21 海洋産アルカロイド・アマサスピラミドFの全合成(口頭発表の部)

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Amathaspiramides A-F are a class of marine alkaloids isolated from a New Zealand collection of the bryozoan Amathia wilsoni. Since these alkaloids were available in a minute quantities from the bryozoan, only preliminary pharmacological evaluation of these alkaloids has been reported, i.e., amathaspiramide E exhibited a potent antiviral activity and both A and E showed moderate antifungal and cytotoxic activities. The structures of amathaspiramides are characterized by their unique spirobicyclic framework containing three contiguous chiral centers and a dibrominated aromatic ring. We have been reported that the enolate-Claisen rearrangement of an α-acyloxy-α-alkenylsilane gave the rearranged product with a complete transfer of the original chirality. Based on these results, we planned the total synthesis of amathaspiramide F (1) using the enolate-Claisen rearrangement of the α-acyloxy-α-alkenylsilane with a Z-olefin geometry 4 (route A) or 5 (route B) as the key step: the reaction would produce 8 or 9 possessing the requisite chiral centers at CS and C9 (erythro relationship) together with a vinylsilane group convertible to the hemiaminal moiety of 1. The enolate-Claisen rearrangement of 4 (route A) having a proline as the acyloxy group was found to give undesired threo isomer 8 as the major product. On the other hand, the acyclic route (route B) underwent stereoselective rearrangement to give desired erythro isomer 9 (threo/erythro = 1: 7). The rearranged product 9 was converted to the spirobicyclic hemiaminal 12 via the pyrrolidine 10. The completion of the total synthesis of 1 was accomplished by the following sequence of reactions: (1) dibromination of the phenol group with n-Bu_4NBrCl_2 (2) methylation of the phenol with MeI, and (3) removal of the TFA group with LiBH_4.
天然有機化合物討論会の論文
2007-08-24

21 海洋産アルカロイド・アマサスピラミドFの全合成(口頭発表の部)

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