Double intramolecular hetero Diels-Alder reactions of α,β-unsaturated hydrazones as 1-azadienes:: a new route to 2,2′-bipyridines

Salicylaldehyde was converted into the O-propynyl- and O-butynyl alpha,beta -unsaturated aldehydes 4 and ketones 6, subsequent reaction of which with NN-dimethylhydrazine and alkyne homocoupling gave the 1,3-diyne bis(hydrazones) 8, substrates for a double intramolecular hetero Diels-Alder reaction. Similar substrates 11, 15a/15c and 15b/15d were prepared from 2-(N-benzoylamino)cinnamaldehyde, hex-5-ynol and hept-6-ynol respectively. Heating the 1,3-diynyl bis(alpha,beta -unsaturated hydrazones) 8a, 8c, 11 and 15a resulted in double intramolecular Diels-Alder reaction to give, after aromatisation by loss of dimethylamine, 2,2'-bipyridines 21-24.