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 correspo nding N,O-trimethylsilylacetals could be obtained using the mixture of LDA and trimethylsilyl chloride in THF. The unexpected reaction seque nce Diels-Alder reaction/acylation between the N-butadienyl-N-alkylket ene N,O-trimethylsilylacetal of propionamide (2a-2b) and N-phenylmalei mide 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 sy nthesis of bicyclic systems of this general structure could only be su ccessfully executed in 26% yield treating the Diels-Alder product rac- 10 with LDA.