Unexpected regioselectivity in the photocyclization of a chiral 2,3-bisbenzylidenesuccinate, leading to a podophyllotoxin related cyclolignan

The Stobbe condensation between 3′,4′,5′-trimethoxyacetophenone and a benzylidenesuccinate derived from piperonal gave the Z,E-2,3-bisbenzylidenesuccinate as the only product. Such regioselectivity was explained by considering the Newman’s projections of the transition states of this reaction. Introducing L-prolinol as a chiral auxiliary, macrolactonization and subsequent photocyclization of the atropoisomeric cyclic amide ester led to new cyclolignan analogue. By comparing the regioselectivity observed in this study with our previous results, some general rules regarding the photocyclization of chiral 2,3-bisbenzylidenesuccinates could be established.