Enantiomerically Enriched .alpha.-Methyl Amino Acids. Use of an Acyclic, Chiral Alanine-Derived Dianion with a High Diastereofacial Bias
作者:David B. Berkowitz、Marianne K. Smith
DOI:10.1021/jo00110a029
日期:1995.3
diast. ratio, 72% yield) constitutes a formal synthesis of the clinically important antihypertensive (S)-α-methyl-DOPA (Aldomet), in enantiomerically enriched from. In all cases studied, yields are markedly improved, yet diastereoselectivities unchanged, by the addition of 10% HMPA to the reaction milieu. The (-)-8-phenylmenthol chiral auxiliary is conveniently recovered via ester cleavage with KO2/18-crown-6
已合成了衍生自N-苯甲酰基丙氨酸和以下手性醇的受阻酯:(1)(-)-异樟脑醇;(2)(-)-反式-2-苯基环己醇和(3)(-)-8-苯基薄荷醇。在-78°C下,用LDA(1.2当量)和正丁基锂(2.4当量)在THF中依次处理这些酯,生成相应的手性二价阴离子。用苄基溴将它们中的每一个烷基化表明只有(-)-8-苯基薄荷基助剂对其衍生的二价阴离子具有很高的非对面偏压。实际上,对于具有九种烷基卤化物的烷基化,该二价阴离子(6)始终显示89:11至94:6的非对映异构体比率。如果包含一个重结晶步骤,则可以得到单一的非对映异构产物,如对6的苄基化所证明的。特别要注意的是,用3的烷基化,4-双(叔丁基二甲基甲硅烷氧基)苄基溴(18)(重量比94:6,产率72%)构成了对临床上重要的降压(S)-α-甲基-DOPA(Aldomet)的形式合成,呈对映体形式富集从。在所有研究的案例中,通过向反应环境中添加10