Asymmetric #alpha#-substitution versus aza Diels-Alder reaction of electron deficient N-sulfonyl imines

摘要

Several N-arylsulfonylglycine esters have been brominated under photolytic conditions to provide the corresponding #alpha#-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 Diels-Alder adducts in varying yields, depending upon the base used. In addition,if organometallic bases were employed (alkyllithiums and alkylaluminium reagents),not only were aza Diels-Alder adducts formed ,but addition to the imine was also observed. In the case of organoaluminum 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.