2270 zyxwvutsrq HELVETICA CHIMICA ACTA zyxwv - Vol.67 (1984) 267. Aromatization of 1,CDihydropyridines by Clay-Supported Metal Nitrates’) by Maria Balogha), Istvan Hermecza), Zoltan Meszaros”), and Pierre Laszlob)* ”) zyxwvutsrq CHINOIN Pharmaceutical and Chemical Works Ltd., Research Centre, P.O. Box 110, H-1325 Budapest b, Institut de Chimie Organique et de Biochimie, Universitk de Li6ge, Sart-Tilman, B-4000 Li6ge (12.X.84) zyxwvu Summary 1,4-Dihydropyridines can be aromatized under very mild conditions by K zy 10 clay- supported ferric and cupric nitrates. 1. Introduction. - Diethyl zyxwv 4-substituted-2,6-dimethyl-3,5-pyridinedicarboxylates z 2 are useful as antihypoxics and antiischemics [2] [3], and some representatives have acaricidal, insecticidal, bactericidal and herbicidal activity [4]. On the other hand these compounds are starting materials for the synthesis of antibacterial 1,6-naphthyridines [5] and 1,2-benzisoazalenes [6]. Several methods are reported for aromatization of dihydropyridines 1, zy e.g. using HNO, at 60°C [6] [7], S at 200°C [6] [7], NaNO, in acidic media [8] [9], chromic anhydride in AcOH [6], chloranil in benzene at reflux temperature [lo], K,Cr,O, in H,SO, [lo], 5 YO Pd/C catalyst in a mixture of toluene and AcOH at reflux temperature [l I], and KMnO, in AcOH [I 11. Studying the utilization of clay-supported Fe(NO,), (‘clayfen’), this reagent was proven to be efficient for oxidation of alcohols to aldehydes or ketones [ 12-14], nitra- tion of phenols [14] and estrone [15]. Thioacetals can be cleaved into aldehydes and ketones by ‘clayfen’ and clay-supported Cu(NO,), (‘claycop’) [ 161. A study on the mechanism of the oxidation process has revealed that nitrosonium cation (NO+) releasing reagents are efficient oxidants for 1 ,4-dihydropyridines [9]. ‘Clayfen’ and ‘claycop’ can serve as a source of nitrosonium ion [13-161. ’) ‘Clay-supported Reagent’; previous publication in this series: [I].