A new strategy for the synthesis of pyridines from N-propargylic beta-enaminothiones

A new method for the synthesis of 2,4,5-trisubstituted pyridines from N-propargylic beta-enaminothiones is reported. beta-Enaminothiones were prepared by thionation of the corresponding beta-enaminones with Lawesson's reagent. When treated with diisopropylamine in DMF at room temperature, N-propargylic beta-enaminothiones yielded 2,4,5-trisubstituted pyridines in moderate to high yields, along with small amounts of 2-methylene-2,3-dihydro-1,4-thiazepines. The reaction was found to be general for a broad range of N-propargylic beta-enaminothiones and tolerated the presence of aromatic, heteroaromatic and aliphatic groups with electron-withdrawing and electron-donating substituents. The method could be widened to the internal alkyne-tethered N-propargylic beta-enaminothiones. This operationally simple method may provide rapid access to a library of functionalized pyridines of pharmacological interest.