β-Selective Glucosylation in the Absence of Neighboring Group Participation: Influence of the 3,4-O-Bisacetal Protecting System

Abstract

A 3,4-O-bisacetal 2,6-di-O-benzyl protected thioglucoside is converted to the corresponding glucosyl triflate with 1-benzenesulfinyl piperidine and trifluoromethanesulfonic anhydride. The moderate to excellent β-selectivity exhibited with this glucosyl triflate with a range of alcohols is generally higher than that observed with the more electronically disarmed corresponding 3,4-O-carbonate, for which a possible reason is advanced.

1. Introduction

The use of cyclic protecting groups to restrict conformational mobility can have dramatic effects on the stereoselectivity of glycosylation reactions.¹ This is illustrated by the glycosyl triflates 1, 2, 3, and 4, as generated from the corresponding thioglycosides with either 1-benzenesulfinyl piperidine (BSP) and triflic anhydride, or with benzenesulfinyl triflate, or from the corresponding sulfoxides with triflic anhydride, typically in the presence of a hindered base such as 2,6-di-tert-butyl-4-methylpyridine or 2,4,6-tri-tert-butylpyrimidine (TTBP).^2–5 With 1, β-mannosides are obtained in an highly selective manner,^6–8 an observation that is attributed to the restriction of the C5–C6 bond to the most deactivating trans-gauche (tg) conformation, with the antiperiplanar C5–O5 and C6–O6 bonds,^9,10 by the presence of the benzylidene acetal.¹¹ This rationale is borne out by 2, which is highly α-selective under the same reactions conditions.¹² Donor 3 is also highly α-selective,¹² a fact that we attribute to the imposition of a half chair conformation on the pyranose ring by the cis-fused carbonate and which is sufficient to override the β-directing effect of the benzylidene acetal.^12,13 The 3,4-di-O-carbonate 4 on the other hand, which was investigated in the L-6-deoxy-mannose (or L-rhamnose series) shows moderate β-selectivity.¹⁴

Different results are observed in the gluco-series wherein the benzylidene-protected system 5 is α-selective,¹⁵ the 2,3-di-O-carbonate 6 moderately β-selective,¹⁶ and the 3,4-di-O-carbonate 7 somewhat unselective.¹⁶ The 3-deoxy manno and gluco donors 8 and 10,¹⁷ as well as their 3-deoxy-3-fluoro counterparts 9 and 11,¹⁸ are all somewhat unselective prompting us to suggest that a key interaction in determining stereoselectivity in this series of glycosylation reactions is that between the C2–O2 and C2–O3 bonds.^17,18

Here we describe our investigations into the influence of the bisacetal type protecting group on glycosylation stereoselectivity in the gluco series through the use of thioglycosides 12 and 14, and the corresponding triflates 13, and 15.

2. Results

Thioglycosides 12 and 14 were prepared as previously described¹⁶ and converted to the corresponding triflates 13 and 15 in the standard manner with BSP and trifluoromethanesulfonic anhydride,^19,20 in the presence of TTBP²¹ at −60 °C. Experiments conducted in deuteriochloroform in the NMR spectrometer revealed the formation of both triflates to be complete within minutes at −60 °C, with 13 characterized by an anomeric hydrogen resonating at δ 6.10, and 15 exhibiting the corresponding signal at δ 6.06. In variable temperature NMR experiments both 13 and 15 were found to undergo decomposition around −5 °C. Quenching of the 2,3-bisacetal protected triflate 15 with isopropanol gave the corresponding glucoside 16 in good yield, but with a disappointing anomeric selectivity of 3/1 in favor the β anomer (Table 1, entry 1). The 3,4-bisacetal protected triflate 13, on the other hand, gave the β-glucoside 17 exclusively and in excellent yield (Table 1, entry 2). On the basis of these results no further couplings were undertaken in the 2,3-bisacetal series, however, a series of glycosylations were carried out with the preformed 3,4-bisacetal protected triflate 13 (Table 1, entries 3–10).

Table 1.

Glycosylation of Thioglycosides 12 and 14

Entry	Donor	Acceptor	Product	βα ratiob	Yield a %
1				3:1	78
2				β only	72
3				β only	70
4				β only	60
5				4:1	80
6				3:1	76
7				2:1	82
8				1:5:1	78
9				1:5:1	75
10				α onlyc	70

^aIsolated yields;

^bDetermined by the crude ¹H NMR analysis;

^cReaction was performed with NIS, TfOH activation.

