Completely reversed stereoselectivity of reduction of methyl 6-deoxy-2,3-O-isopropylidene-3-C-methyl-α-L-mannopyranoside (2) and its deisopropylidenated derivative (7) was observed. Compound 2 gave exclusively the L-talo-isomer (3) with NaBH4 in MeOH, but the reduction of 7 with NaBH4 in acetic acid resulted in the L-manno-derivative 8. It is assumed that in the first case the stereoselectivity is determined by the steric accessibility of the carbonyl group, while in the second case free OH-groups direct the selectivity of the reduction by complexation or ligand exchange.
ASJC Scopus subject areas
- Organic Chemistry