Lythraceous alkaloids. Part VII. The absolute configurations of lythrancines-I–IV and lythrancepines-I–III

Abstract

An n.m.r. investigation of lythrancine-IV provided evidence for a cis-relationship among 1-, 3-, and 4-H, and of a cis-ring-juncture of the quinolizidine ring. The chemical conversion of lythrancepine-II (VIa) into compound (XVI), which was proved to be the antipode of (XVII) derived from lythranidine (IX), established the absolute configuration of C-5, -9, and -11. Consideration of this result together with the relative stereochemistry of C-1 -3, and -4 led to two possible stereostructures (A) and (B). Use of a stereomodel showed a preference for (A). X-Ray analysis of lythrancine-II-O-brosylate rigorously confirmed (A). The absolute stereochemistry of the seven new bases was established as (Ib)–(VIIb).