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

Abstract

Synthetically derived (<i>Z</i>)-ligustilide (<b>1</b>) 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 (<b>4</b>, <b>3</b>, and <b>6</b>, respectively) and riligustilide (<b>5</b>) 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.