非对映选择性自由基偶联是通过手性助剂实现的。这些自由基是通过五种丙二酸衍生物的阳极脱羧反应生成的。它们是由丙二酸苄酯和四种薄荷醇助剂制备的。在未分开的电池中,在铂电极上与甲醇中的3,3-二甲基丁酸进行共电解,可得到杂耦合产物,产率为22-69%,非对映选择性为5至65%de。没有助酸的电解质以21-50%的产率产生非对映异构体均偶联产物,非对映异构体的比率为1.17:2.00:0.81至7.03:2.00。X射线结构分析和13 C NMR数据证实了新的立体异构中心的立体化学。
Diastereoselective Alkylation of 8-Phenylmenthyl Phenylacetate: Aggregated Lithium Enolate versus "Naked" Enolate
摘要:
It is shown that monoalkylation of 8-phenylmenthyl phenylacetate using lithiated bases leads to poor or no diastereoselectivities (50/50 to 69/31) and high yields (75 to 98%) while alkylation using tBu-P4 (a strong and cation free base, known to provide ''naked'' anion) leads to high diastereoselectivities (92/8 to 98/2) and high yields (65 to 95%). It is postulated that, in the case of phenylacetates, the degree of aggregation of the lithium enolate is responsible of the poor diastereoselectivities.