Study of the Regioselectivity of Intra- and Intermolecular Glycosylations of Mannoside Diol Acceptors

Abstract

The intramolecular glycosylation of thiomannoside donors linked through C6′ by a phthaloyl group to C3 of a diol mannoside acceptor is described. The unexpected results led us to undertake a systematic analysis of the factors affecting the regioselectivity of the intermolecular process. The substituents on the diol mannoside acceptor have been found to play an important role.

glycosylation - regioselectivity - stereoselectivity

References

<A NAME="RD31804ST-1">1</A>

Present address: Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco 28049, Spain.

<A NAME="RD31804ST-2">2</A>

Present address: DDW-Medicinal Chemistry II, GlaxoSmithKline, 28770 Tres Cantos, Madrid, Spain.

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<A NAME="RD31804ST-10">10</A>

General Method for the Formation of Diesters 8 and 9. A solution of 6a or 6b, phthalic anhydride (2 equiv) and Et₃N (5 equiv) in 20 mL of CH₂Cl₂ was stirred at r.t. for 4 h. Then, a sat. solution of NH₄Cl was added, the mixture was stirred for 15 min, the aqueous layer was extracted with CH₂Cl₂ (3 × 100 mL), and the combined organic layers were washed with brine, dried over MgSO₄ and evaporated to give the corresponding triethylamine salt. This salt was dissolved in toluene and treated with SOCl₂ (5 equiv) and 3 drops of DMF, and the mixture was stirred under reflux for 4 h. The reaction mixture was then co-evaporated with toluene several times under vacuum to give the corresponding acid chloride as a yellow oil, which was treated with a solution of the tin derivative (1.1 equiv) prepared as follows: methyl 6-O-tert-butyldimethylsilyl-α-d-mannopyranoside (7) was dissolved in anhyd toluene in an Erlenmeyer flask, then dibutyltin oxide (1.1 equiv) was added. The suspension was heated in a microwave oven (domestic microwave oven LG intellowave 700 W) for periods of 2 min until the solid was completely dissolved. The solution was allowed to reach r.t., then was added to the freshly prepared acid chloride, and the mixture was stirred for 20 h. Silica gel was then added, the slurry was stirred for 1 h, the solvent was evaporated under vacuum, and the solid was directly loaded into a column for flash chromatography. Phenyl 2,3,4-tri- O- methyl-6- O -[methyl- O -(6- O - tert -butyldimethylsilyl-α- d -mannopyranos-3-yloxy)-2-carbonylbenzoyl]-1-thio-α- d -mannopyranoside ( 8a): [α]_D +36 (c 0.63, CHCl₃). ¹H NMR (300 MHz, CDCl₃): δ = 7.83-7.11 (m, 19 H, Ar), 5.65 (d, 1 H, J = 1.2 Hz, H_{1 ′}), 5.27 (dd, 1 H, J = 9.8, 3.1 Hz, H₃), 4.72 (d, 1 H, J = 1.3 Hz, H₁), 4.53 (m, 2 H, H_{5 ′} and H_{6 ′}), 4.31 (m, 2 H, H_{4 ′} and H₂), 4.11 (td, 1 H, J = 9.7, 2.6 Hz, H₄), 3.97 (dd, 1 H, J = 9.3, 4.9 Hz, H₆), 3.91 (dd, 1 H, J = 9.3, 4.9 Hz, H₆), 3.89 (br t, 1 H, J = 1.2 Hz, H_{2 ′}), 3.79 (dt, 1 H, J = 9.7, 4.9 Hz, H₅), 3.53-3.41 (m, 2 H, H_{3 ′} and H_{6 ′}), 3.54 (s, 3 H, OMe), 3.52 (s, 3 H, OMe), 3.44 (s, 3 H, OMe), 3.37 (s, 3 H, OMe), 3.14 (d, 1 H, J = 2.6 Hz, C₄OH), 2.91 (d, 1 H, J = 4.7 Hz, C₂OH), 1.05 (s, 9 H, 3 Me). ¹³C NMR (75 MHz, CDCl₃): δ = 167.8 (CO), 167.2 (CO), 135.5 (2 CH), 134.0, 133.1, 133.0, 132.2, 131.1, 130.6, 129.7, 139.6, 129.1, 129.0, 127.6, 127.3, 100.6, 84.1, 81.5, 78.1, 77.0, 76.4, 71.7, 70.5, 68.3, 66.7, 64.8, 64.8, 61.0, 58.0, 57.5, 54.8, 26.8 (3 C), 19.1 (C). MS (ES): m/z (%) = 894 (100) [M + 18], 899 (75) [M + 23].
Phenyl 2,3,4-tri- O- acetyl-6- O -[methyl- O -(6- O - tert -butyldimethylsilyl-α- d -mannopyranos-3-yloxy)-2-carbonylbenzoyl]-1-thio-α- d -mannopyranoside ( 8b): [α]_D +32.2 (c 0.5, CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ = 7.75 (d, 1 H, J = 7.8 Hz, Ar), 7.68-7.51 (m, 7 H, Ar), 7.45-7.19 (m, 9 H, Ar), 7.14-7.11 (m, 2 H, Ar), 5.43-5.41 (m, 2 H, H_{1 ′}, H_{2 ′}), 5.35 (t, 1 H, J = 9.9 Hz, H_{4 ′}), 5.26 (m, 1 H, H₃), 5.22 (dd, 1 H, J = 9.9, 3.1 Hz, H_{3 ′}), 4.65 (d, 1 H, J = 1.5 Hz, H₁), 4.54 (ddd, 1 H, J = 9.9, 7.8, 3.5 Hz, H_{5 ′}), 4.35 (dd, 1 H, J = 7.8, 3.5 Hz, H_{6 ′a} and H_{6 ′b}), 4.20 (m, 1 H, H₂), 4.0 (dt, 1 H, J = 9.7, 2.8 Hz, H₄), 3.91 (dd, 1 H, J = 10.7, 4.7 Hz, H_6a), 3.85 (dd, 1 H, J = 10.7, 5.3 Hz, H_6b), (m, 1 H, H₅) 3.41 (s, 3 H, OMe), 3.14 (d, 1 H, J = 2.9 Hz, C₄-OH), 2.88 (d, 1 H, J = 4.7 Hz, C₂-OH), 2.02 (s, 3 H, Me), 1.94 (s, 3 H, Me), 1.93 (s, 3 H, Me), 0.98 (s, 9 H, t-Bu). ¹³C NMR (50 MHz, CDCl₃): δ = 170.0 (CO), 169.7 (2 × CO), 167.6 (CO), 167.1 (CO), 135.6, 131.8, 130.7, 129.7, 129.5, 129.4, 129.2, 128.0, 127.7, 100.7, 86.6, 77.0, 71.9, 70.9, 69.4, 69.2, 68.6, 66.9, 66.5, 64.9 (C-6), 63.9 (C-6), 54.8 (OMe), 26.8 (t-Bu), 20.7 (2 × Me), 20.6 (Me), 19.2. ESI⁺ [M + Na]⁺: 983.6; ESI^- [M + Cl]^-: 995.4.

