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■ New Synthetic Approach Leading to 1- or 1,3-Disubstituted 2-Thiouracil-5-carboxylates via Dimroth Rearrangement of Isomeric Thiazines

Veronika Hladíková, Richard Kammel, and Jiří Hanusek*

*Institute of Organic Chemistry and Technology, University of Pardubice, Studentská 573, Pardubice, 532 10, Czech Republic

Abstract

Methyl 1-substituted and 1,3-disubstituted-2-thiouracil-5-carboxylates (3b-d) were prepared using new synthetic approach involving base-catalyzed cyclization of easily available S-[3-methoxy-2-(methoxycarbonyl)-3-oxoprop-1-en-1-yl]isothiuronium salts (1a-d) to methyl 2-substituted imino-4-oxo-3,4-dihydro-2H-1,3-thiazine-5-carboxylates (2a-d) and their nucleophile-assisted Dimroth rearrangement to methyl 1-substituted and 1,3-disubstituted-2-thiouracil-5-carboxylates (3b-d). Cyclization step is very sensitive towards the base used – e.g. 1a react with stronger bases or bases in excess to give bis-[3-methoxy-2-(methoxycarbonyl)-3-oxoprop-1-en-1-yl]sulfide (4) as the only isolated product.