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Regio- and stereoselectivity in ene-carbonyl photocycloadditions depend on the spin multiplicity of the excited carbonyl state, although both singlet and triplet states produce the cycloadducts with comparable chemoselectivity. The correlation between selectivity and spin state was evaluated by concentration, temperature, and solvent viscosity studies. The higher selectivity observed for triplet reactions is rationalized by the optimal conformations of the intermediate 2-oxabutane-1,4-diyls for intersystem crossing (ISC) to the singlet manifold, controlled preferentially by spin-orbit coupling. This weak interaction connected with ISC can lead to substantial control of regio- and stereoselectivity. The role of hyperfine coupling is demonstrated by magnetic isotope effects.

The concentration dependence of the Paterno-Buchi photocycloaddition of the two cyclic enolethers 2,3-dihydrofuran and 2,3-dihydropyran, respectively, with aromatic as well as aliphatic aldehydes was studied. For aliphatic aldehydes, a sharp transition from low to high diastereostereoselectivity was observed, indicating a switch from singlet to triplet photocycloaddition with different selectivity controlling mechanisms.

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