Pechmann reaction

What is Pechmann reaction?

This reaction was first reported by Pechmann and Duisberg in 1883. It is an acid-promoted synthesis of coumarin derivatives from the condensation of phenols and -ketoesters or -keto carboxylic acids and is generally known as the Pechmann reaction or Pechmann condensation. Occasionally, this reaction is also referred to as the Pechmann coumarin synthesis, Pechmann cyclization, Pechmann coumarin
condensation, Pechmann cyclocondensation, or the Pechmann synthesis.

Pechmann reaction - general reaction scheme - Pechmann condensation - Pechmann coumarin synthesis - Pechmann cyclization - Pechmann coumarincondensation - Pechmann cyclocondensation - Pechmann synthesis
Pechmann reaction

Pechmann reaction, under acidic conditions, involves an esterification or transesterification, followed by a cyclization and dehydration. It has been found that the outcome of this reaction depends on the nature of phenols, -keto esters, and condensation reagents.

For example, only phenols with electron-donating groups at the meta-position of the hydroxy group readily undergo the Pechman condensation, whereas phenols with electron-withdrawing groups at the meta-positions or with electron-donating groups at other positions fail for this reaction. On the other hand, the α-substituents and γ-substituents on β-keto esters retard this reaction.

Although Pechmann reaction is often carried out with concentrated H2SO4 or 70 % H2SO4, other acids, including Lewis acids, are also effective in promoting this reaction, such as POCl3, P2O5, HF, sulfamic acid, TFA, triflic acid, polyphosphoric acid, KHSO4, dipyridine copper chloride, ZnCl2, AgOTf, AlCl3, molecular iodine, VCl3, BF3·2H2O, BF3, BiCl3, ZrCl4, SiCl4, and Yb(OTf)3. However, the application of the common condensation reagent of P2O5 often results in the formation of chromone derivatives.

References

Von Pechmann, H. and Duisberg, C. (1883), Ueber die Verbindungen der Phenole mit Acetessigäther. [On the compounds of phenols with acetoacetic ethers] Ber. Dtsch. Chem. Ges., 16: 2119-2128. https://doi.org/10.1002/cber.188301602117

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