Atwal modification

The Atwal modification of the Hantzsch synthesis consists of a condensation reaction between a thiouronium salt, an aldehyde and a P-oxoacid derivative to form dihydropyrimidine derivatives as shown in Scheme 1.8.5.9. 

L. D. Robinett, Solid Phase Synthesis of Dehydropyrimidines by the Atwal Modification of the Biginelli Reaction, conference 211th ACS National Meeting, New Orleans, 1996. 

In addition to modification of the catalyst, several variants of the Biginelli reaction have emerged as viable alternatives however, each method requires pre-formation of intermediates that are normally formed in the one-pot Biginelli reaction. First, Atwal and coworkers reported the reaction between aldol adducts 39 with urea 40a or thiourea 40b in the presence of sodium bicarbonate in dimethylformamide at 70°C to give 1,4-dihydropyrimidines 41. DHPM 42 was then produced by deprotection of 41. 

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