626
3
Chemical Glycosylation Reactions
mido-2-deoxy unit in 350, was retrosynthetically correlated with 2-azido-2-deoxy glycosyl
donor 354.
3.3.5 Conversion of 2-Buten-2-yl Glycosides to Other Glycosyl Donors
Treatment of “active” vinyl glycosides with NIS/TMSOTf in CH 2 Cl 2 in the presence of dibenzyl phosphate gives good yield of glycosyl phosphates [184].
2-Buten-2-yl glycosides can also be transformed to glycosyl fluorides and trichloroacetimidates by hydrolysis to the corresponding hemiacetal (HgO, HgBr 2 , aq. acetone) followed by
standard treatment (CCl 3 CN, DBU, CH 2 Cl 2 or DAST, THF, respectively) [183].
3.3.6 Synthesis of 3-Buten-2-yl Glycosides
Being normal alkyl glycopyranosides, 3-buten-2-yl glycosides can be prepared by standard
glycosylation methods, as previously mentioned for NPGs.
3.4 Oxathiines: Vinyl Glycosyl Donors
for the Synthesis of 2-Deoxy Glycosides
Cycloadduct 357, readily available by cycloaddition of tri-O-benyzl glucal (355) with the
electron-poor 3-thioxopentane-2,4-dione (356) [185] has been used by two research groups as
precursor glycosyl for vinyl glycosyl donors 358, 361, and 363 ( > Scheme 58). Franck and coworkers showed that glycoside 358, prepared by methylenation of 357, underwent β-selective
glycosylation with a variety of glycosyl acceptors in the presence of TfOH to give glycosides
359, in good yields [186]. Moreover, Raney nickel desulfurization of 359 granted access to 2deoxy-glycosides 360 [187]. Capozzi and co-workers reported that acetyl [188], and silyl [189]
derivatives 361 and 363, also functioned as glycosyl donors in reactions catalyzed by MeOTf in
nitromethane and TMSOTf in CH 2 Cl 2 , respectively. The timing in the quenching of the reactions is crucial for obtaining completely selective β-glycosylations, and prolonged reaction
times led to α/β anomeric mixtures. The total β-stereoselectivity of the coupling was ascribed
by Capozzi and co-workers to an S N 2 type reaction (361→362, > Scheme 59) that induces
β-stereospecific glycosylation [188]. The observed subsequent α/β–equilibration presumably
proceeds through an oxonium intermediate 364 ( > Scheme 59).
4 DISAL Glycosyl Donors
4.1 Synthesis and Glycosylation Reactions
Petersen and Jensen reasoned that glycosides of phenols (e. g. 366) carrying sufficiently
electron-withdrawing substituents could possibly serve as O-glycosyl donors under neutral
or mildly basic conditions ( > Scheme 60) [190,191]. Carbohydrate hemiacetals have been
used as nucleophiles in aromatic substitutions using activated fluoroarenes [192,193]. Accordingly, glycosides of methyl 2-hydroxy-3,5-dinitrobenzoate (DISAL, a DInitroSALicylic acid
derivative), e. g. 367 ( > Scheme 61), and methyl 4-hydroxy-3,5-dinitrobenzoate (para-isomer)
3
Chemical Glycosylation Reactions
mido-2-deoxy unit in 350, was retrosynthetically correlated with 2-azido-2-deoxy glycosyl
donor 354.
3.3.5 Conversion of 2-Buten-2-yl Glycosides to Other Glycosyl Donors
Treatment of “active” vinyl glycosides with NIS/TMSOTf in CH 2 Cl 2 in the presence of dibenzyl phosphate gives good yield of glycosyl phosphates [184].
2-Buten-2-yl glycosides can also be transformed to glycosyl fluorides and trichloroacetimidates by hydrolysis to the corresponding hemiacetal (HgO, HgBr 2 , aq. acetone) followed by
standard treatment (CCl 3 CN, DBU, CH 2 Cl 2 or DAST, THF, respectively) [183].
3.3.6 Synthesis of 3-Buten-2-yl Glycosides
Being normal alkyl glycopyranosides, 3-buten-2-yl glycosides can be prepared by standard
glycosylation methods, as previously mentioned for NPGs.
3.4 Oxathiines: Vinyl Glycosyl Donors
for the Synthesis of 2-Deoxy Glycosides
Cycloadduct 357, readily available by cycloaddition of tri-O-benyzl glucal (355) with the
electron-poor 3-thioxopentane-2,4-dione (356) [185] has been used by two research groups as
precursor glycosyl for vinyl glycosyl donors 358, 361, and 363 ( > Scheme 58). Franck and coworkers showed that glycoside 358, prepared by methylenation of 357, underwent β-selective
glycosylation with a variety of glycosyl acceptors in the presence of TfOH to give glycosides
359, in good yields [186]. Moreover, Raney nickel desulfurization of 359 granted access to 2deoxy-glycosides 360 [187]. Capozzi and co-workers reported that acetyl [188], and silyl [189]
derivatives 361 and 363, also functioned as glycosyl donors in reactions catalyzed by MeOTf in
nitromethane and TMSOTf in CH 2 Cl 2 , respectively. The timing in the quenching of the reactions is crucial for obtaining completely selective β-glycosylations, and prolonged reaction
times led to α/β anomeric mixtures. The total β-stereoselectivity of the coupling was ascribed
by Capozzi and co-workers to an S N 2 type reaction (361→362, > Scheme 59) that induces
β-stereospecific glycosylation [188]. The observed subsequent α/β–equilibration presumably
proceeds through an oxonium intermediate 364 ( > Scheme 59).
4 DISAL Glycosyl Donors
4.1 Synthesis and Glycosylation Reactions
Petersen and Jensen reasoned that glycosides of phenols (e. g. 366) carrying sufficiently
electron-withdrawing substituents could possibly serve as O-glycosyl donors under neutral
or mildly basic conditions ( > Scheme 60) [190,191]. Carbohydrate hemiacetals have been
used as nucleophiles in aromatic substitutions using activated fluoroarenes [192,193]. Accordingly, glycosides of methyl 2-hydroxy-3,5-dinitrobenzoate (DISAL, a DInitroSALicylic acid
derivative), e. g. 367 ( > Scheme 61), and methyl 4-hydroxy-3,5-dinitrobenzoate (para-isomer)