Ring Enlargements and Ring Contractions in the Reaction of 1,3-Oxazolidine-2,4-diones and 1,3-Thiazolidine-2,4-dione with 3-Amino-2H-azirines

Article abstract of DOI:10.1002/hlca.19900730520

The reaction of 3-amino-2H-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 ₁₅N-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)-2H-imidazol-2-ones 7, 5-amino-3,4-dihydro-2H-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.