<A NAME="RD31804ST-11">11</A>

Procedure for Intramolecular Glycosylation Reactions.
Method A: diester 8a was co-evaporated 3 times with toluene, 4 Å molecular sieves were added, and the residue was dried under vacuum for 3 h. The mixture was dissolved in CH₂Cl₂ under argon, and NIS (3 equiv) was added as solid under a stream of argon at r.t. The reaction mixture was stirred for 48 h, then NaHCO₃ and Na₂S₂O₃ were added as solids. The mixture was stirred at r.t. for 30 min, then filtered through a short pad of celite, the solvent was evaporated, and the crude product was purified by flash chromatography.
Method B: as for method A, but with the addition of 0.1 equiv of TfOAg after the addition of NIS. In this case the reaction mixture was stirred at -40 °C for 2 h, was left to reach r.t. over a period of 12 h, and was then stirred at r.t. for a further 4 h.
Method C: as for method A, but with the addition of 1 equiv of TfOAg after the addition of NIS and stirring the reaction mixture at 0 °C for 10 min.
Methyl O -(2′,3′,4′-tri- O -methyl-α- d -mannopyranosyl)-(1-2)-6- O - tert- butyldiphenylsilyl-α- d -mannopyranoside-3,6′-phthalate ( 10a): [α]_D +43 (c 0.21, CHCl₃). ¹H NMR (300 MHz, CDCl₃): δ = 7.88 (dd, 1 H, J = 6.6, 2.2 Hz, Ar), 7.58-7.33 (m, 13 H, Ar), 5.57 (d, 1 H, J = 4.4, H_{1 ′}), 5.32 (dd, 1 H, J = 10.2, 3.4 Hz, H₃), 4.78-4.74 (m, 2 H, H₁ and H_{6 ′}), 4.58 (m, 1 H, H₂), 4.43-4.33 (m, 2 H, H_{6 ′} and H₄), 4.12 (m, 2 H, 2 H₆), 3.83 (m, 1 H, H_{5 ′}), 3.78 (dt, J = 9.3, 4.1 Hz, 1 H, H₅), 3.53-3.51 (m, 1 H, H_{2 ′}), 3.50 (s, 3 H, OMe), 3.45-3.35 (m, 2 H, H_{4 ′} and H_{3 ′}), 3.41 (s, 3 H, OMe), 3.37 (s, 3 H, OMe), 3.36 (s, 3 H, OMe), 2.82 (d, 1 H, J = 3.5 Hz, C₄OH), 1.08 (s, 9 H, t-Bu). ¹³C NMR (75 MHz, CDCl₃): δ = 168.1 (CO), 168.0 (CO), 136.1 (4 CH), 133.5, 133.4, 132.9, 132.8, 132.0, 131.4, 130.5, 130.1, 129.7, 128.8, 128.1 (2 CH), 128.0 (2 CH), 101.4 (C₁, J _C-H = 166 Hz), 98.0 (C_{1 ′}, J _C-H = 175 Hz), 80.6 (C_{2 ′}), 78.9 (C_{4 ′}), 77.6 (C₃), 76.0 (C_{3 ′}), 74.6 (C_{5 ′}), 73.0 (C₅), 71.8 (C₂), 66.8 (C₄), 66.3 (C₆), 64.8 (C_{6 ′}), 60.5 (OMe), 59.1 (OMe), 58.0 (OMe), 55.1 (OMe), 28.9 (C), 27.2 (3Me). After treatment with pyridine-Ac₂O: ¹H NMR (300 MHz, CDCl₃): δ = 5.55 (t, 1 H, J = 10.0 Hz, H₄). MS (ES): m/z (%) = 809 (14) [M + 1], 826 (100) [M + 18], 831 (28) [M + 23].