With 3β-cholestanol 18 as acceptor the β-glucoside 19 was formed exclusively (Table 1, entry 3), and a comparable result was observed with 1-adamantanol (Table 1, entry 4). With the glucose 4-OH acceptor 22 a ratio of 4:1 in favor the β-glucoside 23 was observed (Table 1, entry 5). In view of this result, the poor ratio observed on coupling to the less hindered glucose 6-OH acceptor 24 was surprising (Table 1, entry 6). With the threonine derivative 26 the selectivity fell to 2:1 in favor of the β-anomer (Table 1, entry 7), and with either the L- or D-rhamnose 4-OH acceptors 28 and 30,^22,23 respectively, the β:α selectivity was only 1.5:1 (Table 1, entries 8 and 9). In the β-mannosylation reactions conducted previously with donor 1, as well as in β-rhamnosylation reactions with 4, and, importantly, β-selective glucosylations with 6 and 7, the secondary glucosyl acceptor 22 performs less well than the L-rhamnosyl acceptor 28, and the primary glucosyl acceptor 24, perhaps suggesting that diastereoselective matching and/or mismatching plays a significant role in some of these glycosylation reactions.^24–26 Finally, donor 12 was activated with the N-iodosuccinimide/trifluoromethanesulfonic acid combination²⁷ in the presence of acceptor 30, when the disaccharide 31 was obtained with complete α-selectivity (Table 1, entry 10). The contrast in selectivity between entries 9 and 10 of Table 1 underlines the importance of the preformation of the glucosyl triflate 13 in the chemistry reported here. The change in selectivity on going from the BSP/trifluoromethanesulfonic anhydride activation method to promotion with N-iodosuccinimide and trifluoromethanesulfonic acid, or its silver salt, parallels similar observations made with a range of other thioglycosides in our laboratory.^28,29

To probe the influence of the methoxy groups in the bisacetal system on the stereoselectivity of glycosylations, thioglycoside 12 was treated with sodium cyanoborohydride and HCl in THF,³⁰ when 32 was obtained in excellent yield, as a single diastereomer. The diequatorial disposition of the methyl groups in 32 follows from the mechanism of reduction and is supported by nuclear Overhauser measurements. The analogous stereoselectivity was previously observed in the mannose series when thioglycoside 33 was reduced cleanly to 34.¹² Activation of 32 with BSP and triflic anhydride at −60 °C in dichloromethane, the standard conditions applied for glycosylation of 12, followed by addition of acceptor 22 gave disaccharide 35 as a 1:1 mixture of α- and β-anomers in 50% yield. The methoxy groups in the bisacetal protecting system of 12, therefore, play a significant role in determining the stereoselectivity of the glycosylation reaction.

3. Discussion

The β-selectivity observed with the 3,4-O-bisacetal-protected glucosyl triflate 13 is significant in so far as it contrasts with the α-selectivity observed earlier¹² with the corresponding α-selective 3,4-O-bisacetal-protected mannosyl triflate 2. Even more significant, however, is the improved β-selectivity observed with triflate 13 over and above that seen previously¹⁶ with the corresponding 3,4-O-carbonate 7 as the more disarmed^31,32 carbonate was expected to be the more β-selective donor. Applying our standard mechanistic hypothesis for these glycosyl triflate based glycosylations, which is based on the classical Lemieux mechanism,³³ and for which we have provided substantial evidence,^7,34,35 the explanation must be found in the effect of the protecting groups on the equilibria between the covalent α-triflates, a transient contact ion pair (CIP) comprised of the glycosyl oxacarbenium ions and the triflate counter ion, and related solvent separated ion pair (SSIP). In this hypothesis the covalent α-triflate serves as a reservoir for the transient CIP, which, with the triflate shielding the α-face from which it has just departed, is the source of the β-glycosides. In the SSIP the anomeric effect takes over, leading to the formation of the α-glycosides. β-Selectivity is the result of a protecting system that destabilizes the glycosyl oxacarbenium shifting the whole series of equilibria toward the covalent glycsoyl triflate and thereby decreasing the concentration of the SSIP. α-Selectivity is the result of a higher concentration of the SSIP arising from increased glycosyl cation stability. In the case of triflate 13 (Scheme 1) the covalent triflate collapses to the CIP with its glycosyl oxacarbenium ion in a conformation approximating to a ⁴H₃ half-chair^36,37 the C2–O2 bond necessarily rotates down below the nominal pyranose plane leading to an increased steric interaction with the methoxy group of the bisacetal. We argue that it is this increased steric interaction, which is absent in the corresponding mannosyl triflate 2, that destabilizes the glycosyl cation sufficiently to bring about the observed βselectivity. With the 2,3-O-bisacetal-protected triflate 15 this interaction is absent in the derived oxacarbenium ion, whether it adopts the ⁴H₃ or the closely related, isoenergetic ³E conformer,^36,37 and the chemistry reverts to the more normal pattern with the more highly disarmed 2,3-O-carbonate being more β-selective than the acetal.

Scheme 1.

Glycosylation Mechanism Illustrating the Influence of the 3,4-O-Bisacetal Group

Strong support for this hypothesis is provided by the loss of stereoselectivity on glycosylation of the des-methoxy system 32, when there is no longer a destabilizing steric interaction between the O2 protecting group and the cyclic protecting group spanning O3 and O4. Once again, it is appropriate to note the contrast with the mannose series wherein both the standard donor 33 and the des-methoxy analog 34 were α-selective.

4. Conclusion

Although several β-selective glucosylation reactions have been developed recently relying on the use of non-traditional methods of neighboring group participation,^38–40 the chemistry described here, together with that of the 2,3-O-carbonate 6, provides a rare example of such a reaction that completely avoids such types of participation. The increased β-selectivity of 13, as compared to the more electronically disarmed 6, illustrates how subtle conformational effects and remote protecting groups can sometimes have major effects on reactivity.

5. Experimental

5.1. General Experimental

NMR spectra were recorded in CDCl₃ solution, with chemicals shifts in parts per million downfield from tetramethylsilane at 500 and 125 MHz for ¹H and ¹³C, respectively. Specific rotations were measured in CHCl₃ at room temperature. Mass spectra were recorded with electrospray ionization.

