Chiral Synthesis of Erythrina Alkaloids. (2). Synthesis of enantio-Type Erythrinan Alkaloids Utilizing Asymmetric Acylation and Kinetic Resolution of Diastereomers.
作者:Yoshisuke TSUDA、Shinzo HOSOI、Kazuya ISHIDA、Masako SANGAI
DOI:10.1248/cpb.42.204
日期:——
Oxalylation of the enamino-ester 13 derived from the L-dopa derivative 12 gave the dioxopyrroline of (6S)-configuration (15A) diastereoselectively (50-60% diastereomer excess). This was converted to a mixture of erythrinans of (5S, 6R, 7R, 10S) and (5R, 6S, 7S, 10S) configuration by cyclization with BF3·Et2O. The de of the major diastereomer (A) was elevated to 82% by application of a kinetic resolution of diastereomers (partial hydrolysis of the ethylene acetal group), where the minor diastereomer (B) was hydrolyzed more rapidly. The acetal 17A which remained unchanged was converted, in several steps, to the enantio-type erythrinan alkaloid, (-)-3-demethoxyerythratidinone (-)-7, and also to the 1, 7-cycloerythrinan (-)-9, a key intermediate to Erythrina alkaloids. The more easily hydrolyzable diastereomer (B) was similarly converted to the enantiomer (+)-9. The mechanism of partial racemization, sometimes observed in the product, is discussed.
由 L-多巴衍生物 12 衍生出的烯酰胺酯 13 通过草酰化反应,非对映选择性地得到了 (6S) 构型的二氧吡咯啉 (15A)(非对映异构体过量率为 50-60%)。通过与 BF3-Et2O 环化,将其转化为(5S, 6R, 7R, 10S)和(5R, 6S, 7S, 10S)构型的赤藓红混合物。通过非对映异构体的动力学解析(乙烯缩醛基团的部分水解),主要非对映异构体(A)的de 提高到 82%,而次要非对映异构体(B)的水解速度更快。保持不变的乙缩醛 17A 经过几个步骤转化为对映体型红花楠生物碱 (-)-3-demethoxyerythratidinone (-)-7,以及红花楠生物碱的关键中间体 1,7-环红花楠 (-)-9。更容易水解的非对映异构体(B)也同样转化为对映体(+)-9。本文讨论了有时在产品中观察到的部分消旋化机制。