190, CH-8057 Ziirich (23.V.90) ~~ Ring Enlargements and Ring Contractions in the Reaction of 1,3-0xazolidine-2,4-diones and 1,3-Thiazolidine-2,4-dione with 3-Amino-W-azirines
The reaction of 3-amino-W-azirines 1 and 1,3-oxazolidine-2,4-diones 2 in MeCN at room temperature leads to 3,4-dihydro-3-(2-hydroxyacetyl)-W-imidazol-2-ones 3 in good yield (Scheme 2, Table I ) . 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 '5N-labelled la*: in the isolated product 3a*, only N(3) is labelled (Scheme 2). 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)-W-imidazol-2-ones 7, 5-amino-3,4-dihydro-2H-imidazol-2ones of type 8 and/or N-(1,4-thiazin-2-ylidene)ureas 9 are formed. In the case of 2-(dimethylamino)-1azaspiro[2.3]hex-l-ene (Id), 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.
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