Methyl O -(2′,3′,4′-tri- O -methyl-β- d -mannopyranosyl)-(1-2)-4- O -acetyl-6- O - tert- butyldiphenylsilyl-α- d -mannopyranoside-3,6′-phthalate ( 11a): [α]_D -19 (c 1.07, CHCl₃). ¹H NMR (300 MHz, CDCl₃): δ = 7.78-7.62 (m, 5 H, Ar), 7.56-7.28 (m, 9 H, Ar), 5.45 (dd, 1 H, J = 16.0, 4.8 Hz, H₃), 5.36 (t, 1 H, J = 15.0, H₄), 5.07 (dd, 1 H, J = 16.0, 6.0 Hz, H_{6 ′}), 4.77 (d, 1 H, J = 2.7 Hz, H₁), 4.67 (d, 1 H, J = 1.6 Hz, H_{1 ′}), 4.49 (dd, 1 H, J = 15.0, 2.7 Hz, H₂), 4.22 (dd, 1 H, J = 16.0, 2.7 Hz, H_{6 ′}), 3.88 (ddd, 1 H, J = 13.3, 5.9, 3.8 Hz, H₅), 3.73 (m, 2 H, 2 H₆), 3.71 (dd, 1 H, J = 4.2, 1.6 Hz, H_{2 ′}), 3.59 (s, 3 H, OMe), 3.53 (dd, 1 H, J = 13.5, 13 Hz, H_{4 ′}), 3.46 (s, 3 H, OMe), 3.44 (s, 3 H, OMe), 3.43 (s, 3 H, OMe), 3.44-3.37 (m, 1 H, H_{5 ′}), 3.19 (dd, 1 H, J = 13.0, 4.2, H_{3 ′}), 1.96 (s, 3 H, Me), 1.02 (s, 9 H, t-Bu). ¹³C NMR (75 MHz, CDCl₃): δ = 169.9 (CO), 168.8 (CO), 167.4 (CO), 136.0 (2 CH), 136.0 (2 CH), 134.2, 133.6, 131.9, 131.6, 130.6, 130.1, 128.5, 128.1 (2 CH), 128.0 (2 CH), 99.3 (C₁, J _C-H = 168 Hz), 98.6 (C_{1 ′}, J _C-H = 156 Hz), 83.1 (C_{3 ′}), 76.0 (C_{4 ′}, C_{2 ′}), 73.3 (C_{5 ′}), 71.9, 71.8, 71.6 (C₃, C₂ and C₅), 67.6 (C₄), 63.4 (C₆), 62.0 (C_{6 ′}), 61.4 (OMe), 61.1 (OMe), 57.3 (OMe), 55.2 (OMe), 27.0 (3 Me), 21.1 (Me), 19.6 (C). MS (ES): m/z (%) = 809(7) [M + 1], 826 (100) [M + 18], 831 (63) [M + 23].
Methyl O -(2′,3′,4′-tri- O -methyl-α-d-mannopyranosyl)-(1-4)-2- O -acetyl-6- O - tert- butyldiphenylsilyl-α- d -mannopyranoside-3,6′-phthalate ( 12a): ¹H NMR (300 MHz, CDCl₃): δ = 7.96 (dd, 1 H, J = 7.4, 1.1 Hz, Ar), 7.75-7.35 (m, 13 H, Ar), 5.89 (dd, 1 H, J = 10.7, 3.6 Hz, H₃), 5.68 (t, 1 H, J = 2.2 Hz, H₂), 5.19 (d, 1 H, J = 3.7 Hz, H_{1 ′}), 5.04 (d, 1 H, J = 9.3 Hz, H₆ or H_{6 ′}), 4.72 (d, 1 H, J = 2.2 Hz, H₁), 5.58 (t, 1 H, J = 10.3 Hz, H₄), 4.04 (dd, 1 H, J = 10.7, 3.0, H₆ or H_{6 ′}), 3.96-3.85 (m, 5 H, 2 H₆ or 2 H_{6 ′}, H_{5 ′} H₅ and H_{4 ′}), 3.51 (s, 3 H, OMe), 3.42 (s, 3 H, OMe), 3.41 (s, 3 H, OMe), 