Eine überraschende Ringerweiterung von 3‐(Dimethylamino)‐2,2‐dimethyl‐2 H ‐azirin zu 4,5‐Dihydropyridin‐2(3 H )‐on‐Derivaten

An Unexpected Ring Enlargement of 3‐(Dimethylamino)‐2,2‐dimethyl‐2 H ‐azirine to 4,5‐Dihydropyridin‐2(3 H )‐one Derivatives The reaction of 3‐(dimethylamino)‐2,2‐dimethyl‐2 H ‐azirine ( 1a ) and 4,4‐disubstituted 2‐(trifluoromethyl)‐1,3‐oxazol‐5(4 H )‐ones 7 in MeCN at 70° afforded 5‐(dimethylamino)‐3,6‐dihydropyrazin‐2(1 H )‐ones 10 ( Scheme 4 ), whereas no reaction could be observed between 1a and 2‐allyl‐4‐phenyl‐2‐(trifluoromethyl)‐1,3‐oxazol‐5(2 H )‐one ( 8a ) or 4,4‐dibenzyl‐2‐phenyl‐1,3‐oxazol‐5(4 H )‐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(4 H )‐ones bearing electron‐withdrawing substituents do react as electrophiles with 1a . The amino‐azirine 1a and 2,4‐disubstituted 1,3‐oxazol‐5(4 H )‐ones 2b – e in refluxing MeCN undergo a novel ring enlargement to 4,5‐dihydropyridin‐2(3 H )‐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 ).