Stereochemical consequences of the lewis acid-promoted 3-aza-cope rearrangement of N-alkyl- N-allyl enamines

Abstract

Internal and relative asymmetric induction were examined for the electrophile promoted 3-aza-Cope rearrangement of substituted N-alkyl- N-allyl enamines. In general, internal asymmetric induction was highly variable, and was dependent both upon the nature of the electrophilic reagent and substrate. However, substitution at C-4 of the substrate served to anchor the transition state of the substrates, and product selectivity was typically > 95:5. When the N-alkyl substituent was tethered to C-4, ring expansion from a five- to a nine-membered ring was obtained. The key features necessary for the stereoselective [3,3] rearrangement of N-alkyl- N-allyl enamies were determined through these studies. In the absence of a substituent at C-4, internal asymmetric induction that resulted from chair:boat transition state selectivity was highly variable, and was dependent both on the nature of the electrophilic reagent and substrate. In general, poor selectivity was obtained in these studies, and placement of a chiral peripheral substituent on the nitrogen did not improve selectivity through relative asymmetric induction. Substitution at C-4 was critical to the stereoselective rearrangement of these substrates, and incorporation of a methyl group at C-4 served to anchor the transition state of the acyclic substrates in the chair conformation. In these examples, chair:boat selectivity was typically > 95:5, and resulted in selective product formation. Rearrangement of a cyclic substrate, tethered at the C-4 and nitrogen positions, also generated products with a high degree of control over transition state geometry. Based on these studies, the scope and limitations of the charge accelerated [3,3] rearrangement of N-alkyl- N-allyl enamies has been outlined for future use.