3.30 (m, 1 H, H_{2 ′}), 3.27 (s, 3 H, OMe), 3.19 (dd, 1 H J = 9.3, 5.5 Hz, H_{3 ′}), 2.17 (s, 3 H, Me), 1.05 (s, 9 H, t-Bu). ¹³C NMR (75 MHz, CDCl₃): δ = 170.3 (CO), 168.2 (CO), 165.5 (CO), 135.7 (2 CH), 135.6 (2 CH), 135.2, 133.5, 133.3, 132.4, 131.2, 130.0, 129.7, 129.6, 127.6 (2 CH), 127.6 (2 CH), 98.2 (C₁, J _C-H = 172 Hz), 92.6 (C_{1 ′}, J _C-H = 160 Hz), 79.2 (C_{2 ′}), 77.1 (C_{3 ′}), 71.1, 69.8, 69.7, 69.2, 68.8 (H₄), 64.9 (CH₂), 62.3 (CH₂), 59.2 (OMe), 58.2 (OMe), 57.9 (OMe), 55.3 (OMe), 26.7 (3 Me), 20.7 (Me), 19.4 (C). After desilylation of 12 with TBAF and treatment with pyridine-Ac₂O: ¹³C NMR (75 MHz, CDCl₃): δ = 98.6 (C₁, J _C-H = 177 Hz), 93.3 (C_{1 ′}, J _C-H = 166 Hz). MS (ES): m/z (%) = 809 (2) [M + 1], 826 (100) [M + 18], 831 (40) [M + 23].
Methyl O -(2′,3′,4′-tri- O -methyl-β- d -mannopyranosyl)-(1-4)-2- O -acetyl-6- O - tert -butyldiphenylsilyl-α- d -mannopyranoside-3,6′-phthalate ( 13a): [α]_D -15 (c 0.17, CHCl₃), ¹H NMR (300 MHz, CDCl₃): δ = 7.70-7.30 (m, 14 H, Ar), 5.69 (dd, 1 H, J = 6.3, 3.7 Hz, H₃), 5.46 (t, 1 H, J = 3.7 Hz, H₂), 4.95 (dd, 1 H, J = 11.5, 2.7 Hz, H_{6 ′}), 4.78 (d, 1 H, J = 4.7 Hz, H₁), 4.41 (s, 1 H, H_{1 ′}), 4.28 (dd, 1 H, J = 9.8, 6.3 Hz, H₄), 4.12 (dd, 1 H, J = 11.5, 6.6 Hz, H_{6 ′}), 4.00-3.88 (m, 2 H, 2 H₆), 3.85-3.78 (m, 1 H, H₅), 3.52 (s, 3 H, OMe), 3.46 (s, 3 H, OMe), 3.44 (s, 3 H, OMe), 3.43 (s, 3 H, OMe), 3.50-3.26 (m, 2 H, H_{5 ′} and H_{2 ′}), 3.34 (t, 1 H, J = 8.8 Hz, H_{4 ′}), 3.01 (dd, 1 H, J = 8.8, 2.9 Hz, H_{3 ′}), 2.07 (s, 3 H, Me), 1.09 (s, 9 H, 3 Me). ¹³C NMR (75 MHz, CDCl₃): δ = 170.7 (CO), 167.8 (CO), 167.1 (CO), 136.4 (2 CH), 136.1 (2 CH), 134.3, 133.6, 133.1, 131.8, 131.7, 131.3, 130.4, 129.6, 128.3 (2 CH), 128.2 (2 CH), 102.4 (C_{1 ′}, J _C-H = 157 Hz), 99.1 (C₁, J _C-H = 172 Hz), 83.8 (C_{3 ′}), 77.1 (C_{4 ′}), 76.8 (C_{5 ′}), 74.9 (C₄), 74.0 (C₃), 73.8 (C_{2 ′}), 72.1 (C₅), 70.2 (C₂), 64.1 (C_{6 ′}), 63.7 (C₆), 61.8 (OMe), 61.2 (OMe), 57.9 (OMe), 55.7 (OMe), 27.1 (3 Me), 21.4 (Me), 19.7 (C). MS (ES): m/z (%) = 809 (5) [M + 1], 826 (100) [M + 18], 831 (16) [M + 23].
