Regiospecific Allylation of Benzoyl-Substituted Heterocyclic Ketene Aminals and Their Zinc Chloride-Promoted 3-Aza-Cope Rearrangement

Article abstract of DOI:10.1021/jo00114a033

The regiospecific allylation of benzoyl-substituted heterocyclic ketene aminals 2 is reported.Compound 2 reacted with allyl bromide in refluxing acetonitrile to give, exclusively, C-allylated products 5 in moderate to good yields, while only N-allylated compounds 7 were obtained in dimethylformamide with the use of sodium hydride.Under neutral conditions, reaction between 2 and allyl bromide proceeded via a six-membered transition state involving the secondary enamine segment (HNC=C) and carbon-bromine bond (C-Br).Without the secondary amino group, C-allylation was inhibited, which was illustrated by the fact that 1,3-dimethyl-2-(aroylmethylene)imidazolidines did not react with allyl bromide.No 3-aza-Cope rearrangement of N-allylated heterocyclic ketene aminals 7 was observed at temperatures up to 140 deg C.Only with the use of zinc chloride did the rearrangement take place at 140 deg C to give allyl-shifted products 5.N-Benzylated heterocyclic ketene aminal analogues did not undergo the rearrangement under the same conditions, indicating that the 3-aza-Cope rearrangement proceeds through a charge-accelerated concerted mechanism.