α,β-Unsaturated carbonyls are essential structural motifs for nucleophiles of disease-related proteins. Methods for stereoselective functionalizations at the α-position include the Morita-Baylis-Hillman, Negishi, Sonogashira, Stille, and Rauhut-Currier reactions. Described here is a method for the diastereodivergent α-homoallylation of cyclic enones via a sequence of conjugate addition, aldol condensation, and diastereoselective [3,3]-sigmatropic rearrangement. Mechanistic investigations revealed that the [3,3]-sigmatropic rearrangement proceeds with transfer of chirality. These inspire a photocatalyzed olefin isomerization of the aldol condensation product leading to a highly diastereoselective [3,3]-sigmatropic rearrangement to furnish the α-homoallylation of cyclic enones. Importantly, this photocatalyzed olefin isomerization/diastereoselective [3,3]-sigmatropic rearrangement reaction sequence permits a full stereocontrol of the exo-β-position featuring an allyl group as a synthetic functional handle.