5.2. General Procedure for Glycosylation Using the BSP/TTBP/Tf₂O System

To a stirred solution of phenyl 2,6-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-1-thio-β-D-glucopyranoside (1 equiv), BSP (1.1 equiv), TTBP (1.5 equiv) and 4Å molecular sieves in CH₂Cl₂ (0.05 M in substrate) at −60 °C under an Ar atmosphere, was added Tf₂O (1.2 equiv). After stirring for 30 mins at the same temperature, a solution of glycosyl acceptor (1.5 equiv) in CH₂Cl₂ (0.02 M in acceptor) was added slowly at −60 °C. The reaction was further stirred for 3 h at the same temperature and then allowed to reach room temperature before dilution with dichloromethane, filtration, and washing with excess CH₂Cl₂. The organic layer was then washed with saturated sodium bicarbonate solution, followed by saturated sodium chloride solution, then was dried over sodium sulfate, evaporated to dryness and purified by silica gel column chromatography using EtOAc/Hexanes as eluant to afford the corresponding β and α glucopyranosides.

5.2.1.1. Isopropyl 4,6-di-O-benzyl-2,3-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranoside (16β)

[α]¹⁷_D – 103.0° (c, 1.0); ¹H NMR δ:1.19 (d, J = 6.0 Hz, 3H), 1.27 (d, J=6.0 Hz, 3H), 1.32 (s, 3H), 1.36 (s, 3H), 3.29 (s, 3H), 3.30 (s, 3H), 3.48–3.55 (m, 2H), 3.60 (t, J = 9.5 Hz, 1H), 3.65–3.68 (m, 1H), 3.74 (d, J= 10.5 Hz, 1H), 3.88 (t, J = 9.5 Hz, 1H), 3.95–3.98 (m, 1H), 4.53 (d, J = 8.0 Hz, 1H), 4.56 (s, 2H), 4.60 (d, J = 12.0 Hz, 1H), 4.92 (d, J = 11.0 Hz, 1H), 7.23–7.33 (m, 10H); ¹³C NMR δ: 17.7, 17.8, 22.3, 23.5, 47.8, 47.9, 69.4, 69.5, 72.5, 73.4, 74.0, 74.9, 75.0, 75.6, 99.3, 99.4, 99.5, 127.5, 127.6, 127.7, 128.0, 128.3, 128.4, 138.3, 138.4; HRMS Calcd for C₂₉H₄₀O₈Na [M+Na] ⁺: 539.2615, found 539.2610.

5.2.1.2. Isopropyl 4,6-di-O-benzyl-2,3-O-(2,3-dimethoxybutane-2,3-diyl)-α-D-glucopyranoside (16α)

[α]¹⁷_D + 10.0° (c, 1.0); ¹H NMR δ:1.18 (d, J = 6.0 Hz, 3H), 1.22 (d, J=6.5 Hz, 3H), 1.32 (s, 3H), 1.35 (s, 3H), 3.26 (s, 3H), 3.30 (s, 3H), 3.63 (d, J = 10.0, Hz, 1H), 3.75–3.79 (m, 3H), 3.85 (d, J= 9.5 Hz, 1H), 3.91–3.93 (m, 1H), 4.19 (t, J = 10.0 Hz, 1H), 4.76 (d, J = 8.5 Hz, 1H), 4.49 (d, J = 7.0 Hz, 1H), 4.64 (d, J = 12.5 Hz, 1H), 4.91 (d, J = 11.5 Hz, 1H), 4.93 (d, J = 4.0 Hz, 1H) 7.19–7.36 (m, 10H);¹³C NMR δ: 17.7, 18.1, 21.3, 23.1, 47.8, 47.9, 68.3, 68.5, 69.7, 70.5, 70.6, 73.4, 75.0, 75.3, 95.2, 99.3, 99.7, 127.6, 127.7, 127.9, 128.1, 128.3, 138.2, 138.5; HRMS Calcd for C₂₉H₄₀O₈Na [M+Na] ⁺: 539.2615, found 539.2610.

5.2.2. Isopropyl 2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranoside (17β)

White Solid, Mp 98 °C; [α]¹⁷_D + 94.0° (c, 1.0); ¹H NMR δ:1.23 (d, J = 6.0 Hz, 3H), 1.29 (s, 3H), 1.31 (d, J = 6.5 Hz, 3H), 1.35 (s, 3H), 3.19 (s, 3H), 3.30 (s, 3H), 3.41 (dd, J = 8.7,7.5 Hz, 1H), 3.61–3.68 (m, 3H), 3.77–3.81 (m, 2H), 3.98–4.00 (m, 1H), 4.46 (d, J = 7.5 Hz, 1H), 4.61 (q, J = 4.0 Hz, 2H), 4.77 (d, J = 11.0 Hz, 1H), 4.86 (d, J = 11.5 Hz, 1H), 7.25–7.28 (m, 2H), 7.30–7.35 (m, 6H), 7.39–7.40 (m, 2H);¹³C NMR δ: 17.7, 17.8, 22.3, 23.7, 47.9, 48.0, 66.4, 68.7, 72.5, 72.7, 73.5, 73.7, 74.7, 79.2, 99.5, 102.6, 127.4, 127.5, 127.7, 128.2, 128.3, 138.5, 139.0; HRMS Calcd for C₂₉H₄₀O₈Na [M+Na] ⁺: 539.2615, found 539.2618.

