2 光学活性テルペン類の不斉合成 : α-cyclocitralの不斉合成を中心として

抄録

In vivo, acyclic terpenes having no asymmetric center are generally converted to cyclic terpenes carrying several asymmetric carbons by the enzymic cyclization reaction. In order to achive the same kinds of transformations as those obeserved in vivo by chemical means, we, at first, developed a new acid-catalyzed biogenetic-type cyclization of the citral pyrrolidine enamine (VII)a, which was found to give α-cyclocitral (V) in at most 41% yield with concomitant formation of the trace amount of β-cyclocitral (VI). When this novel reaction was applied to optically active enamines (VII)b〜f prepared from citral and several kinds of optically active pyrrolidines (X)b〜f, easily prepared from commercially available L-proline, optically active α-cyclocitral was obtained in at most 33.3% optical yield. Studies on the reaction mechanism for this novel cyclization reaction clearly demonstrated that the asymmetric induction had occurred at the stage of the cyclization of the imminium salt (VIII) which was, in situ, prepared from the enamine (VII). Optically active α-cyclocitral thus prepared was successfully used for the synthesis of the famous perfume, trans-α-damascone (XIV). Other possible method for the preparation of the optically active α-cyclocitral and approaches for the synthesis of optically active abscisic acid (XVIII) were also discussed.