Generation of a Thiazole o-Quinodimethane from an Imino Derivative and Its Intermolecular Diels-Alder Trapping with Alkynes or Quinoes.

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(E)-5-(N-Benzylformimidoyl)-4-methylthiazole 3 was prepared in good yield from 5-formyl-4-methylthiazole and benzylamine. Treatment of 3 with methylchloroformate in the presence of ethyldiisopropylamine in refluxing toluene has generated o-quinodimethane (o-QDM) 4. In situ trapping of the latter with acetylenic or quinonoid dienophiles directly afforded the aromatized products 5, 6, 7 and 9. An unprecedented elimination of N-car-bomethoxybenzylamine occurred from the primary dihydro or tetrahydro cycloadducts. Starting from ethyl-propynoate or juglone 8a, the single regioisomer 6 or 9a was obtained, while when using 2- or 3-bromojuglone 8b or 8c the reaction regiospecifically gave 9a or 9b. Calculations by the semi-empirical method PM3 indicated that the regiochemistry observed connot be predicted by the frontier orbital coefficients theory for a concerted electrocyclic reaction, but is better explained by an attack of the nucleophilic end of diene 4 on the more electron deficient carbon of the dienophiles.

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