A highly selective, radical-mediated (4 + 2) coupling reaction of aldehydes and conjugated olefins has been achieved through asymmetric SOMO-catalysis. A radical-polar crossover mechanism is proposed wherein olefin addition to a transient enamine radical cation and oxidation of the resulting radical furnishes a cation which is vulnerable to nucleophilic addition. A range of aromatic aldehydes are shown to couple with styrenes and dienes to provide cyclic products with high chemical efficiency, regioselectivity, and stereoselectivity.