CH), 62.5 (CH2), 61.9 (CH2), 48.2 (CH2), 31.8 (CH2), 20.8 (CH3), 20.8 (CH3), 20.7 (CH3), 20.7 (CH3), 14.1 (CH3). HRMS (ESI) m/z: [M + Na]+ Calcd for C26H32O12Na 559.1791; Found 559.1781.

General Coupling Protocol with Donor 11.

A mixture of phosphate donor 11, acceptor (5 equiv) and activated 4Å acid-washed pulverized molecular sieves (2 g/mmol of donor) in dry CH2Cl2/CH3CN (2:1) (0.05 M) was stirred under argon at room temperature for 1 h before cooling to -78 °C. The reaction mixture was then treated with TMSOTf (1 equiv.) and stirred at -78 °C for 5 h before quenching with triethylamine (2 equiv). The crude mixture was diluted with CH2Cl2, filtered through a pad of Celite, washed with 20% aqueous Na2S2O3, dried over Na2SO4 and concentrated in vacuo. The anomeric ratio of the products was determined by integration of the 1H NMR spectra of crude reaction mixtures.

Methyl [Methyl (4,8-di-O-acetyl-5,7-di-azido-3,5,7,9-tetra-deoxy-L-glycero-β-L-manno-non2-ulopyranosid)onate]-(2→3)-2,4,6-tri-O-benzyl-β-D-galactopyranoside (13).

Glycoside 13 was prepared according to the general coupling protocol with donor 11 (3.8 mg, 0.01 mmol) and methyl 2,4,6-tri-O-benzyl-β-D-galactopyranoside (6 mg, 0.01 mmol) in CH2Cl2/CH3CN (2:1) (0.1 mL). The crude reaction mixture was purified by column chromatography eluting with hexane/ethyl acetate (75:25) to afford 13 (3.9 mg, 72%) as a white foam. [α]22D +11 (c 0.08, CH2Cl2). 1H NMR (600 MHz, CDCl3): δ 7.40 – 7.27 (m, 15H), 5.39 (dt, J = 8.1, 4.1 Hz, 1H), 4.96 – 4.88 (m, 2H), 4.83 (d, J = 10.9 Hz, 1H), 4.64 (d, J = 10.8 Hz, 1H), 4.59 – 4.47 (m, 3H), 4.32 (d, J = 7.8 Hz, 1H), 4.01 – 3.88 (m, 3H), 3.86 (d, J = 3.1 Hz, 1H), 3.73 (s, 3H), 3.71 – 3.60 (m, 4H), 3.57 (s, 3H), 3.38 (d, J = 9.9 Hz, 1H), 2.40 (t, J = 12.9 Hz, 1H), 2.29 (dd, J = 13.5, 5.0 Hz, 1H), 2.04 (s, 3H), 2.00 (s, 3H), 1.28 (d, J = 6.4 Hz, 3H). 13C{1H} NMR (151 MHz, CDCl3): δ 169.7, 169.5, 168.0 (s, 3JC1−H3axial = 7.5 Hz), 139.1, 138.5, 129.5, 128.4, 128.3, 128.0, 127.8, 127.5, 127.5, 127.2, 104.8, 100.4, 77.6, 77.5, 76.3, 75.2, 74.8, 74.0, 73.6, 71.5, 70.2, 69.7, 69.5, 63.5, 57.9, 57.1, 53.0, 29.7, 21.0, 12.7. HRMS (ESI) m/z: [M + Na]+ Calcd for C42H50N6NaO13 869.3334; Found 869.3340.

Methyl (2-C-allyl-4,8-di-O-acetyl-5,7-di-azido-3,5,7,9-tetra-deoxy-L-glycero-β-L-manno-non-2-ulopyranitol)onate (18).

