Synthesis of (−)-Sedinine by Allene Cyclization and Iminium Ion Chemistry

2010-09-03T00:00:00Z (GMT) by Roderick W. Bates Yongna Lu
A synthesis of the <i>sedum</i> alkaloid sedinine has been achieved employing silver(I)-catalyzed allenic hydroxylamine cyclization and ring-closing metathesis to form a bicyclic <i>N</i>,<i>O</i>-acetal. Ring opening of this acetal with a silyl enol ether under Lewis acidic conditions is exclusively <i>trans</i> selective, leading to the natural product after reduction. On the other hand, conversion of the bicyclic <i>N</i>,<i>O</i>-acetal to a semicyclic <i>N</i>,<i>O</i>-acetal results in no stereoselectivity during such a reaction. The contrasting results can be rationalized by consideration of the conformation of the iminium ions.