Ringerweiterungen und Ringverengungen bei der Umsetzung von 1, 3‐Oxazolidin‐2, 4‐dionen und 1, 3‐Thiazoiidin‐2, 4‐dion mit 3‐Amino‐2 H ‐azirinen

Ring Enlargements and Ring Contractions in the Reaction of 1, 3‐Oxazolidine‐2, 4‐diones and l, 3‐Thiazolidine‐2, 4‐dione with 3‐Amino‐2 H ‐azirines The reaction of 3‐amino‐2 H ‐azirines 1 and 1, 3‐oxazolidine‐2, 4‐diones 2 in MeCN at room temperature leads to 3, 4‐dihydro‐3‐(2‐hydroxyacetyl)‐2H‐imidazol‐2‐ones 3 in good yield ( Scheme 2 , Table 1 ). A reaction mechanism proceeding via ring enlargement of the bicyclic zwitterion A to give B, followed by transannular ring contraction to C, is proposed for the formation of 3 . This mechanism is in accordance with the result of the reaction of 2a and the 15 N‐labelled 1a *: in the isolated product 3a *, only N(3) is labelled ( Scheme 1 ). The analogous reaction of 1 and 1, 3‐thiazolidine‐2, 4‐dione ( 5 ) is more complex ( Schemes 4 and 5 , Table 2 ). Besides the expected 3, 4‐dihydro‐3‐(2‐mercaptoacetyl)‐2 H ‐imidazol‐2‐ones 7, 5‐amino‐3, 4‐dihydro‐2 H ‐imidazol‐2‐ones of type 8 and/or N‐(1, 4‐thiazin‐2‐ylidene)ureas 9 are formed. In the case of 2‐(dimethylamino)‐1‐azaspiro[2. 3]hex‐1‐ene ( 1d ), the postulated eight‐membered intermediate 6d could be isolated. Its structure as well as that of 9f has been determined by X‐ray structure analysis. A reaction mechanism for the formation of the 1, 4‐thiazine derivatives of type 9 is proposed in Scheme 6 .