5.2.3. 3β-Cholestanyl 2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranoside (19β)

White Solid, Mp 128 °C; [α]¹⁷_D + 84.0° (c, 0.75); ¹H NMR δ:0.59–0.63 (m,1H), 0.65 (s, 3H), 0.83 (s, 3H), 0.86–1.39 (m, 36H), 1.47–1.57 (m, 4H), 1.64–1.73 (m, 2H), 1.80–1.81 (m, 1H), 1.96–1.98 (m, 2H), 3.19 (s, 3H), 3.30 (s, 3H), 3.40 (dd, J = 9.5,7.5 Hz, 1H), 3.61–3.67 (m, 4H), 3.78 (t, J = 9.5 Hz, 2H), 4.50 (d, J = 7.5 Hz, 1H), 4.60 (q, J = 4.0 Hz, 2H), 4.77 (d, J = 11.5 Hz, 1H), 4.86 (d, J = 11.5 Hz, 1H), 7.24–7.33 (m, 8H), 7.39–7.41 (m, 2H);¹³C NMR δ: 12.1, 12.3, 17.7, 17.8, 18.7, 21.3, 22.6, 22.8, 23.8, 24.2, 28.0, 28.3, 28.8, 29.7, 32.1, 34.8, 35.5, 35.6, 35.8, 36.2, 37.1, 39.5, 40.1, 42.6, 44.8, 47.8, 47.9, 54.4, 56.3, 56.5, 66.5, 68.7, 72.5, 73.5, 73.7, 74.7, 79.4, 99.5, 102.4, 127.4, 127.5, 127.8, 128.2, 128.3, 138.5, 139.0; HRMS Calcd for C₅₃H₈₀O₈Na [M+Na] ⁺: 867.5751, found 867.5758.

5.2.4. 1-Adamantyl 2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranoside (21β)

[α]¹⁷_D + 85.0° (c, 1.0);¹H NMR δ: 1.28 (s, 3H), 1.34 (s, 3H), 1.59–1.62 (m, 6H), 1.81–1.84 (m, 3H), 1.91–1.93 (m, 3H), 2.14 (s, 3H), 3.19 (s, 3H), 3.29 (s, 3H), 3.39 (dd, J = 9.5,7.5 Hz, 1H), 3.59–3.63 (m, 3H), 3.78–3.80 (m, 2H), 4.59 (s, 2H), 4.69 (d, J = 7.5 Hz, 1H), 4.75 (d, J = 11.5 Hz, 1H), 4.87 (d, J = 1.5 Hz, 1H), 7.24–7.27 (m, 2H), 7.29–7.34 (m, 6H), 7.39–7.41 (m, 2H); ¹³C NMR δ: 17.6, 17.8, 30.7, 36.3, 42.7, 47.9, 66.7, 69.0, 72.7, 73.3, 73.4, 74.7, 75.2, 79.2, 96.5, 99.5, 127.3, 127.4, 127.8, 128.1, 128.2, 138.5, 139.1; HRMS Calcd for C₃₆H₄₈O₈Na [M+Na] ⁺: 631.3241, found 631.3243.

5.2.5.1. Methyl 2,3,6-tri-O-benzyl-4-O-[2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranosyl]-α-D-glucopyranoside (23β)

[α]¹⁷_D + 72.0° (c, 1.0); ¹H NMR δ:1.27 (s, 3H), 1.34 (s, 3H), 3.19 (s, 3H), 3.29 (s, 3H), 3.37 (s, 3H), 3.43–3.47 (m, 4H), 3.53–3.58 (m, 1H), 3.68–3.81 (m, 5H), 3.88–3.92 (m, 2H), 4.34 (d, J = 12.5 Hz, 1H), 4.43 (q, J = 7.5 Hz, 2H), 4.49–4.60 (m, 4H), 4.71–4.78 (m, 3H), 4.87 (d, J = 11.5 Hz, 1H), 5.06 (d, J = 11.5 Hz, 1H), 7.37–7.19 (m, 25H); ¹³C NMR δ:17.6, 17.8, 47.9, 48.0, 55.3, 66.2, 68.2, 68.4, 70.0, 73.1, 73.2, 73.4, 73.6, 74.1, 74.7, 75.3, 78.9, 79.6, 80.5, 98.4, 99.6, 102.8, 127.0, 127.2, 127.3, 127.4, 127.5, 127.6, 127.7, 127.8, 127.9, 128.1, 128.4, 138.0, 138.4, 138.7, 138.9, 139.7; HRMS Calcd for C₅₄H₆₄O₁₃Na [M+Na] ⁺: 943.4239, found 943.4260.

