Oligomeric procyanidins containing 4&agr;-linked epicatechin units are rare in nature and have hitherto not been accessible through stereoselective synthesis. Provided herein is the preparation of the prototypical dimer, epicatechin-4&agr;,8-epicatechin, by reaction of the protected 4-ketones with aryllithium reagents derived by halogen/metal exchange from the aryl bromides. Removal of the 4-hydroxyl group from the resulting tertiary benzylic alcohols is effected by tri-n-butyltin hydride and trifluoroacetic acid in a completely stereoselective manner, resulting in hydride delivery exclusively from the &bgr; face.
含有 4&agr;-连接
表儿茶素单元的低聚
原花青素在自然界中非常罕见,迄今为止还无法通过立体选择性合成获得。本文提供了一种制备原型二聚体
表儿茶素-4&agr;,8-
表儿茶素的方法,该方法是将受保护的 4-酮与通过卤素/
金属交换从芳基
溴化物中得到的芳基
锂试剂进行反应。氢化三正
丁基锡和
三氟乙酸以完全立体选择性的方式从生成的叔
苄醇中去除
4-羟基,结果
氢化物完全从&bgr;面传递。