Pechman condensation

The syntheses of coumarins by organolithiation reaction are of interest because they provide compounds with methoxyl substitution pattern not available by the usual acid catalyzed methods such as Pechman condensation. 

Pechman condensation essentially comprise of the synthesis of coumarins by the interaction of phenols with (3-keto esters particularly in the presence of Lewis acid catalysts as given below ... 

Pechman condensation may also be exemplified by the S3mthesis of a 4-substituted coumarin, wherein a dihydroxy phenol (e.g., resorcinol) may be condensed with ethyl acetoacetate under the influence of sulphuric acid to give rise to the formation of 7-hydroxy-4-methyl coumarin. 

Add this solution (2) very slowly into the sulphuric acid (1) in small lots at intervals taking care that the temperature of the reaction mixture should not rise above 10°C by any means. Continue further stirring for a duration of 30-40 of minutes with a view to complete the Pechman condensation reaction. 

Rahmatpour, A. Mohammadian, S. An environmentally friendly, chemoselective, and efficient protocol for the preparation of coumarin derivatives by Pechman condensation reaction using new and reusable heterogeneous Lewis acid catalyst polystyrene-supported GaCl3. C. R. Chim. 2013,16, 271-278. 

De la Hoz et al. have studied the microwave-assisted synthesis of substituted coumarins by the condensation of phenol, 1,3-dihydroxybenzene/l,3,5-trihydroxy-benzene and propynoic/propenoic acids55. The examples illustrate that the application of heterogeneous supported catalysts can eliminate the production of acidic waste streams, associated with conventional Lewis acid catalysts. This synthetic approach constitutes an interesting alternative to the Pechman reaction (Scheme 3.34). 

It has subsequently been used in a wide diversity of reactions including Pechman reaction in water [60], esterification [61], Prins reaction [62], Biginelli condensation [63], protection of carbonyl compounds [64], or quinoxaline derivatives synthesis [65] (Fig. 23). 

The von Pechman-Duisberg condensation, illustrated for pheuol iu equatiou 11, was applied to the S-lactam phenolic antibiotics amoxicillin (151), cefadroxil (13) and cefop-erazone (acid form of 14) to yield the corresponding coumarin derivatives. The determination was spectrofluorometric, with X x at 401 to 467 mu aud Xg at 465 to 503 nm. The method is of advautage as compared to established procedures . 

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