5.2.5.2. Methyl 2,3,6-tri-O-benzyl-4-O-[2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-α-D-glucopyranosyl]-α-D-glucopyranoside (23α)

[α]¹⁷_D + 82.5° (c, 0.8); ¹H NMR δ: 1.28 (s, 3H), 1.30 (s, 3H), 3.17 (s, 3H), 3.27 (s, 3H), 3.38 (s, 3H), 3.52 (dd, J = 9.5,3.5 Hz, 2H), 3.55–3.59 (m, 2H), 3.68 (d, J = 9.0 Hz, 1H), 3.76–3.83 (m, 4H), 3.94 (t, J = 9.5Hz, 1H), 4.02 (t, J = 9.0Hz, 1H), 4.11 (t, J = 9.5 Hz, 1H), 4.38 (d, J = 12.0 Hz, 1H), 4.47–4.58 (m, 6H), 4.67 (d, J = 12.5Hz, 1H), 4.81–4.88 (m, 3H), 5.66 (d, J = 4.0 Hz, 1H), 7.16–7.31 (m, 25H); ¹³C NMR δ: 17.7, 17.9, 47.9, 48.0, 55.2, 65.8, 67.8, 69.4, 69.6, 70.4, 73.3, 73.3, 73.5, 73.7, 74.1, 74.4, 76.3, 80.1, 81.8, 97.7, 98.1, 99.3, 99.4, 126.8, 126.9, 127.3, 127.4, 127.5, 127.6, 127.8, 128.1, 128.2, 128.2, 128.4, 138.0, 138.1, 138.3, 138.6, 139.2; HRMS Calcd for C₅₄H₆₄O₁₃Na [M+Na] ⁺: 943.4239, found 943.4258.

5.2.6.1. Methyl 2,3,6-tri-O-benzyl-6-O-[2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranosyl]-α-D-glucopyranoside (25β)

[α]¹⁷_D + 61.1° (c, 1.65); ¹H NMR δ: 1.28 (s, 3H), 1.32 (s, 3H), 3.17 (s, 3H), 3.29 (s, 3H), 3.33 (s, 3H), 3.47–3.59 (m, 4H), 3.65–3.69 (m, 3H), 3.75–3.79 (m, 3H), 3.97 (t, J = 9.5 Hz, 1H), 4.15 (d, J = 11.0 Hz, 1H), 4.35 (d, J = 7.0 Hz, 1H), 4.51 (d, J = 11.0 Hz, 1H), 4.58 (d, J = 6.0 Hz, 2H), 4.62 (d, J = 3.5 Hz, 1H), 4.65–4.69 (m, 3H), 4.78 (dd, J = 12.5,4.5 Hz, 1H), 4.82 (s, 2H), 4.96 (d, J = 11.0 Hz, 1H), 7.13–7.36 (m, 25H); ¹³C NMR δ;13.8, 13.9, 44.0, 44.1, 51.3, 62.3, 64.7, 64.8, 66.0, 69.0, 69.5, 69.6, 70.1, 70.9, 71.0, 71.8, 73.9, 74.1, 74.8, 75.9, 78.2, 94.2, 95.7, 100.1, 123.4, 123.5, 123.6, 123.7, 123.8, 123.9, 124.0, 124.1, 124.3, 124.4, 124.5, 124.6, 134.4, 134.5, 134.6, 134.9, 135.1; HRMS Calcd for C₅₄H₆₄O₁₃Na [M+Na] ⁺: 943.4239, found 943.4208.

5.2.6.2. Methyl 2,3,6-tri-O-benzyl-6-O-[2,6-di-O-benzyl-3,4-di-O-(2,3-dimethoxybutane-2,3-diyl)-α-D-glucopyranosyl]-α-D-glucopyranoside (25α)

[α]¹⁷_D + 81.0° (c, 0.5); ¹H NMR δ: 1.27 (s, 3H), 1.32 (s, 3H), 3.18 (s, 3H), 3.24 (s, 3H), 3.29 (d, J = 8.0 Hz, 1H), 3.33 (s, 3H), 3.39 (dd, J = 10.0,4.0 Hz, 1H), 3.57–3.69 (m, 4H), 3.73–3.77 (m, 3H), 3.89–3.96 (m, 2H), 4.19 (t, J = 10.0 Hz, 1H), 4.49–4.57 (m, 4H), 4.63–4.70 (m, 3H), 4.78–4.86 (m, 3H), 4.93 (d, J = 11.0 Hz, 1H), 5.03 (d, J = 4.0 Hz, 1H), 7.16–7.34 (m, 25H); ¹³C NMR δ: 17.7, 17.9, 47.8, 47.9, 54.9, 65.6, 66.2, 68.1, 68.9, 69.8, 70.5, 72.8, 73.3, 73.4, 75.0, 75.7, 76.6, 77.9, 80.2, 82.1, 97.8, 97.9, 99.4, 99.5, 127.2, 127.3, 127.4, 127.5, 127.6, 127.8, 127.9, 128.0, 128.2, 128.3, 128.4, 128.4, 138.2, 138.3, 138.5, 138.9, 139.0; HRMS Calcd for C₅₄H₆₄O₁₃Na [M+Na] ⁺: 943.4239, found 943.4227.

5.2.7.1. N-Benzyloxycarbonyl 2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranosyl–L-threonine methyl ester (27β)

[α]¹⁷_D + 66.9° (c, 1.3); ¹H NMR δ: 1.28 (d, J = 6.5 Hz, 6H), 1.34 (s, 3H), 3.18 (s, 3H), 3.29 (s, 3H), 3.34–3.37 (m, 1H), 3.50–3.51 (m, 1H), 3.65 (s, 3H), 3.67–3.78 (m, 4H), 4.34 (dd, J = 9.0,3.5Hz, 1H), 4.38 (d, J = 7.5Hz, 1H), 4.41–4.43 (m, 1H), 4.50 (q, J = 12.0 Hz, 2H), 4.77 (s, 2H), 5.13 (dd, J = 17.5,12.5 Hz, 2H), 5.76 (d, J = 9.0Hz, 1H), 7.23–7.37 (m, 15H); ¹³C NMR δ: 17.7, 17.8, 47.9, 48.0, 52.4, 58.8, 65.6, 67.1, 68.2, 72.6, 73.6, 73.7, 74.8, 75.0, 78.6, 99.6, 101.8, 127.5, 127.6, 127.8, 128.1, 128.3, 128.5, 136.4, 138.2, 138.6, 156.8, 170.8; HRMS Calcd for C₃₉H₄₉NO₁₂Na [M+Na] ⁺: 746.3147, found 746.3166.

