The chromone derivatives 1, 2, and 4give with Iv-phenylmaleimide the cycloadducts 5, 12, and 13 which are converted by pelladised charcoal into P-azaxanthone 6, xanthones 14 and 15, respectively. Treatmsnt of the azaxanthone 5 with acidic ethanol produces a mixture of salicyloylpyridines 7-9. h,kDimethylhydrazones of d,&unsaturated aldehydes undergo [4+2]cycloaddition with a number of representative dienophiles and the resultant cycloadducts on aromatisation uith elimination of dimethylamine provide substituted pyridines2. Several attempts to synthesise pyridine fused with a heteroaromatic ring by similar cyclisation involving hetaryl aldehyde-hr,hcdimethylhydrazones as the azadienes have, however, been abortive3. Recently, the indolo[2,1-clpyridine system has been constructed from the hr,k-dimethylhydrazone of indole-3-carboxaldehyde and Iv-methylmaleimide4. So far as the [llbenzopyran system is concerned, 4-oxo-4f-l-1-benzopyran-3-carboxaldehyde (3-formylchromone) has its pyran 2,3-and aldehydic double bonds in the cisoid configuration5 and its heterodiene activity in the DieJs-Alder reaction is welI demonstrated6. [4+2]Cycloaddition of the Schiff base of 3-formylchromone with highly reactive dienophiles like ketenes is also successfu17. Low azadiene activity of the Schiff base corresponding to 3-formylchromone is due to electron withdrawing effect of the pyrone carbonyl group.
We contended that in the hydrazones 1 and 2, the strongly electron donating dimethylamino group was likely to compensate for, if not overcome, the electron withdrawing effect of the carbonyl group so that these two hydrazones 1 and 2 might 7 CONPhCO 6 C02Et CONHPh 9 Hc CONHPh 12 R3= NMe2 14 X=H 13 R3m C6H4Me-p 15 x = NH CsH4Me-p I-15 a b c d R' H Me Cl Br