O-Glycosyl Donors
3.4
619
⊡ Scheme 51
Reaction of armed and disarmed isopropenyl glycopyranosides
armed β-294 relative to that from disarmed glycoside β-307. In general, factors which favor
the formation of the glycosyl oxocarbenium ion (strong electrophile, polar solvent, electronreleasing protecting groups on the glycosyl donor) lead to transglycosylation. Factors which
retard the formation of the glycosyl cation (weak electrophile, nonpolar solvent, electron-withdrawing protecting groups on the glycosyl donor) lead to addition across the isopropenyl ether
double bond.
The ability of various electrophiles to promote transglycosylation of disarmed isopropenyl
glycosides is outlined in > Table 10. NIS/TfOH, TMSOTf, and Tf 2 O in MeCN, all led to
the formation of disaccharide 319 in good yield ( > Table 10, entries i–iii). Reactions were
carried out at 0 °C and were complete within 2–5 min. With silver triflate (AgOTf) the reaction
was slower and gave a lower yield of disaccharide 319 ( > Table 10, entry iv). When TfOH,
NIS, or NBS were used alone β-307 failed to react and the glycosyl donor was recovered
unchanged ( > Table 10, entries vi, viii). Thus, neither NIS/TfOH, TMSOTf, Tf 2 O, nor AgOTf
seem to activate isopropenyl glycosides by acting as a source of TfOH. Dimethyl(methylthio)sulfonium triflate (DMTST) was the only promoter that led exclusively to the formation of
disaccharide 319 from β-307 when CH 2 Cl 2 was used as the solvent.
In terms of glycosyl donors, either α-307 or β-307 gave the same results in terms of yields.
Likewise, isopropenyl galactopyranosides reacted in a similar manner to glucopyranosides.
Acylated isopropenyl donors gave lower yields than pivaloyl analogs, presumably because of
complications due to orthoester formation [175].
Isopropenyl glycosides could be activated selectively in the presence of armed NPGs, and that
allowed a one-pot synthesis of trisaccharide 322 involving the successive glycosyl coupling of
a vinyl glycoside β-307, and an NPG, 321 ( > Scheme 52).
3.4
619
⊡ Scheme 51
Reaction of armed and disarmed isopropenyl glycopyranosides
armed β-294 relative to that from disarmed glycoside β-307. In general, factors which favor
the formation of the glycosyl oxocarbenium ion (strong electrophile, polar solvent, electronreleasing protecting groups on the glycosyl donor) lead to transglycosylation. Factors which
retard the formation of the glycosyl cation (weak electrophile, nonpolar solvent, electron-withdrawing protecting groups on the glycosyl donor) lead to addition across the isopropenyl ether
double bond.
The ability of various electrophiles to promote transglycosylation of disarmed isopropenyl
glycosides is outlined in > Table 10. NIS/TfOH, TMSOTf, and Tf 2 O in MeCN, all led to
the formation of disaccharide 319 in good yield ( > Table 10, entries i–iii). Reactions were
carried out at 0 °C and were complete within 2–5 min. With silver triflate (AgOTf) the reaction
was slower and gave a lower yield of disaccharide 319 ( > Table 10, entry iv). When TfOH,
NIS, or NBS were used alone β-307 failed to react and the glycosyl donor was recovered
unchanged ( > Table 10, entries vi, viii). Thus, neither NIS/TfOH, TMSOTf, Tf 2 O, nor AgOTf
seem to activate isopropenyl glycosides by acting as a source of TfOH. Dimethyl(methylthio)sulfonium triflate (DMTST) was the only promoter that led exclusively to the formation of
disaccharide 319 from β-307 when CH 2 Cl 2 was used as the solvent.
In terms of glycosyl donors, either α-307 or β-307 gave the same results in terms of yields.
Likewise, isopropenyl galactopyranosides reacted in a similar manner to glucopyranosides.
Acylated isopropenyl donors gave lower yields than pivaloyl analogs, presumably because of
complications due to orthoester formation [175].
Isopropenyl glycosides could be activated selectively in the presence of armed NPGs, and that
allowed a one-pot synthesis of trisaccharide 322 involving the successive glycosyl coupling of
a vinyl glycoside β-307, and an NPG, 321 ( > Scheme 52).