8 (+)-ヒルスチン酸の高選択的全合成

抄録

A simple, highly stereocontrolled synthesis of (+)-Hirsutic acid 1, a representative of a novel tricyclic sesquiterpene class, is described. In the first part of this presentation, the asymmetric syntheses of (1S, 5R, 6S)-6-hydroxybicyclo[3,3,0]octan-3-one, (+)-2, a key intermediate for the present synthesis, are reported. One route consists of the conversion of the known hydroxy-ester, (+)-27, to optically pure (+)-2 by employing a series of efficient reactions. The other route, which is capable of providing (+)-2 in large quantities and high enantiomeric purity, involves the reaction of dl-3 with (+)-di-3-pynanylborane as a key step. Using this reaction, followed by oxidation and hydrogenolysis, (+)-2 with high optical purity is readily obtained as a major product. The undesired ketone, (-)-5, is also convertible to (+)-2. In the second, the highly stereocontrolled transformation of (+)-2 with 80% optical purity to (+)-1, which includes a methylene transfer reaction with very high stereochemical control(>98%) by the participation of a rather remote hydroxy group as a key step (7→10) is presented. As a result, optically pure (+)-1, [α]^<25>_D+114°(CHCl_3), m.p. 174°〜176℃, could be obtained by recrystallization from ether for two times. In the third, an alternative route to (+)-1 from (+)-2, which involves the chelation-controlled methylation of an ester enolate from the sterically more crowded face (14→16) as a key step, is discussed.