5.2.7.2. N-Benzyloxycarbonyl 2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-α-D-glucopyranosyl–L-threonine methyl ester (27α)

[α]¹⁷_D +100.0° (c, 1.0); ¹H NMR δ: 1.28 (d, J = 6.0 Hz, 6H), 1.31 (s, 3H), 3.17 (s, 3H), 3.25 (s, 3H), 3.54 (s, 3H), 3.64 (dd, J = 11.0,2.0 Hz, 2H), 3.67–3.72 (m, 1H), 3.74 (d, J = 10.0 Hz, 1H), 3.93–3.96 (m, 1H), 4.07 (t, J = 10.0 Hz, 1H), 4.24–4.25 (m, 1H), 4.32–4.33 (m, 1H), 4.54 (dd, J = 12.0,6.5 Hz, 2H), 4.66 (d, J = 12.0 Hz, 1H), 4.77 (d, J = 12.0 Hz, 1H), 4.86 (d, J = 3.5 Hz, 1H), 5.07–5.14 (m, 2H), 5.96 (d, J = 8.0Hz, 1H), 7.23–7.34 (m, 15H); ¹³C NMR δ: 17.6, 17.8, 19.2, 47.9, 52.3, 58.9, 65.9, 66.9, 67.9, 69.3, 70.2, 73.4, 73.5, 75.7, 76.2, 98.9, 99.5, 99.6, 1274, 127.5, 127.7, 127.9, 128.0, 128.2, 128.4, 136.4, 138.0, 138.5, 156.7, 171.0; HRMS Calcd for C₃₉H₄₉NO₁₂Na [M+Na] ⁺: 746.3147, found 746.3171.

5.2.8.1. Methyl 4-O-[2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranosyl]-2,3-O-isopropylidene-α-L-rhamnopyranoside (29β)

[α]¹⁷_D + 47.0° (c, 0.85); ¹H NMR δ: 1.28 (s, 3H), 1.31 (s, 3H), 1.32 (d, J = 6.5 Hz, 3H), 1.34 (s, 3H), 1.45 (s, 3H), 3.21 (s, 3H), 3.29 (s, 3H), 3.36 (d, J = 9.5 Hz, 1H), 3.39 (s, 3H), 3.57–3.55 (m, 1H), 3.66–3.63 (m, 1H), 3.77–3.69 (m, 4H), 3.82 (t, J = 10.0 Hz, 1H), 4.07 (d, J = 6.0 Hz, 1H), 4.21 (t, J = 6.5 Hz, 1H), 4.56 (d, J = 12.0 Hz, 1H), 4.62 (d, J = 12.5 Hz, 1H), 4.77 (d, J = 12.0 Hz, 1H), 4.84 (d, J = 11.5 Hz, 2H), 4.94 (d, J = 6.5 Hz, 1H), 7.40–7.41 (m, 2H), 7.25–7.32 (m, 8H); ¹³C NMR δ: 17.6, 17.7, 17.8, 26.4, 27.8, 47.9, 48.0, 54.8, 64.3, 66.1, 68.2, 72.4, 73.5, 73.8, 74.3, 75.9, 77.9, 78.2, 79.1, 98.1, 99.6, 101.7, 109.2, 127.2, 127.4, 127.5, 128.0, 128.3, 138.5, 139.3; HRMS Calcd for C₃₆H₅₀O₁₂Na [M+Na] ⁺, 697.3194, found: 697.3176.

5.2.8.2. Methyl 4-O-[2,6-di-O-benzyl-3,4-dO-(2,3-dimethoxybutane-2,3-diyl)-α-D-glucopyranosyl]-2,3-O-isopropylidene-α-L-rhamnopyranoside (29α)

[α]¹⁷_D + 88.0° (c, 0.37); ¹H NMR δ: 1.22 (s, 3H), 1.29 (d, J = 6.5 Hz, 3H), 1.30 (s, 3H), 1.35 (s, 3H), 1.39 (s, 3H), 3.21 (s, 3H), 3.31 (s, 3H), 3.33 (s, 3H), 3.61–3.67 (m, 2H), 3.69–3.73 (m, 2H), 3.74–3.76 (m, 1H), 3.92 (t, J = 10.0 Hz, 1H), 4.04 (d, J = 5.5 Hz, 1H), 4.10–4.17 (m, 3H), 4.57 (s, 2H), 4.69 (d, J = 12.0 Hz, 1H), 4.81 (s, 1H), 4.90 (d, J = 12.0 Hz, 1H), 4.99 (d, J = 3.5 Hz, 1H), 7.25–7.35 (m, 10H); ¹³C NMR δ: 17.4, 17.8, 18.0, 26.3, 28.1, 48.0, 48.1, 54.6, 64.8, 65.7, 67.3, 68.8, 70.5, 73.6, 74.1, 75.9, 80.8, 97.9, 98.8, 99.5, 99.6, 108.8, 127.4, 127.5, 127.8, 128.2, 128.3, 138.2, 138.7; HRMS Calcd for C₃₆H₅₀O₁₂Na [M+Na] ⁺, 697.3194, found: 697.3183.