Methyl O -(2′,3′4′-tri- O -acetyl-α-d-mannopyranosyl)-(1-2)-α- d -mannopyranoside-3,6′-phthalate: formed by treatment of 10b with TBAF. ¹H NMR (400 MHz, CDCl₃): δ = 7.94-7.93 (m, 1 H, Ar), 7.55-7.50 (m, 3 H, Ar), 5.69 (d, 1 H, J = 4.7 Hz, H_{1 ′}), 5.33 (dd, 1 H, J = 10.6, 3.7 Hz, H₃), 5.24 (dd, 1 H, J = 7.0, 3.8 Hz, H_{3 ′}), 5.06-5.00 (m, 2 H, H_{2 ′}, H_{4 ′}), 4.78 (dd, 1 H, J = 12.3 , 5.0 Hz, H_{6 ′a}), 4.73 (d, 1 H, J = 1.3 Hz, H₁), 4.46 (dd, 1 H, J = 3.5, 1.2 Hz, H₂), 4.28-4.22 (m, 2 H, H₄, H_{6 ′b}), 4.17-4.14 (m, 1 H, H_{5 ′}), 3.82-3.78 (m, 2 H, H_6a and H_6b), 3.64 (ddd, 1 H, J = 9.7, 6.7, 3.4 Hz, H₅), 3.34 (s, 3 H, OMe), 2.81 (d, 1 H, J = 5.3 Hz, C₄OH), 2.01 (s, 3 H, Me), 2.00 (s, 3 H, Me), 1.78 (s, 3 H, Me). ¹³C NMR (75 MHz, CDCl₃): δ = 165.7 (CO), 165.5 (CO), 165.1 (CO), 163.7 (CO), 162.6 (CO), 128.7 (C), 127.7, 126.6, 126.4, 124.7 (C), 123.4, 96.6, 89.1, 72.9, 72.7, 72.5, 72.1, 68.8, 68.4, 68.3, 65.5, 64.4, 63.8, 60.6, 60.4, 57.4, 50.6 (OMe), 16.3 (2 × Me), 15.9 (Me).
Methyl O -(2′,3′4′-tri- O -acetyl-α-d-mannopyranosyl)-(1-4)-α- d -mannopyranoside-3,6′-phthalate: formed by treatment of 12b with TBAF. ¹H NMR (400 MHz, CDCl₃): δ = 7.98 (m, 1 H, Ar), 7.62 (dt, 1 H, J = 7.5, 1.5 Hz, Ar), 7.55 (dt, 1 H, J = 7.5, 1.5 Hz, Ar), 7.43 (m, 1 H, Ar), 5.70 (dd, 1 H, J = 10.7, 3.5 Hz, H₃), 5.33 (dd, 1 H, J = 10.0, 3.3 Hz, H_{3 ′}), 5.33 (s, 1 H, H_{1 ′}), 5.24 (t, 1 H, J = 10.0 Hz, H_{4 ′}), 5.01 (dd, 1 H, J = 3.3, 1.5 Hz, H_{2 ′}), 4.83 (dd, 1 H, J = 11.5, 1.2 Hz, H_{6 ′a}), 4.75 (d, 1 H, J = 2.0 Hz, H₁), 4.41-4.34 (m, 2 H, H₂, H₄), 4.20 (br t, 1 H, J = 10.0 Hz, H_{5 ′}), 3.95-3.87 (m, 3 H, H_{6 ′b}, H_6a, H_6b), 3.83- 3.77 (m, 1 H, H₅), 3.38 (s, 3 H, OMe), 2.16 (s, 3 H, Me), 2.10 (s, 3 H, Me), 2.08 (s, 3 H, Me), 2.07 (s, 3 H, Me), 2.11 (s, 3 H, Me), 2.06 (s, 3 H, Me), 1.96 (s, 3 H, Me). ¹³C NMR (75 MHz, CDCl₃): δ = 166.0 (CO), 165.8 (CO), 165.3 (CO), 164.2 (CO), 160.7 (CO), 130.5 (C), 128.3, 125.8, 125.6, 123.1, 122.9 (C), 95.8, 87.5, 66.5, 66.4, 66.2, 66.0, 64.4, 64.2, 63.5, 61.7, 59.2 (C₆), 57.3 (C₆), 51.1 (OMe), 16.4 (Me), 16.2 (Me), 16.1 (Me). After treatment with pyridine-Ac₂O: ¹H NMR (300 MHz, CDCl₃): δ = 5.56 (m, 1 H, H₂). ¹³C NMR (75 MHz, CDCl₃): δ = 98.2 (J _C-H = 170 Hz), 91.7 (J _C-H = 167 Hz).

