Abstract
An Unexpected Ring Enlargement of 3-(Dimethylamino)-2,2-dimethyl-2H-azirine to 4,5-Dihydropyridin-2(3H)-one Derivatives The reaction of 3-(dimethylamino)-2,2-dimethyl-2H-azirine (la) and 4,4-disubstituted 2-(trifluoromethyl)-1,3-oxazol-5(4H)-ones 7 in MeCN at 70° 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-pheny1-1,3-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).
Hugener, Martin