We have previously developed a novel method for the diastereomeric acetals 14a, 15a and 14b, 15b were prepared from 9 + 10/7 and 11 + 12/13, respectively. Investigations by stereocontrolled synthesis of β-manno-glycoside. Starting from 2-O-PMB (p-methoxybenzyl)-protected mannosyl donor means of NMR and a computational approach using DADAS 90 for quantifying steric hindrance, resulted in the conclusion 1, conversion into the mixed acetal 3 under oxidative conditions followed by the activation of the anomeric position that 14a/15a derived from 2-O-PMB-protected 9 has an (S) configuration and 14b/15b derived from 2-O-unprotected 11 affords β-manno-glycoside as a single stereoisomer. Although the utility of this method has been further has an (R) configuration. Based on the characteristic 1 H-NMR patterns inherent to the (S) isomers, 4,6-O-benzylidene-demonstrated in the synthesis of the core structure of Asnlinked glycan chains, there remained uncertainty with protected 30-35, derived from thiomannosides 5, 23, 24, 26, 27, were also revealed to have the (S) configuration. respect to the stereochemistry of the mixed acetal. In order to make a stereochemical assignment of this intermediate,