Claycop solid support

Aromatization of Hantzsch dihydropyridines to pyridines can be effected by the solid-supported reagent "Claycop" (cupric nitrate on montmorillonite). The reaction is strongly accelerated by sonication, but no change in the selectivity occurs. The method is successful only when an aryl group substitutes the 4-position.202 No mechanism is proposed, although the authors mention that nitric acid or nitronium or nitrosonium ions could be involved (Fig. 36). 

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