Pechman reaction

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). 

The synthesis of 5,7-dimethoxy coumarin 39 started from phloroglucinol through coumarin construction by Pechman reaction followed by methylation via two steps. The resulting 8-tigloylated coumarin 40 (Scheme 8-9) was obtained by treatment of the coumarin (39) with tigloyl chloride according to the preparation method described in Scheme 8-8. 

One of the mildest conditions for the von Pechman reaction was developed by D.S. Bose and co-workers who used indium(lll) chloride as the catalyst. A large number of 4-substituted coumarins were prepared in high yield by this method. Under the reaction conditions most functional groups are tolerated. In the typical procedure the substrates are heated in the presence of 10 mol% of InCIs and the reaction mixture was poured onto crushed ice which caused the product to precipitate. 

Photochemotherapy is an efficient way to treat hyperproliferative diseases. Especially the so-called PUVA therapy (psoralen + UVA light) is very common in which the psoralen is irradiated with UVA light to give rise to a covalent adduct with the pyrimidine bases of DNA by means of a photoaddition reaction. There are several undesired side effects for the patients as a result of this therapy, so the synthesis and photobiological evaluation of novel benzosporalen derivatives was undertaken by the research team of L.D. Via. The key step in their synthetic sequence was the von Pechman reaction of 2-methoxyresorcinol with ethyl 2-oxocyclohexanecarboxylate. 

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). 

Phenol alkylation, with alkenes Coumarin synthesis (Pechman reaction)... 

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. 

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. 

---