Dehydrotryptophans (9) having various substituents on the indole ring were synthesized by vinylation of the corresponding indole derivatives (6) with N-acetyldehydroalanine methyl ester (7a) by the use of a stoichiometric amount of palladium salt in acetic acid (stoichiometric condition). Although vinylation of 4-, 5-, and 7-bromoindoles (6b, g, l, m) with 7a occurred chemoselectively at the C3-position to give the corresponding bromodehydrotryptophans (9b, g, l, m) under the stoichiometric condition, the palladium-catalyzed vinylation of 4-bromoindoles (6b, i) occurred at the bromine-bearing carbon at the C4-position [0.1 eq PdCl2(PPh3)2 in Et3N-dimethylformamide]. Vinylation of 3-bromoindoles (17a, b) with 7a, or ethoxycarbonyl-(7b) or tert-butoxycarbonyldehydroalanine methyl ester (7c) also gave the dehydrotryptophan (18a-d) in good yields under catalytic conditions. One of two possible regioisomers of the vinylated products was isolated in all cases, and the geometry was determined as Z based on the coupling constant of the ester carbonyl carbon and the olefinic proton in the 13C-NMR spectrum.
通过将相应的
吲哚衍
生物(6)与N-乙酰脱氢丙
氨酸甲酯(7a)在
乙酸中使用
化学计量的
钯盐(
化学计量条件)进行
乙烯基化,合成了在
吲哚环上带有各种取代基的脱氢色
氨酸(9)。尽管在
化学计量条件下,4-、5-和
7-溴吲哚(6b、g、l、m)与7a在C3位置上发生
化学选择性
乙烯基化,生成相应的
溴脱氢色
氨酸(9b、g、l、m),但
钯催化的
4-溴吲哚(6b、i)的
乙烯基化发生在C4位置上带
溴的碳上[0.1 eq PdCl2(PPh3)2 在Et3N-二甲基甲酰胺中]。
3-溴吲哚(17a、b)与7a或乙氧羰基(7b)或叔丁氧羰基脱氢丙
氨酸甲酯(7c)的
乙烯基化在催化条件下也以良好产率生成脱氢色
氨酸(18a-d)。在所有情况下,分离出两种可能的区域异构体中的一种,根据13C-NMR谱中酯羰基碳和烯烃质子的耦合常数,几何构型确定为Z。