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Preparation of Coumarins: the Pechmann Condensation

In 1883 Hans von Pechmann and Carl Duisberg {H. v Pechmann, and C. Duisberg, Ber., 1883, 16, 2119} found that phenols condense with beta-ketonic esters in the presence of sulphuric acid, giving coumarin derivatives.

Pechman condensation for coumarin synthesis

With R1=OH we have 4-hydroxycoumarin, the starting material for the preparation of Warfarin

The reaction is also catalysed by the presence of a Lewis acid such aluminium(III) chloride or other strong Brönstedt acids such as methanesulphonic acid to form a coumarin. The acid catalyses trans-esterification as well as keto-enol tautomerisation.

Bismuth(III) chloride, also a Pechmann catalyst, provides a recent procedure for 4-substituted coumarins.{ An Efficient and Practical Procedure for the Synthesis of 4-Substituted Coumarins Surya K. De*, Richard A. Gibbs, Synthesis, 2005, 1231.}

In another Pechmann condensation synthesis, the ionic liquid 1-butyl-3-methylimidazolium chloroaluminate ([bmim]Cl.2AlCl3) plays the dual role of solvent and Lewis acid catalyst for the reaction of phenols with ethyl acetoacetate leading to coumarin derivatives. Here, the reaction time is reduced drastically even at ambient conditions. {M. K. Potdar, S. S. Mohile, M. M. Salunkhe, Tetrahedron Lett., 2001, 42, 9285}

Solid acid catalysts with the H+ attached to the polymer surface such as Nafion 417 or Amberlyst IR120 can be used. Thus resorcinol reacts with ethyl acetoacetate in boiling toluene in the presence of Nafion sheet to form the coumarin 7-hydroxy-4-methylcoumarin. This preparation forms the basis of a student organic chemistry experiment at Penn State University. In this case the coumarin, {also named, 7-hydroxy-4-methyl-2H-benzo[b]-pyran-2-one} is not a blood thinner but is a drug used in bile therapy, Hymecromone. The material is also, in highly purified form a laser dye, and the starting material for some insecticides!

The Preparation of Warfarin

warfarin synthesis Reaction of 4-hydroxycoumarin with benzylacetone under Michael reaction conditions gives racaemic warfarin.

assymetric synthesis via MacMillan catalyst
Imidazolidinone compounds - MacMillan organocatalysts - enable a stereoselective preparation for this reaction

There has been a recent flurry of interest in such assymetric preparation, well cataloged by Wikipedia, references 17 to 22. The last reference even puts the stereoselective preparation into the second year undergraduate chemistry laboratory as an innovative 'green chemistry' experiment:

The enantioselective synthesis of drugs is of fundamental importance in the pharmaceutical industry. In this experiment, students synthesize either enantiomer of warfarin, a widely used anticoagulant, in a single step from inexpensive starting materials. Stereoselectivity is induced by a commercial organocatalyst, (R,R)- or (S,S)-1,2-diphenylethylenediamine. The environmentally friendly microscale reaction is performed at ambient temperature, and the product can be purified by recrystallization or column chromatography. Product characterization includes thin-layer chromatography, NMR spectroscopy, and polarimetry. {T.C.Wong, C.M.Sultana and D.A.Vosburg, Department of Chemistry, Harvey Mudd College, Claremont, California 91711, J. Chem. Educ., 2010, 87(2), 194}