Synthesis of Erythrina And Related Alkaloids: XXX: Photochemical Approach: (1): Synthesis of Key Intermediates To Erythrina Alkaloids By Intermolecular [2+2] Photocycloaddition Followed By 1,3-shift

Abstract

A novel synthetic route to Erythrina alkaloids consisting in [2 + 2] intermolecular photocycloaddition of the isoquinolinodioxopyrroline 4 to 2-trimethylsilyloxybutadienes and the subsequent ring enlargement reaction of the trimethylsilyloxyvinylcyclobutane 5 by thermal or tetrabutylammonium fluoride (TBAF)-induced 1,3-shift was developed. The TBAF method was particularly useful, offering a good yield under mild reaction conditions. Thus, photoannulation of 4 with l-methoxy-3-trimethylsilyloxybutadiene, followed by hydride reduction, TBAF-induced 1,3-shift, and hydrogenation gave the 7oc-hydroxy-2,8-dioxoerythrinan 19 in overall 45% yield, and this product was converted, in 70% yield, into the 1,7-cycloerythrinan 24, a key intermediate for the total synthesis of Erythrina alkaloids. © 1992, The Pharmaceutical Society of Japan. All rights reserved.