5.2.9.1. Methyl 4-O-[2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-β-D-glucopyranosyl]-2,3-O-isopropylidene-α-D-rhamnopyranoside (31β)

[α]¹⁷_D +78.0° (c, 1.0); ¹H NMR δ: 1.26 (s, 3H), 1.28 (s, 3H), 1.33 (d, J = 6.0 Hz, 3H), 1.34 (s, 3H), 1.46 (s, 3H), 3.19 (s, 3H), 3.29 (s, 3H), 3.37 (s, 3H), 3.45–3.51 (m, 2H), 3.58–3.61 (m, 1H), 3.67–3.82 (m, 5H), 4.09 (d, J = 5.5 Hz, 1H), 4.32 (t, J = 6.5 Hz, 1H), 4.55–4.62 (m, 3H), 4.79 (s, 2H), 4.82 (s, 1H), 7.27–7.38 (m, 10H); ¹³C NMR δ: 17.6, 17.8, 17.9, 26.2, 28.0, 47.9, 48.0, 54.8, 64.6, 66.0, 68.4, 72.9, 73.4, 73.9, 74.8, 75.8, 79.3, 82.2, 98.2, 99.5,99.6, 103.3, 108.9, 127.4, 127.5, 127.6, 127.8, 128.2, 128.2, 138.4, 138.7; HRMS Calcd for C₃₆H₅₀O₁₂Na [M+Na] ⁺, 697.3194, found 697.3191.

5.2.9.2. Methyl 4-O-[2,6-di-O-benzyl-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-α-D-glucopyranosyl]-2,3-O-isopropylidene-α-D-rhamnopyranoside (31α)

[α]¹⁷_D + 132.0° (c, 1.0); ¹H NMR δ: 1.29 (s, 3H), 1.30 (d, J = 6.5 Hz, 3H), 1.34 (s, 3H), 1.35 (s, 3H), 1.53 (s, 3H), 3.19 (s, 3H), 3.32 (s, 3H), 3.34 (s, 3H), 3.55–3.65 (m, 3H), 3.69–3.78 (m, 3H), 3.91–3.92 (m, 1H), 4.04–4.08 (m, 2H), 4.33 (t, J = 6.0 Hz, 1H), 4.56 (q, J = 8.5 Hz, 2H), 4.84 (s, 1H), 4.77–4.81 (m, 2H),5.59 (d, J = 4.0 Hz, 1H), 7.24–7.32 (m, 8H), 7.41–7.42 (m, 2H); ¹³C NMR δ: 17.7, 18.0, 18.5, 26.3, 28.0, 47.9, 48.1, 54.7, 63.7, 66.2, 67.9, 69.4, 69.8, 73.2, 73.6, 76.0, 76.4, 78.5, 78.6, 96.5, 98.1, 99.4, 99.6, 109.2, 127.4, 27.5, 127.6, 128.2, 128.3, 138.1, 138.9; HRMS Calcd for C₃₆H₅₀O₁₂Na [M+Na] ⁺, 697.3194, found 697.3198.

5.3. Activation of Donor 12 with N-Iodosuccinimide/Trifluoromethanesulfonic Acid

To a stirred 0.05M solution of 12 and acceptor 30 (1.5 equiv) in dry dichloromethane under argon was added N-iodosuccinimide (1.2 equiv) at 0 °C, followed by trifuoromethanesulfonic acid (0.1 equiv). After stirring for 0.5h at 0 °C, the reaction mixture was then diluted with dichloromethane, and washed with saturated aqueous sodium bicarbonate. The combined organic portion was washed with aqueous sodium thiosulfate, then brine, dried over sodium sulfate and evaporated to dryness. Purification by silica gel chromatography gave 31α in 70% yield, with spectroscopic data identical to those described above.

