General Method for Synthesis of Erythrinan and Homoerythrinan Alkaloids (1) : Synthesis of a Cycloerythrinan, as a Key Intermediate to Erythrina Alkaloids, by Pummerer-Type Reaction

Abstract

A new synthetic route to Erythrina alkaloids by formation of ring C utilizing a Pummerer-type intramolecular cyclization was developed. The N-(2-phenylthioethyl)-dioxopyrroline (3) was converted to 13 in five steps with satisfactory overall yield (69%) by means of [2+2] photocycloaddition followed by 1, 3-anionic rearrangement as crucial reactions. Treatment of the sulfoxide (13) with trifluoroacetic anhydride gave the cyclization product, 11-phenylthioerythrinan (15), in 87% yield. This was converted to the cycloerythrinan (22) by reductive elimination of the PhS group, thus constituting a formal total synthesis of (±)-erysotrine.