Asymmetric α-substitution versus aza Diels–Alder reaction of electron deficient N-sulfonyl imines
作者:Paul E. Morgan、Ray McCague、Andrew Whiting
DOI:10.1039/a909116e
日期:——
Several N-arylsulfonylglycine esters have been brominated under photolytic conditions to provide the corresponding α-bromoglycine. These bromo esters can be treated with a range of bases to generate N-sulfonyl imino esters in situ; attempts to isolate the imines in a pure state were universally unsuccessful. Once generated, the imines can be trapped with cyclopentadiene to provide the corresponding aza DielsâAlder adducts in varying yields, depending upon the base used. In addition, if organometallic bases were employed (alkyllithiums and alkylaluminium reagents), not only were aza DielsâAlder adducts formed, but addition to the imine was also observed. In the case of organoaluminium reagents, imine addition was the major product. This process could be transformed into a stoichiometric asymmetric version, by generating a chiral aluminium reagent in situ to form a trialkyl (or trialkoxy) aluminium reagent, which when reacted with an N-sulfonyl bromoglycinate resulted in 19 to 62% enantiomeric excess of the corresponding substituted glycinate product.
在光解条件下,几种 N-芳磺酰甘氨酸酯被溴化,生成相应的 δ-溴甘氨酸。这些溴化酯可以用一系列碱处理,在原位生成 N-磺酰亚胺酯;试图分离出纯净状态的亚胺的尝试普遍不成功。亚胺一旦生成,就可以与环戊二烯反应,生成相应的氮杂 DielsâAlder 加合物,根据所用碱的不同,产率也不同。此外,如果使用有机金属碱(烷基锂和烷基铝试剂),不仅可以生成氮杂 DielsâAlder 加合物,还可以观察到亚胺的加成反应。在有机铝试剂中,亚胺加成是主要产物。通过在原位生成手性铝试剂,形成三烷基(或三烷氧基)铝试剂,该试剂与 N-磺酰基溴甘氨酸反应后,相应的取代甘氨酸产物的对映体过量率为 19% 至 62%,从而将这一过程转化为化学计量不对称版本。