5.4. Phenyl 2,6-di-O-benzyl-3,4-O-(butane-2,3-diyl)-1-thio-β-D-glucopyranoside (32)

Sodium cyanoborohydride (0.33 g, 15.0 mmol), was added to a stirred solution of donor 12 (0.2 g, 1.0 mmol) in THF (10.0 mL) under an inert atmosphere. The solution was cooled to 0 °C, and 2.0 M HCl in diethyl ether was added until the pH was 3–4. The reaction mixture was stirred at room temperature for 24 h, maintaining the same pH, then was quenched by the addition of saturated NaHCO₃ The aqueous phase was extracted with EtOAc (50 mL), and the organic layer was washed with water then brine. After drying over sodium sulfate, the extracts were evaporated to dryness and purified by column chromatography using EtOAc/Hexanes as eluant to give 32 (0.13 g, 70%). [α]¹⁷_D – 4.7° (c, 1.0); ¹H NMR δ:1.10 (d, J = 6.0 Hz, 3H), 1.17 (d, J = 6.0 Hz, 3H), 3.30–3.39 (m, 3H), 3.43 (t, J = 9.5 Hz, 1H), 3.54 (t, J = 9.0 Hz, 1H), 3.58 (d, J = 8.0 Hz, 1H), 3.65 (dd, J = 11.0,4.5 Hz, 1H), 3.82 (d, J = 10.0 Hz, 1H), 4.55 (d, J = 12.0 Hz, 1H), 4.63 (d, J = 12.5 Hz, 1H), 4.72 (d, J = 9.5 Hz, 1H), 4.76 (d, J = 11.0 Hz, 1H), 4.83 (d, J = 11.0 Hz, 1H), 7.58–7.60 (m, 2H), 7.45–7.47 (m, 2H), 7.23–7.37 (m, 11H);¹³C NMR δ: 17.3, 17.4, 68.8, 73.4, 73.9, 75.1, 77.6, 77.7, 77.8, 82.9, 87.2, 127.4, 127.5, 127.6, 127.7, 128.2, 128.3, 128.8, 132.1, 133.7, 138.4, 138.5; HRMS Calcd for C₃₀H₃₄O₅S: C₃₀H₃₄O₅S [M+Na] ⁺: 529.2019, found 529.2019

5.5. Glycosylation of Desmethoxy Donor 32

Activation of 32 according to the standard BSP protocol and treatment with acceptor 22 gave 35 (50%) as a 1:1 α/β mixture. Column Chromatography on silica gel eluting with EtOAc/Hexanes gave 35 β and 35 α.

5.5.1. Methyl 2,3,6-tri-O-benzyl-4-O-[2,6-di-O-benzyl-3,4-O-(butane-2,3-diyl)-β-D-glucopyranosyl]-α-D-glucopyranoside (35β)

[α]²¹_D + 20.0° (c, 0.45); ¹H NMR δ:1.07 (d, J = 6.0 Hz, 3H), 1.15 (d, J = 5.5 Hz, 3H), 3.29–3.33 (m, 4H), 3.37 (s, 3H), 3.39–3.46 (m, 4H), 3.58 (d, J = 10.5 Hz, 1H), 3.65 (d, J = 9.5, 1H), 3.69 (d, J = 11.5 Hz, 1H), 3.82 (d, J = 3.0 Hz, 1H), 3.86 (t, J = 9.0 Hz, 2H), 3.91 (d, J = 10.0 Hz, 1H), 4.37–4.49 (m, 5H), 4.55 (d, J = 3.5 Hz, 1H), 4.59 (d, J = 12.0 Hz, 1H), 4.69 (d, J = 12.0 Hz, 1H), 4.76 (dd, J = 11.0,7.5 Hz, 1H), 4.83 (d, J = 11.5 Hz, 1H), 5.06 (d, J = 11.0 Hz, 1H), 7.21–7.41 (m, 25H);¹³C NMR δ: 17.3, 17.4, 55.3, 68.2, 68.5, 69.9, 73.1, 73.3, 73.7, 74.2, 74.3, 74.5, 75.3, 77.5, 78.9, 79.6, 80.5, 80.9, 98.4, 102.6, 127.1, 127.2, 127.4, 127.5, 127.6, 127.7, 127.7, 127.8, 128.0,128.1, 128.2, 128.4, 138.0, 138.4, 138.8, 138.9, 139.6; HRMS Calcd for C₅₂H₆₀O₁₁: C₅₂H₆₀O₁₁ [M+Na] ⁺: 883.4028, found 883.4021.

5.5.2. Methyl 2,3,6-tri-O-benzyl-4-O-[2,6-di-O-benzyl-3,4-O-(butane-2,3-diyl)-α-D-glucopyranosyl]- α-D-glucopyranoside (35α)

[α]²¹_D + 21.0° (c, 0.50); ¹H NMR δ:1.06 (d, J = 6.0 Hz, 3H), 1.13 (d, J = 6.0 Hz, 3H), 3.27–3.45 (m, 4H), 3.37 (s, 3H), 3.49–3.55 (m, 2H), 3.64–3.74 (m, 3H), 3.76 (t, J = 9.5, 1H), 3.84–3.87 (m, 1H), 3.93 (t, J = 10.0 Hz, 1H), 4.03 (t, J = 9.0 Hz, 1H), 4.32 (d, J = 12.5 Hz, 1H), 4.48 (q, J = 9.5 Hz, 2H), 4.54–4.60 (m, 5H), 4.67 (d, J = 12.0 Hz, 1H), 4.78 (d, J = 12.0 Hz, 1H), 4.82 (d, J = 11.5 Hz, 1H), 4.94 (d, J = 11.5 Hz, 1H), 5.74 (d, J = 3.5 Hz, 1H), 7.18–7.33 (m, 25H);¹³C NMR δ: 17.3, 17.5, 55.1, 67.3, 69.5, 69.8, 73.1, 73.2, 73.3, 73.4, 73.5, 73.7, 74.3, 75.9, 77.3, 77.9, 80.3, 82.0, 97.7, 97.9, 126.8, 127.0, 127.4, 127.4, 127.5, 127.5, 127.7, 127.9, 128.1,128.2, 128.3, 128.4, 138.0, 138.2, 138.3, 138.4, 139.1; HRMS Calcd for C₅₂H₆₀O₁₁: C₅₂H₆₀O₁₁ [M+Na] ⁺: 883.4028, found 883.4027.