Glycoside 18 was prepared according to the general coupling protocol with donor 11 (8.5 mg, 0.01 mmol) and allyltributylstannane (22 μL, 0.07 mmol) in CH2Cl2/CH3CN (2:1) (0.3 mL). The crude mixture was purified via silica gel column chromatography eluting with hexane/ethyl acetate (75:25) to give 18 (4.9 mg, 80%) as a colorless syrup. [α]22D -70 (c 0.2, CH2Cl2). 1H NMR (600 MHz, CDCl3): δ 5.74 (ddt, J = 17.4, 10.2, 7.3 Hz, 1H), 5.42 (qd, J = 6.4, 2.7 Hz, 1H), 5.15 (dd, J = 17.0, 2.0 Hz, 1H), 5.10 (dd, J = 17.0, 1.7 Hz, 1H), 4.93 (ddd, J = 12.5, 4.6, 3.2 Hz, 1H), 4.08 – 4.03 (m, 1H), 3.99 (dd, J = 9.9, 2.6 Hz, 1H), 3.74 (s, 3H), 3.50 (dd, J = 9.9, 1.7 Hz, 1H), 2.48 (d, J = 7.8 Hz, 2H), 2.31 (ddd, J = 12.9, 4.6, 1.2 Hz, 1H), 2.15 (s, 3H), 2.12 (s, 3H), 2.04 (t, J = 12.9 Hz, 1H), 1.37 (d, J = 6.4 Hz, 3H). 13C{1H} NMR (151 MHz, CDCl3): δ 171.9, 169.9, 169.9 (s, 3JC1−H3axial = 6.8 Hz), 130.9, 119.5, 80.9, 71.7, 70.9, 69.9, 64.1, 58.7, 52.4, 44.2, 31.6, 21.1, 20.7, 12.9. HRMS (ESI) m/z: [M + Na]+ Calcd for C17H24N6NaO7 447.1604; Found 447.1611.

Methyl [2-C-(2-oxo-2-phenylethyl)-4,8-di-O-acetyl-5,7-di-azido-3,5,7,9-tetra-deoxy-L-glycero-β-L-manno-non-2-ulopyranitol]onate (19β).

Glycoside 19β was prepared according to the general coupling protocol with donor 11 (4.1 mg, 0.01 mmol) and [(3,3-dimethylbut-1-en-2-yl)oxy]trimethylsilane (6.2 μL, 0.03 mmol) in CH2Cl2/CH3CN (2:1) (0.1 mL). The crude mixture was purified via silica gel column chromatography eluting with hexane/ethyl acetate (60:40) to afford 19β (2.89 mg, 83%) as a colorless syrup in a 15:85 α:β mixture of anomers. Further chromatography over silica gel eluting with hexanes/EtOAc 90:10 gradient to 70:30 gave an analytical sample of the 19β. [α]22D -29 (c 0.2, CH2Cl2). 1H NMR (500 MHz, CDCl3): δ 7.92 – 7.87 (m, 2H), 7.56 (td, J = 7.3, 1.5 Hz, 1H), 7.46 (t, J = 7.9 Hz, 2H), 5.14 (ddt, J = 8.4, 6.5, 3.5 Hz, 2H), 4.09 – 4.05 (m, 1H), 3.88 (dd, J = 9.9, 2.8 Hz, 1H), 3.72 (s, 3H), 3.67 (dd, J = 9.9, 1.6 Hz, 1H), 3.54 (d, J = 15.8 Hz, 1H), 3.38 – 3.29 (m, 1H), 2.52 – 2.32 (m, 1H), 2.11 (s, 4H), 2.04 (s, 3H), 1.24 (d, J = 6.4 Hz, 3H). 13C{1H} NMR (126 MHz, CDCl3): δ 195.9, 171.5, 169.9, 169.8 (s, 3JC1−H3axial = 8.0 Hz), 136.8, 133.6, 128.8, 128.4, 124.5, 78.7, 71.8, 70.9, 69.5, 63.7, 58.7, 52.7, 48.4, 20.8, 12.9. HRMS (ESI) m/z: [M + Na]+ Calcd for C22H26N6O8Na 525.1710; Found 525.1718. The minor isomer 19α was not obtained in sufficient quantity for characterization: it was identified in the crude reaction mixture by the following diagnostic signals: 1H NMR (600 MHz, CDCl3): δ 7.93 – 7.87 (m, 2H), 7.65 – 7.57 (m, 1H), 7.51 – 7.48 (m, 2H), 5.30 – 5.21 (m, 2H), 3.73 (s, 3H), 3.53 (d, J = 16.8 Hz, 1H), 3.44 (dd, J = 9.6, 1.6 Hz, 1H), 2.48 (t, J = 13.0 Hz, 1H), 2.17 (s, 3H), 2.00 (s, 3H), 1.15 (d, J = 6.3 Hz, 3H).

Supplementary Material

supp material

Acknowledgments

We thank the NIH (GM62160) for support of this work.

Footnotes

Supporting Information

Copies of 1H and 13C NMR spectra for the synthesis of all new compounds.

References

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