Synthesis of Indolizidines and Pyrrolizidines through the [2 + 2]Cycloaddition of Five-Membered Endocyclic Enecarbamates to Alkyl Ketenes. Unusual Regioselectivity of Baeyer−Villiger Ring Expansions of Alkyl Aza-Bicyclic Cyclobutanones
作者:Antonio R. de Faria、Elias L. Salvador、Carlos Roque D. Correia
DOI:10.1021/jo016189u
日期:2002.5.1
regioisomeric lactones in a ratio of approximately 1.5 to 1. It is hypothesized that the steric strain built into the Criegee cyclobutane intermediate is the regioselective controlling factor in these oxidations, overriding any stereoelectronic bias for ring expansion. A rationale for the mechanism of the [2 + 2]cycloaddition involving enecarbamates and ketenes is presented, which seems to involve the participation
从一个普通的五元环内烯甲氨基甲酸酯开始,以简明的方式完成了两个吲哚并立定骨架和烟碱(+/-)-platynecine的总合成。这些合成的特征是五元内环烯氨基甲酸酯5的[2 + 2]环加成到烷基烯酮上,后者以高收率和高立体选择性进行,以提供内烷基环加合物作为主要产物或唯一产物。可以在某些[2 + 2]环加成物中观察到的次要的exo烷基环加合物,似乎是由内环加合物通过差向异构化而推定的动力学产物而得。对于7-烷基-2-氮杂双环环丁酮的Baeyer-Villiger氧化观察到异常的区域选择性。Endo-7-烷基环加合物仅扩环成一个γ-内酯,其中氧优先于桥头C5-C6键插入C6-C7键中。有了exo-7-烷基环加合物,Baeyer-Villiger氧化反应的区域选择性大大降低,导致区域异构内酯的混合物比例约为1.5:1。这些氧化中的区域选择性控制因素,覆盖了环扩展的任何立体电子偏压。提出了一个关于[2