On the Influence of the C2−O2 and C3−O3 Bonds in 4,6-O-Benzylidene-Directed β-Mannopyranosylation and α-Glucopyranosylation

Abstract

The synthesis of 4,6-O-benzylidene protected 2-deoxy-arabino, 3-deoxy-arabino-, and 3-deoxy-ribothioglycosides is described and their glycosylation reactions, with activation by either 1-benzenesulfinyl piperidine/trifluoromethansulfonic anhydride or diphenyl sulfoxide/ trifluoromethanesulfonic anhydride, studied. In contrast to the corresponding 4,6-O-benzylidene protected glucosyl and mannosyl donors, which are α-and β-selective, respectively, poor diastereoselectivity is observed in all cases. The reasons for this poor selectivity are discussed in terms of the interaction between the C2-O2 and C3-O3 bonds in the glucosyl and mannosyl donors and of the influence of this interaction on the ease of formation of the intermediate glycosyl oxacarbenium ions.

Is Donor−Acceptor Hydrogen Bonding Necessary for 4,6-O-Benzylidene-directed β-Mannopyranosylation? Stereoselective Synthesis of β-C-Mannopyranosides and α-C-Glucopyranosides

“…With the help of a series of 2- or 3-deoxy and the corresponding deoxy fluoro gluco and mannosyl donors, we have rationalized the glucose/mannose stereoselectivity shift on the basis of the differing O2-C2-C3-O3 torsional interactions as the covalent triflates collapse to the ion pairs 11. In mannose this torsion angle is compressed as the oxacarbenium ion is formed, whereas in glucose it is relaxed.…”

Formation of the Glycosidic Linkage

“…225 Indeed, as applied to a-D-mannopyranosyl sulfoxides having nonparticipating groups at C2 and C3 and a 4,6-O-benzylidene protecting group (presumably to discourage glycosyl cation formation), 253 there now exists a powerful method for the synthesis of b-D-mannopyranosides from virtually any sort of acceptor alcohol: [257][258][259][260] The stereoselectivity of the method is excellent, with any 'leakage' to the a-Danomer presumably arising via the 'loose' ion pair. 261 Even this successful method had limitations and has now been superseded by the aforementioned combinations of 1-benzenesulfinylpiperidine or diphenyl sulfoxide and triflic anhydride: [262][263][264][265] A recent report approaches the synthesis of b-D-mannopyranosides from the hemiacetal, using Gin's dehydrative glycosidation methodology; 266 another uses the ubiquitous TCA approach. 261 Even this successful method had limitations and has now been superseded by the aforementioned combinations of 1-benzenesulfinylpiperidine or diphenyl sulfoxide and triflic anhydride: [262][263][264][265] A recent report approaches the synthesis of b-D-mannopyranosides from the hemiacetal, using Gin's dehydrative glycosidation methodology; 266 another uses the ubiquitous TCA approach.…”

Synthesis and Immunological Screening of β‐Linked Mono‐ and Divalent Mannosides

“…Owing to the pioneering work of Crich and co-workers and others, several methods have now become available for the synthesis of β-mannosides, either by direct glycosylation [46][47][48][49][50][51] or by indirect approaches. Owing to the pioneering work of Crich and co-workers and others, several methods have now become available for the synthesis of β-mannosides, either by direct glycosylation [46][47][48][49][50][51] or by indirect approaches.…”