O-Glycosyl Donors
3.4
651
ed with primary hydroxyl acceptors in the presence of catalytic TfOH at −78 °C to give mainly β-glucosides, e. g. 492 ( > Scheme 83b). Although, a highly stereoselective α-glucosylation (α/β = 88:12) was carried out in high yield using 20 mol% of HClO 4 in tert-BuOMe
( > Scheme 83c). 6-Nitro-2-benzothiazolyl α-mannosides (e. g. 494) effected stereoselective
β-mannosylation with several glycosyl acceptors [234,235]. The highest β-stereoselectivity
was achieved when tetrakis(pentafluorophenyl)boric acid [HB(C 6 F 5 ) 4 ] [236] was employed
as catalyst ( > Scheme 83d). BF 3 ·Et 2 O, a weaker Lewis acid, showed a reversed stereoselectivity [237] ( > Scheme 83e). The β-selective coupling was employed by Mukaiyama and coworkers in the formation of the β-Man(1→4)GlcN linkage, e. g. 498, that exists in N-linked
glycans ( > Scheme 83f ) [238,239].
7 Miscellaneous O-Glycosyl Donors
Noyori and Kurimoto [240] described that hydroxyl-protected and -unprotected glycosyl aryloxides reacted with alcohols under mild electrolytic conditions to give the corresponding
glycosides. They hypothesized that the glycosylation reaction proceeded via oxocarbenium
ion intermediates generated from the radical cation of the easily oxidizable aryloxy substrate
( > Scheme 84).
A combination of trimethylsilyl bromide and zinc triflate promoted the glycosylation of
benzyl-, isopropyl-, and methyl glycosides with several glycosyl acceptors in moderate to
good yields [241,242].
2-Deoxyglycosides, e. g. 501, were obtained by DDQ oxidation of 3,4-dimethoxybenzyl
glycosides 500, in MeCN in the presence of primary, secondary, and tertiary alcohols
( > Scheme 85) [243].
Davis and co-workers [244] examined the self-activating properties of unprotected and acetylated bromobutyl glycosides 505 and 508, respectively ( > Scheme 86). These readily available
compounds reacted with galactose acceptor 40 (1 equiv.) in the presence of a halophilic Lewis
⊡ Scheme 84
Electrochemical glycosylation of glycosyl aryloxides
3.4
651
ed with primary hydroxyl acceptors in the presence of catalytic TfOH at −78 °C to give mainly β-glucosides, e. g. 492 ( > Scheme 83b). Although, a highly stereoselective α-glucosylation (α/β = 88:12) was carried out in high yield using 20 mol% of HClO 4 in tert-BuOMe
( > Scheme 83c). 6-Nitro-2-benzothiazolyl α-mannosides (e. g. 494) effected stereoselective
β-mannosylation with several glycosyl acceptors [234,235]. The highest β-stereoselectivity
was achieved when tetrakis(pentafluorophenyl)boric acid [HB(C 6 F 5 ) 4 ] [236] was employed
as catalyst ( > Scheme 83d). BF 3 ·Et 2 O, a weaker Lewis acid, showed a reversed stereoselectivity [237] ( > Scheme 83e). The β-selective coupling was employed by Mukaiyama and coworkers in the formation of the β-Man(1→4)GlcN linkage, e. g. 498, that exists in N-linked
glycans ( > Scheme 83f ) [238,239].
7 Miscellaneous O-Glycosyl Donors
Noyori and Kurimoto [240] described that hydroxyl-protected and -unprotected glycosyl aryloxides reacted with alcohols under mild electrolytic conditions to give the corresponding
glycosides. They hypothesized that the glycosylation reaction proceeded via oxocarbenium
ion intermediates generated from the radical cation of the easily oxidizable aryloxy substrate
( > Scheme 84).
A combination of trimethylsilyl bromide and zinc triflate promoted the glycosylation of
benzyl-, isopropyl-, and methyl glycosides with several glycosyl acceptors in moderate to
good yields [241,242].
2-Deoxyglycosides, e. g. 501, were obtained by DDQ oxidation of 3,4-dimethoxybenzyl
glycosides 500, in MeCN in the presence of primary, secondary, and tertiary alcohols
( > Scheme 85) [243].
Davis and co-workers [244] examined the self-activating properties of unprotected and acetylated bromobutyl glycosides 505 and 508, respectively ( > Scheme 86). These readily available
compounds reacted with galactose acceptor 40 (1 equiv.) in the presence of a halophilic Lewis
⊡ Scheme 84
Electrochemical glycosylation of glycosyl aryloxides