Pericyclic Transformations at the Periphery of Chromen‐4‐one (=4 H ‐1‐Benzopyran‐4‐one): An Unusual Preference for a 1,5‐Shift of Allylic Moieties over the Ene Reaction

Quite unlike the reported facile ene reactions on the periphery of many related heterocyclic systems, similarly disposed moieties on the periphery of the chromen‐4‐one (=4 H ‐1‐benzopyran‐4‐one) system fail to undergo an ene reaction and display a rather unusual preference for an overall [1,5] shift of the allylic C‐atom. Thus, heating xylene solutions of 2‐( N ‐allylanilino)‐, 2‐( N ‐crotylanilino)‐, and 2‐( N ‐cinnamylamino)‐substituted ( E )‐(oxochromenyl)propenoates 9a – c and 2‐[allyl(benzyl)amino]‐, 2‐[benzyl(crotyl)amino]‐, and 2‐[benzyl(cinnamyl)amino]‐substituted ( E )‐(oxochromenyl)propenoates 16a – c in a sealed tube at 220–230° leads to a [1,5] shift of the allylic moieties (allyl, crotyl, cinnamyl), which is followed by intramolecular cyclization involving the N‐atom and the ester function, to give the 3‐allyl‐3‐crotyl‐, and 3‐cinnamyl‐substituted‐1‐phenyl‐ or 1‐benzyl‐2 H ‐[1]benzopyrano[2,3‐ b ]pyridine‐2,5(1 H )‐diones 10a – c and 17a – c . The anticipated carbonyl–ene reaction in the 2‐( N ‐allylanilino)‐, 2‐( N ‐crotylanilino)‐, 2‐( N ‐cinnamylanilino)‐, 2‐[allyl(benzyl)amino]‐, 2‐[benzyl(crotyl)amino]‐, and 2‐[benzyl(cinnamyl)amino]‐substituted 4‐oxochromene‐3‐carboxaldehydes 8a – c and 15a – c is also not observed, and these molecules remain untransformed under identical conditions. No [1,5] shifts of benzyl, phenyl, or methyl groups are observed, even in the absence of allylic moieties, though facile [1,5]‐H shift occurs in 2‐(benzylamino)‐ and 2‐(phenylamino)‐substituted ( E )‐(oxochromenyl)propenoates 23a , b , which is followed by a similar intramolecular cyclization leading to the 2 H ‐[1]benzopyrano[2,3‐ b ]pyridine‐2,5(1 H )‐diones 24a , b .