Direct and Metal-Catalyzed Photochemical Dimerization of the Phthalide (Z)-Ligustilide Leading to Both [2+2] and [4+2] Cycloadducts: Application to Total Syntheses of Tokinolides A-C and Riligustilide

Synthetically derived (Z)-ligustilide (1) has been subjected to photochemically-promoted dimerization processes under a range of conditions. By such means, varying distributions of the dimeric natural products tokinolides A-C (4, 3, and 6, respectively) and riligustilide (5) as well certain related (isomeric) compounds have been obtained. The structures of three of them have been confirmed by single-crystal X-ray analysis. The biosynthetic implications of the outcomes of this study are discussed.