An Unexpected Ring Enlargemant of 3-(Dimethylamino)-2,2-dimethyl-2H-azirine to 4,5-Dihydropyridin-2(3H)-one Derivatives

Article abstract of DOI:10.1002/hlca.19950780720

The reaction of 3-(dimethylamino)-2,2-dimethyl-2H-azirine (1a) and 4,4-disubstituted 2-(trifluoromethyl)-1,3-oxazol-5(4H)-ones 7 in MeCN at 70 deg afforded 5-(dimethylamino)-3,6-dihydropyrazin-2(1H)-ones 10 (Scheme 4), whereas no reaction could be observed between 1a and 2-allyl-4-phenyl-2-(trifluoromethyl)-1,3-oxazol-5(2H)-one (8a) or 4,4-dibenzyl-2-phenyl-1,3-oxazol-5(4H)-one (9).The formation of 10 is rationalized by a mechanism via nucleophilic attack of 1a onto 7.The failure of a reaction with 9 shows that only activated 1,3-oxazol-5(4H)-ones bearing electron-withdrawing substituents do react as electrophiles with 1a.The amino-azirine 1a and 2,4-disubstituted 1,3-oxazol-5(4H)-ones 2b-e in refluxing MeCN undergo a novel ring enlargement to 4,5-dihydropyridin-2(3H)-ones 11 (Scheme 5).Several side products were observed in these reactions.Two different reaction mechanisms for the formation of 11 are proposed: either 1a undergoes a nucleophilic addition onto the open-chain ketene tautomer of 2 (Scheme 6), or 2 reacts as CH-acidic compound (Scheme 7).