Total syntheses of the angularly fused triquinane sesquiterpenoids (±)-methyl cantabrenonate (10) and (±)-methyl epoxycantabronate (11) are described. The bicyclic enone, (RS)-5-methylbicyclo-[3.3.0]oct-1 (8)-en-2-one (18), was readily obtained from 3-methyl-2-cyclopenten-1-one (20). The highly stereoselective conversion of 18 into the tricyclic oxo alkene, (1R*,5R*,8R*)-5-methyl-9-methylenetricyclo [6.3.0.01,5]undecan-2-one (19), was achieved via a one-pot process involving the reaction of 18 with the novel lower order bifunctional cuprate reagent lithium (4-chloro-1-buten-2-yl)-cyanocuprate (15). The key intermediate 19 was transformed into the tricyclic enone (1R*,5R*,8S*, 9R*)-2,5,9-trimethyltricyclo-[6.3.0.01,5]undec-2-en-4-one (44), by means of a straightforward sequence of reactions. Hydrogenation of 44 gave a single ketone 45, which was converted, in two synthetic operations, into (±)-10. Treatment of 10 with hydrogen peroxide/sodium hydroxide provided, stereoselectively, (±)-11.
角接融合的
三环倍半萜三喹烷(±)-甲基坎塔布雷诺酸酯(10)和(±)-甲基环氧坎塔布罗酸酯(11)的全合成方法被描述。双环烯酮,(RS)-5-甲基双环-[3.3.0]辛-1(8)-烯-2-酮(18),易于从
3-甲基-2-环戊烯-1-酮(20)获得。高度立体选择性地将18转化为
三环氧代烯烃,(1R*,5R*,8R*)-5-甲基-9-亚甲基
三环[6.3.0.01,5]十一烷-2-酮(19),是通过一步过程实现的,该过程涉及18与新型低阶双官能团
铜锂酸盐试剂
锂(
4-氯-1-丁烯-2-基)
氰基
铜锂酸盐(15)的反应。关键中间体19通过一系列直接的反应序列转化为
三环烯酮(1R*,5R*,8S*,9R*)-2,5,9-三甲基
三环[6.3.0.01,5]十一烷-2-烯-4-酮(44)。将44氢化得到单一的酮45,该酮在两次合成操作中转化为(±)-10。将10与
过氧化氢/
氢氧化钠处理,立体选择性地得到(±)-11。