formation of the new stereocenter on the hydrogenation of allylic alcohol 4. The diastereoselectivity (syn/anti ratio) of the process was 86:14, attained with Raney nickel. Hydrogen migration seems not to be the sole factor lowering the diastereoselectivity, as nickel is known not to promote double-bond migration. Instead, the moderate stereocontrol is attributed to the mobility of the side chain of 4, which
从
D-丝氨酸开始合成Fmoc保护的反式-4-甲基脯
氨酸。用烯化的加纳醛3形式的
丝氨酸手性支架控制
烯丙醇4加氢时新立体中心的非对映选择性形成。该过程的非对映选择性(顺/反比)为86:14,达到阮内
镍。氢迁移似乎不是降低非对映选择性的唯一因素,因为已知
镍不会促进双键迁移。取而代之的是,适度的立体控制归因于4侧链的迁移率,它允许氢攻击烯烃的两个表面(开放过渡态)。一系列转化产生环前体8,其在重结晶后得到dr = 95:5的顺式非对映异构体。