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.
观察到甲基 6-脱氧-2,3-O-异丙叉-3-C-甲基-α-L-
吡喃
甘露糖苷 (2) 及其脱异丙叉衍
生物 (7) 还原的完全逆转的立体选择性。化合物 2 与 NaBH4 在 MeOH 中仅生成 L-talo-异构体 (3),但用 NaBH4 在
乙酸中还原 7 得到 L-
甘露糖衍
生物 8。假设在第一种情况下确定了立体选择性通过羰基的空间可及性,而在第二种情况下,游离的羟基通过络合或
配体交换指导还原的选择性。