Photoinduced [4 + 4], [4 + 2], and [2 + 2] cycloadditions of o-quinones with oxazoles: chemo-, regio-, and diastereoselectivity.

Photoinduced reactions of the 1,2-dicarbonyl compounds phenanthrenequinone (PQ), 1-acetylisatin (IS), and benzil (BZ) with the oxazoles 1a-j have been investigated. In photoreactions of PQ with the oxazoles, in addition to the 1,4-dioxins derived from [4 + 2] cycloaddition and the oxetanes from the Paternó-Büchi [2 + 2] reactions, [4 + 4] cycloaddition products are formed in the reactions with 1a, 1c, 1g, 1i, and 1j, with the quinone's dicarbonyl unit (O═C-C═O) and the oxazole's C═N-C═C moiety as two 4π addends. Photoreactions of IS with the oxazoles 1f and 1g give the [4 + 4] cycloaddition products exclusively, while in photoreactions of IS with 1a, 1c, 1e, 1h, and 1i, [4 + 4] products are formed together with the [2 + 2] products. Reaction pathway partitioning in these photocycloaddtions strongly depends on the substitution pattern on the oxazole ring. The presence of a substituent at the oxazole's C2 atom hampers the [4 + 4] pathway by causing steric hindrance to radical pair recombination in the corresponding 1,7-diradical intermediate to form the [4 + 4] cycloaddition products. A substituent at the C4 atom results in steric hindrance for ring closure of the 1,4-diradicals in the [2 + 2] cycloaddition pathway, therefore favoring the [4 + 4] and [4 + 2] cycloaddition pathways. Regio- and diastereoselectivity in the [2 + 2] and [4 + 4] cycloadditions have been discussed based on the thermodynamic stability of the relevant triplet diradical intermediates and the conformations of these diradicals suitable for the intersystem crossing process. Photoreactions of BZ with the oxazoles afford only [2 + 2] cycloaddition products.