Formal intramolecular [5 + 2] photocycloaddition reactions of maleimides: a novel approach to the CDE ring skeleton of (-)-cephalotaxine.

A concise approach to the cephalotaxine CDE ring skeleton based on the intramolecular formal [5 + 2] photocycloaddition of cyclopentenyl-substituted maleimides is described. An investigation of the diastereoselectivity afforded by various protected alkoxy groups demonstrated that the best selectivity (3.5:1) was afforded by the free hydroxyl group, strongly suggesting a hydrogen-bonded excited state. Reaction: see text.