[Total synthesis of crassostreaxanthin B directed toward the biomimetic synthesis of carotenoids].

Marine carotenoids halocynthiaxanthin 2, mytiloxanthin 3 and crassostreaxanthin B 4 have characteristic structures, commonly possessing a monoacetylenic end group. The cyclopentyl end group of 3 is believed to be formed in nature from the epoxide end group of 5,6-epoxy carotenoids such as 2 (Chart 1, route a). It is also conceivable that 4 including a novel tetrasubstituted olefinic end group arises from epoxy carotenoids by the opening of the C-6-oxygen bond of the oxirane ring and the subsequent migration of the methyl group at the C-1 position (route b). We found that treatment of the epoxide 6a having a partial structure of epoxy carotenoids with Lewis acids gave the cyclopentyl ethyl ketone 9 possessing the same configuration as 3, and the acyclic tetrasubstituted olefinic methyl ketone 11 including a partial structure of 4 (Chart 2). It supported the proposed metabolic pathway of 5,6-epoxy carotenoids. Toward the biomimetic synthesis of 4, we examined the reaction of epoxides having several substituents at C-6 position with Lewis acids. Among these epoxides, an epoxide 60 was found to provide a tetrasubstituted compound 61 as a major product. This could be converted into an aldehyde 73 in 8 steps which was transformed into a compound 75 through the coupling reaction with vinyllithium 63. Then, the first total synthesis of 4 was accomplished via the double Wittig condensation of 75 with phosphonium salts 76 and 77.