Engineering Acyclic Stereocontrol in the Alkylation of Vinylglycine-Derived Dianions: Asymmetric Synthesis of Higher α-Vinyl Amino Acids
作者:David B. Berkowitz、Jill M. McFadden、Marianne K. Sloss
DOI:10.1021/jo9918091
日期:2000.5.1
auxiliary may be recovered in high yield (generally 90%) using a modification of Gassman's "anhydrous hydroxide" conditions, in which considerably higher temperatures are employed. Among the side chains introduced directly are those of butyrine, leucine, ornithine, phenylalanine, aspartate, valine, and norvaline. The lysine side chain is elaborated via a 4-step sequence from the alkylation product obtained
提出了高级 L-α-乙烯基氨基酸的通用合成。这里采用的策略涉及通过手性、乙烯基甘氨酸衍生的双阴离子二烯醇盐的烷基化引入氨基酸侧链,该二烯醇盐带有(-)-8-(β-萘基)薄荷基(d'Angelo)辅助剂。提出了一个模型,假设该二烯醇盐具有有利的“外向”构象,从而导致 si 面上的 C(α) 烷基化。该模型调用内部酰胺化物螯合来控制酯烯醇化物的几何形状以及助剂的可极化β-萘基环和二烯醇化物的扩展π系统之间的软-软相互作用以屏蔽再面。计算了该二价阴离子的四种构象异构体的半经验优化几何形状 (PM3) 的形成热。结果支持这样的观点,即在这些乙烯基甘氨酸衍生的双阴离子二烯醇化物中,“外型”构象的能量比它们的“内型”对应物低得多,其中“外型”构象是最有利的。事实上,与使用异丁基碘和苄基的 (-)-8-苯基薄荷基 (Corey) 和反式-2-(β-萘基)环己基助剂相比,d'Angelo 助剂在该体系中提