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

Abstract

Salicylaldehyde was converted into the O-propynyl- and O-butynyl α,β-unsaturated aldehydes 4 and ketones 6, subsequent reaction of which with N,N-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(α,β-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.