Maria Balogh

Cleavage of thioacetals by clay-supported metal nitrates

Abstract Bis(methylseleno)acetals and bis(phenylseleno)acetals are cleaved into aldehydes or ketones, under very mild conditions, by K10 clay-supported ferric or cupric nitrate.

Solid acid catalyzed reaction of aminals with methyl 3-aminocrotonate

Solid acid catalyzed reaction of aminals 1, with methyl 3-aminocrotonate 2 affords dimethyl 4-aryl-2,6-dimethyl-l,4-dihydropyridine-3,5-dicarboxylates 3. 2-Methyl-4//-pyrido[ 1,2-a]pyrimidin-4-one 4_ is formed from 2-aminopyridine moiety of the aminals. Recently attention has been focused on the use of solid acids in organic syntheses as environmentally acceptable alternatives to conventional Brönsted and Lewis acids. Clays and zeolites are effective catalysts for a wide variety of organic reactions (1-3). The prowess of acid-treated montmorillonite clay (K10) and ZF520 zeolite, as strong Brönsted acidic catalysts, has been shown in carbon-carbon and carbon-nitrogen bond formation processes, such as addition and cyclization reactions (4-6). Azines (4) derived from aromatic aldehydes and iV-benzylideneanilines (6) undergo clay-mediated cyclocondensation reaction with methyl 3-aminocrotonate to give symmetrically substituted 1,4-dihydropyridines, which are highly effective calcium antagonists (7). Condensation reaction between aromatic aldehydes and 2-aminopyridine leads to the formation of aminals instead of the expected Schiff bases (8-11). The easy access to aminals I prompted us to examine their reaction with methyl 3-aminocrotonate 2. The presence of an acidic catalyst is a prerequisite for the success of the reaction. Solid acids tested include K10 montmorillonite, kaolinite, ZF520 zeolite and silica gel. The acid strength of a solid surface is defined as its proton-donating ability, quantitatively expressed by Hammett Ho function (Ho = -3 to -6 for kaolinite and Ho = -6 to -8 for acid treated montmorillonite) (12). K10 montmorillonite is a mesoporous solid having pores with the majority in the 60-100 Ä diameter range (13). The Brönsted acidity in zeolites is linked to the Si/Al ratio (14). ZF520 is a highly acidic Y zeolite (Si/Al = 20). Besides microporosity of the faujasite structure (15) with diameters around 8 Ä there is a secondary porous system in the range of 50-200 Ä. Vol. 5, No. 1, 1999 Solid acid catalyzed reactin of aminals with methyl 3-aminocrotonate

Clay-catalyzed reactions of imidazole and benzimidazoles with propiolic esters

Michael addition of imidazole ( 1 ) and benzimidazole ( 5 ) to alkyl propiolate ( 2 ) can be promoted by K10 montmorillonite clay affording alkyl 3-(imidazol-l-yl)acrylate ( 3 ) and alkyl 3-(benzimidazol-1-yl)acrylates ( 6 ), respectively. Michael adduct of 2-mercaptobenz-imidazole ( 8 ) undergoes cyclization in the presence of clay catalyst to 4-oxo-4H-[1,3]thiazino[3,2-a]benzimidazole ( 9 ).