The rate constants for the inversion and for the 14C-acetoxy exchange of the diastereomeric 1,2,3,4,5-penta-O-acetyl S-ethyl monothioacetals show that the substitution is not anchimerically assisted by the C2-acetoxy group. The substitution of acyclic poly-O-acetyl sugar monoacetals is best explained by the formation of the acyclic intermediate (X = S or O) in the rate-determining step.
非对映异构体 1,2,3,4,5-五-O-乙酰基 S-乙基单
硫缩醛的反转和 14C-乙酰氧基交换的速率常数表明,C2-乙酰氧基不支持该取代。无环聚-O-乙酰糖单
缩醛的取代最好通过在限速步骤中形成无环中间体(X = S 或 O)来解释。