91(P-48) 保護基なしの全合成 : 無保護アミノ酸の反応を利用した光学活性Clavicipitic Acidの全合成(ポスター発表の部)

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The protection of reactive functional groups is essential in modern organic synthesis. The protection, however, caused serious problems; that is, increasing the reaction steps and difficulty of the deprotection of unstable compounds. We wish to present here the three-step synthesis of optically active clavicipitic acid (6) from 4-bromoindole (7) and dl-serine (dl-2). This quite short synthesis was made possible by omission of the protection and deprotection steps in the synthetic route. Racemic N-acetyl-4-bromotryptophan (dl-8) was obtained in 72% yield by heating the mixture of 7 with dl-2 in the presence of Ac_2O in AcOH. Various substituted indoles (10) also gave N-acetyltryptophan derivatives (11) in good to moderate yields. The reaction mechanism was also discussed base on the results of the reaction of 11 (X=5-OMe) with various serine derivatives (12-16) and that of NMR experiment. Optically pure (S)-9 (>99% ee) was obtained in 49% yield by kinetic resolution of dl-8 with inexpensive acylase from Aspergillus. Palladium-catalyzed vinylation (Heck reaction) of (S)-9 with 3,3-dimethylallylalcohol (20) in the presence of water soluble ligand, TPPTS in aqueous solution and subsequent treatment with 50% aqueous AcOH (50℃, 3h) gave clavicipitic acids (6) in one-pot. During this synthesis, we have found that the reactive site could be controlled by changing the reaction condition; Heck reaction occurred under strong basic condition to give C_4-vinylated product (18), while N-allylated product (19) was formed under weak basic condition. This selectivity was also observed in the reaction of bromoanilines (21) with (20). The most important observation for this selectivity was that the N-allylation did not occur in organic solvent but only proceeded under neutral condition in aqueous media.
天然有機化合物討論会の論文
2001-09-01

91(P-48) 保護基なしの全合成 : 無保護アミノ酸の反応を利用した光学活性Clavicipitic Acidの全合成(ポスター発表の部)

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