All four stereoisomers of (2S, 3R)-(+)-piscidic acid were synthesized with high optical purity via Sharpless catalytic asymmetric dihydroxylation of (Z)- and (E)-trisubstituted olefins in 6 steps from (4-hydroxyphenyl)pyruvic acid. The Wittig reaction of methyl (4-hydroxyphenyl)pyruvate with (carbomethoxymethylene)triphenylphosphorane gave (Z)- and (E)-trisubstituted olefins in a 3:1 ratio. After protecting the phenolic hydroxyl group as the tert-butyldimethylsilyl ether, the (Z)-olefin was subjected to asymmetric dihydroxylation by using the chiral ligand, dihydroquinidine 1,4-anthraquinonediyl diether, and the reaction proceeded with 89% e.e. Desilylation and subsequent alkaline hydrolysis gave (2S, 3R)-(+)-piscidic acid. The optical purity was increased to >99% e.e. by recrystallization. The use of dihydroquinine 1,4-anthraquinonediyl diether enable (2R, 3S)-(-)-piscidic acid to be obtained. In the asymmetric dihydroxylation of the (E)-olefin, phthalazine ligands (dihydroquinidine and dihydroquinine 1,4-phthalazinediyl diethers) gave high e.e. values. Via the same deprotection procedure, (2S, 3S)-(+)-3-epi-piscidic acid and (2R, 3R)-(-)-2-epi-piscidic acid were respectively obtained.
通过 Sharpless 催化,以 (4-hydroxyphenyl)pyruvic acid 为原料,分 6 步对 (Z)- 和 (E)- 三取代烯烃进行不对称二羟基化反应,合成了 (2S,3R)-(+)-piscidic acid 的所有四种立体异构体,且光学纯度很高。(
4-羟基苯基)
丙酮酸甲酯与(碳氧亚甲基)
三苯基膦的维蒂希反应以 3:1 的比例生成(Z)-和(E)-三取代烯烃。用叔丁基二甲基
硅醚保护
酚羟基后,(Z)-烯烃通过使用手性
配体二氢
喹啉-1,4-
蒽醌基
二醚进行不对称二羟基化反应,反应进行率为 89%。通过重结晶,光学纯度提高到大于 99%。使用二氢
喹啉-1,4-
蒽醌二
乙醚可以得到 (2R,3S)-(-)-piscidic acid。在(E)-烯烃的不对称二羟基化过程中,
酞嗪配体(二氢
喹啉和二氢
喹啉-1,4-
酞嗪二
乙醚)具有很高的等效值。通过相同的脱保护程序,分别得到了 (2S, 3S)-(+)-3-epi-piscidic acid 和 (2R, 3R)-(-)-2-epi-piscidic acid。