An Unexpected Tandem Reaction between N-Butadienyl-N-Alkylketene N,O-Trimethylsilylacetals of Propionamide and Activated Dienophiles like N-Phenylmaleimide or Acryloyl Chloride

Starting from the N-butadienyl-N-alkylpropionamides 1a-1c the corresponding N,O-trimethylsilylacetals could be obtained using the mixture of LDA and trimethylsilyl chloride in THF. The unexpected reaction sequence Diels-Alder reaction/acylation between the N-butadienyl-N-alkylketene N,O-trimethylsilylacetal of propionamide (2a-2b) and N-phenylmaleimide produced tricyclic products rac-5a -rac-5b and bicyclic products rac-6a - rac-6b with high diastereoselectivity. The reaction of the N,O-trimethylsilylacetals 2a and 2c with acryloyl chloride in a similar sequence gave the bicyclic products rac-8a and rac-8c. The stepwise synthesis of bicyclic systems of this general structure could only be successfully executed in 26% yield treating the Diels-Alder product rac-10 with LDA.