3-4′-Bipiperidines via Sequential [4 + 4]−[3,3]−Retro-Mannich Reactions

An approach to assembling unsymmetrically coupled piperidines is described, involving initial [4 + 4] photocycloaddition of 2-pyridones, followed by Cope rearrangement and retro-Mannich reaction. In these reactions, four stereogenic centers set during cycloaddition are relayed or erased during the subsequent steps. Two methods for retro-Mannich reaction are demonstrated.