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<A NAME="RD31804ST-14A">14a</A> Stork G. Kim G. J. Am. Chem. Soc.  1992,  114:  1087 

<A NAME="RD31804ST-14B">14b</A> Barresi F. Hindsgaul O. Can. J. Chem.  1994,  72:  1447 

<A NAME="RD31804ST-14C">14c</A> Ito Y. Ohnishi Y. Ogawa T. Nakahara Y. Synlett  1998,  1102 

<A NAME="RD31804ST-14D">14d</A> Fairbanks AJ. Synlett  2003,  1945 

<A NAME="RD31804ST-14E">14e</A> Abdel-Rahman AAH. El Ashry El SH. Schmidt RR. Carbohydr. Res.  2002,  337:  195 

<A NAME="RD31804ST-14F">14f</A> For a review of intramolecular glycosylation see: Jung K.-H. Müller M. Schmidt RR. Chem. Rev.  2000,  100:  4423 

<A NAME="RD31804ST-15">15</A> Kametani T. Kawamura K. Honda T. J. Am. Chem. Soc.  1987,  109:  3010 

<A NAME="RD31804ST-16">16</A> Methyl 3-O-benzoyl-6-O-tert-butyldimethylsilyl-α-d-mannopyranoside (15) was prepared starting from methyl-α-d-mannopyranoside by silylation of the C6 hydroxyl group followed by benzoylation of the dialkylstannylene derivative formed by microwave irradiation. This compound had been prepared previously by: Chung S.-K. Yu S.-H. Bioorg. Med. Chem. Lett.  1996,  6:  1461 

<A NAME="RD31804ST-17">17</A> Intermolecular glycosylations were carried out following procedure B, previously described for an intramolecular glycosylation reaction using 1 equiv each of the corresponding glycosyl donor and acceptor, at the temperature and for the reaction times specified in the text. The structures of all oligosaccharides were assigned by means of 2D NMR spectroscopy (COSY, HMQC) and mass spectrometry. Regioisomers were identified based on variations in the ¹H chemical shift of the free hydroxyl groups after acetylation. The assignment of the configuration (α or β) was based on the CH coupling constant at the anomeric center (J _CH ª 160Hz for β-mannosides and J _CH ª 170 Hz for α-mannosides)

<A NAME="RD31804ST-18">18</A> Compound 19 was synthesized from methyl-α-d-mannopyranoside under standard benzoylation conditions in the presence of two equivalents of benzoyl chloride. This methylmannoside 19 had been previously prepared by: Meyers HV. Ojika M. Wiesler WT. Nakanishi K. Carbohydr. Res.  1990,  197:  15 

<A NAME="RD31804ST-19">19</A> Alais JO. Veyrières A. J. Chem. Soc., Perkin Trans. 1  1981,  377