<i>Amino-Claisen</i> Rearrangements and <i>Diels-Alder</i> Reactions of Ketene N,O-Acetals: Reactivity Studies. On the Way to a Novel Tandem Process?

We report the synthesis of <i>N</i>-benzyl-<i>N</i>-[(<i>E</i>)-buta-1,3-dienyl]propanamide (<b>6</b>) and its corresponding O-silyl-substituted ketene N,O-acetal <b>7</b> and their <i>Diels-Alder</i> reaction. Propanamide <b>6</b> reacted smoothly, whereas the yield obtained from <b>7</b> was low, probably due to polymerization of the dienophile induced by electron transfer. The ketene N,O-acetals <b>27a - g</b> were synthesized starting from the corresponding benzamides <b>25a - e</b> (<i>Scheme 9</i>). The ketene N,O-acetals <b>27a - g</b> showed increased stabilities and underwent amino-<i>Claisen</i> rearrangements under thermal conditions. Using catalysts, interesting side reactions leading either to the annulated systems <i>rac</i>-<b>35 - 37</b> or to a β-lactam <i>rac</i>-<b>34</b> were observed.