Enantioselective Aldol and Michael Additions of Achiral Enolates in the Presence of Chiral Lithium Amides and Amines
作者:Eusebio Juaristi、Albert K. Beck、Jesper Hansen、Thomas Matt、Triptikumar Mukhopadhyay、Malgorzata Simson、Dieter Seebach
DOI:10.1055/s-1993-26041
日期:——
It is now well established that lithium enolates and analogous derivatives generally exist as complex structures held together by noncovalent bonds ("supramolecules"). In particular, Li enolates aggregate to give dimers, tetramers, and higher oligomers, whose metal centers may be complexed by solvent molecules or chelating ligands. In addition, the anionoid part of the enolates may hydrogen-bond to weak acids such as secondary amines. Furthermore, such supramolecules can be product-forming species in synthetic reactions of Li enolates. This paper describes our observations of the temporary incorporation of chiral amines or chiral lithium amides into achiral lithium enolate aggregates (an interaction which is simply broken during aqueous workup!) to give enantiomerically enriched products. In particular, enantioselective aldol and Michael additions between achiral enolates and achiral aldehydes or achiral nitroolefins have been achieved in the presence of several chiral amines (or their lithium amides) derived from (S)-valine or (R,R)-tartaric acid. Finally, this report demonstrates the potential usefulness in asymmetric synthesis of ortho lithiation directed by chiral α-aminoalkoxides.
目前已经充分确定,锂烯醇盐及其类似衍生物通常以非共价键结合而成的复杂结构(“超分子”)存在。特别是,锂烯醇盐聚集形成二聚体、四聚体和高阶寡聚体,其金属中心可能被溶剂分子或螯合配体络合。此外,烯醇盐的阴离子部分可能与二级胺等弱酸形成氢键。而且,这些超分子在锂烯醇盐的合成反应中可能是产物形成物种。本文描述了我们观察到的将手性胺或手性锂酰胺暂时掺入非手性锂烯醇盐聚集体(在水中处理时这种相互作用被简单破坏!)从而产生手性富集产物的现象。特别是,在几种由(S)-缬氨酸或(R,R)-酒石酸衍生的手性胺(或它们的锂酰胺)存在下,实现了非手性烯醇盐与非手性的醛或非手性硝基烯烃的立体选择性羟醛缩合反应和迈克尔加成反应。最后,本报告展示了手性α-氨基烷氧基导向的邻位锂化在对映选择性合成中的潜在应用价值。