Synthesis of a novel tetrahydroisoquinoline pentacyclic framework

N-Acyliminium cyclization of 6-substituted (3R*,6R*,11aS*)-3-arylmethyl-pyrazino[1,2-b]isoquinoline-1,4-diones gave with very good yields a novel tetrahydroisoquinoline pentacyclic core framework (29), while this reaction failed in all-cis-isomers to give instead conjugated enamines by deprotonation. Electronic and steric factors that govern the approach to both diastereomers from 6-substituted pyrazino[1,2-b]isoquinoline-1,4-diones have been studied. (C) 2009 Elsevier Ltd. All rights reserved.