Synthesis of 2,3-dihydro-1,3-thiazin-4(1H)- ones and their remarkably facile recyclization to 2,3-dihydropyrimidin-4(1H)-ones

Functionalized 2,3-dihydro-1,3-thiazin-4(1H)-one derivatives have been synthesized by cyclocondensation of 3-alkyl(aryl)amino-2-cyano-3-mercaptoacrylamides with aldehydes and ketones under acidic catalysis. 6-Alkyl(aryl)amino-5-cyano-2,3-dihy- dro-1,3-thiazin-4(1H)-ones, when treated with a dilute solution of potassium hydroxide, are converted into the potassium salts of isomeric compounds, 1-alkyl- (aryl)-5-cyano-6-mercapto-2,3-dihydropyrimidin- 4(1H)-ones. Alkylation of the latter with dimethyl sulfate in situ furnishes 1-alkyl(aryl)-6-alkylthio-5- cyano-2,3-dihydropyrimidin-4(1H)-ones, whereas boiling them in ethanol with an excess of hydrochloric acid leads to starting 2,3-dihydro-1,3-thiazin-4(1H)-ones. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:426–436, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20129