Systematic study on acylation of methyl 3-aminocrotonate with acid chlorides of aliphatic, aromatic and α, β-unsaturated acids: A comparative evaluation of the preference for regio- and stereoselectivity vis-à-vis 3-aminocrotononitrile

Abstract

Acylation of methyl 3-aminocrotonate 1a in benzene with a variety of aliphatic and aromatic acid chlorides including α, β-unsaturated acid chloride in the presence of an added organic base, (either pyridine or triethylamine) is reported. The preferred N, C-site selectivity in these reactions has been compared with the terminal selectivity of the products obtained previously on acylation of methyl 3-aminocrotononitrile 1b. A strong preference either for N- or C- selectivity in N,
C-acylation has been observed for both 1a and 1b based on the choice of acid chlorides and added organic base. Interestingly, irrespective of the enamine 1a or 1b, acylation with α, β-unsaturated acid chlorides in the presence of triethylamine afforded 3,4-dihydropyridin-(2H)-one via [3.3] sigmatropic rearrangement of the corresponding intermediary N(E)-enamide. Accrued results show methyl 3-aminocrotonate to be a better precursor for preparation of enamides
(N-acylated products) whereas 3-aminocrotononitrile is found to be a preferred choice for preparation of enaminones
(C-acylated products). An attempt is made to offer a preliminary theoretical interpretation for observed site selectivity.