Synthesis and Structure Assignment of 2-(4-Methoxybenzyl)cyclohexyl β-D-Galactopyranoside Stereoisomers

Several promoters were used in the Koenigs-Knorr synthesis of the title alkyl β-D-galactopyranosides, both in their diastereoisomeric forms (5a/5b and 6a/6b), resulting from the synthesis performed with the respective racemic cis and trans isomers of 2-(4-methoxybenzyl)cyclohexan-1-ol, and in their enantiomerically pure forms 5a and 6a, starting only from the (1S,2S)- and (1S,2R)-enantiomers of 2-(4-methoxybenzyl)cyclohexan-1-ol. The aim of the study was to find convenient modification(s) of the Koenigs-Knorr synthesis of alkyl β-D-galactopyranosides from more hindered and more complex 2-substituted cycloalkanols. Separation of the diastereoisomeric compounds using HPLC on a chiral Nucleodex-β-OH column was used to obtain small quantities of all possibly existing enantiomerically pure products for unambiguous structure assignment by NMR analysis. The (1S,2S)- and (1S,2R)- enantiomers of 2-(4-methoxybenzyl)cyclohexan-1-ol (1a and 2a) were prepared by a reduction of 2-(4-methoxybenzyl)cyclohexan-1-one with Saccharomyces cerevisiae in enantiomeric purities: ee = 98.5% ((1S,2S)-enantiomer (1a)), and ee ≥ 99% ((1S,2R)-enantiomer (2a)).