Synthesis of enamides from aldehydes and amides

A range of double unsaturated amides (15, 19, and 21), 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 42, 45, and 47. 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 (30_cis), lansiumamide-I (31) and lansiumamide-B (32).