An Unexpected Tandem Reaction between <i>N</i>-Butadienyl-<i>N</i>-alkylketene<i>N,0</i>-Trimethylsilylacetals of Propionamide and Activated Dienophiles like <i>N</i>-Phenylmaleimide or Acryloyl Chloride

Starting from the <i>N</i>-butadienyl-<i>N</i>-alkylpropionamides <b>1a-1c</b> the corresponding <i>N,0</i>-trimethylsilylacetals could be obtained using the mixture ofLDA and trimethylsilyl chloride in THF. The unexpected reaction sequence <i>Diels-Alder</i> reaction/acylation between the <i>N</i>-butadienyl-<i>N</i>-alkylketene <i>N,0</i>-trimethylsilylacetal ofpropionamide (<b>2a-2b</b>) and <i>N</i>-phenylmaleimide produced tricyclic products <i>rac</i>-<b>5a</b> -<i>rac</i>-<b>5b</b> and bicyclic products <i>rac</i>-<b>6a</b> - <i>rac</i>-<b>6b</b> with high diastereoselectivity. The reaction of the <i>N,0</i>-trimethylsilylacetals <b>2a</b> and <b>2c</b> with acryloyl chloride in a similar sequence gave the bicyclic products <i>rac</i>-<b>8a</b> and mc-<b>8c</b>. The stepwise synthesis of bicyclic systems of this general structure could only be successfully executed in 26 yield treating the <i>Diels-Alder</i> product <i>rac</i>-<b>10</b> with LDA.