Direct Chemical Synthesis of the β-Mannans: Linear and Block Syntheses of the Alternating β-(1→3)-β-(1→4)-Mannan Common to <i>Rhodotorula </i><i>glutinis</i>, <i>Rhodotorula </i><i>mucilaginosa</i>, and <i>Leptospira </i><i>biflexa</i>

Abstract

Two stereocontrolled syntheses of a methyl glycoside of an alternating beta-(1-->4)-beta-(1-->3)-mannohexaose, representative of the mannan from Rhodotorula glutinis, Rhodotorula mucilaginosa, and Leptospira biflexa, are described. Both syntheses employ a combination of 4,6-O-benzylidene- and 4,6-O-p-methoxybenzylidene acetal-protected donors to achieve stereocontrolled formation of the beta-mannoside linkage. The first synthesis is a linear one and proceeds with a high degree of stereocontrol throughout and an overall yield of 1.9%. The second synthesis, a block synthesis, makes use of the coupling of two trisaccharides, resulting in a shorter sequence and an overall yield of 4.4%, despite the poor selectivity in the key step.