O-Glycosyl Donors
3.4
587
allyloxycarbonyl group is noteworthy. Related n-pentenoyl derivatives have been reported as
efficient protecting groups for amines by its mild deprotection with iodine in THF-water [77].
Rojas and co-workers [78] described a novel synthetic route to α-linked 2-deoxy-2-mannosamine derivatives, which involved a stereocontrolled glycosidation step of NPG oxazolidinones (e. g. 124, > Table 4) and N-CBz NPGs (e. g. 127, > Table 5). The authors found
a striking difference in reactivity between α- and β-anomers of oxazolidinones 124. α-NPG
oxazolidinones served as highly stereoselective donors ( > Table 4, entries iii, v–vii), whereas the β-anomer was nearly inert ( > Table 4, entries ii, iv). However, regioselective N-CBz
oxazolidinone ring opening to 127, prior to glycosylation permitted elaboration of either NPG
anomer to the desired α-Man-NCBz products 128 ( > Table 5).
2.7 Semi-Orthogonal Couplings of NPGs
The term “semi-orthogonality” between glycosyl donors (e. g. A and B, > Scheme 16) was
introduced by Demchenko [79]. It indicates that whereas selective activation of armed and disarmed glycosyl donor A can be effected in the presence of either armed or disarmed donor B
( > Scheme 16a), the opposite is not feasible. Thus, in semi-orthogonal donors, the selective
activation of disarmed glycosyl donor B in the presence of glycosyl donor A can not be accomplished ( > Scheme 16c).
2.7.1 Semi-Orthogonality of O-Pentenyl and S-Ethyl Glycosides
Demchenko and De Meo found conditions for the selective activation of NPGs and ethyl
1-thioglycosides ( > Scheme 17) [79]. They demonstrated that armed NPGs (e. g. β-12)
⊡ Scheme 16
Semi-orthogonality of glycosyl donors
3.4
587
allyloxycarbonyl group is noteworthy. Related n-pentenoyl derivatives have been reported as
efficient protecting groups for amines by its mild deprotection with iodine in THF-water [77].
Rojas and co-workers [78] described a novel synthetic route to α-linked 2-deoxy-2-mannosamine derivatives, which involved a stereocontrolled glycosidation step of NPG oxazolidinones (e. g. 124, > Table 4) and N-CBz NPGs (e. g. 127, > Table 5). The authors found
a striking difference in reactivity between α- and β-anomers of oxazolidinones 124. α-NPG
oxazolidinones served as highly stereoselective donors ( > Table 4, entries iii, v–vii), whereas the β-anomer was nearly inert ( > Table 4, entries ii, iv). However, regioselective N-CBz
oxazolidinone ring opening to 127, prior to glycosylation permitted elaboration of either NPG
anomer to the desired α-Man-NCBz products 128 ( > Table 5).
2.7 Semi-Orthogonal Couplings of NPGs
The term “semi-orthogonality” between glycosyl donors (e. g. A and B, > Scheme 16) was
introduced by Demchenko [79]. It indicates that whereas selective activation of armed and disarmed glycosyl donor A can be effected in the presence of either armed or disarmed donor B
( > Scheme 16a), the opposite is not feasible. Thus, in semi-orthogonal donors, the selective
activation of disarmed glycosyl donor B in the presence of glycosyl donor A can not be accomplished ( > Scheme 16c).
2.7.1 Semi-Orthogonality of O-Pentenyl and S-Ethyl Glycosides
Demchenko and De Meo found conditions for the selective activation of NPGs and ethyl
1-thioglycosides ( > Scheme 17) [79]. They demonstrated that armed NPGs (e. g. β-12)
⊡ Scheme 16
Semi-orthogonality of glycosyl donors