Synthesis of enamides from aldehydes and amides

A range of double unsaturated amides (, , and ), obtained by cross-coupling reactions was reacted with aldehydes to hemiaminals. Heating the hemiaminals in the presence of Ac2O and pyridine affected clean conversion to the corresponding enamides, such as , , and . Alternatively, N,S-acetals were prepared which were oxidized to the sulfones. Treatment with base also gave the enamides, favoring the cis-isomer. However, this method is less general. Application of these methods led to the natural products lansiumamide-A (), lansiumamide-I () and lansiumamide-B ().