SYNTHESIS OF ARISTOTELIA-TYPE ALKALOIDS .7. SYNTHESES OF (+1-)-SORELLINE, (+1-)-SERRATENONE, AND (+1-)-ARISTOTELIN-19-ONE

The imine obtained by condensing indole-protected 2-(indol-3-yl)acetaldehyde (5) with the terpinylamine derivative (+/-)-4 was cyclized in 51% yield to the 19-substituted hobartine derivative (+/-)-20 upon exposure to anhydrous HCOOH. This pivotal intermediate was further elaborated into the indole alkaloids (+/-)-serratenone ((+/-)-22) and (+/-)-soreline ((+/-)-29). In the course of these investigations, a novel rearrangement was uncovered: a Lewis acid-catalyzed 1,3-migration of an arylsulfonyl group from the indole N-atom into the benzene ring. The discovery that synthetic (+/-)-aristotelin-19-one ((+/-)34) has decidedly different spectroscopic properties than aristolasicone, a metabolite for which the structure has been recently proposed, led to a revision of the structure of the latter.