Acid hydrolysis of 5,6-dihydro-2-methyl-I,4-oxathiin-3-carboxanilide gives 2-(2-hydroxyethy1thio)-acetoacetanilide enol. 3-Carbonyl-substituted 5.6-dihydro-l,4-oxathiins were found to undergo ring cleavage by nucleophilic nitrogen attack on C-2. Thus the following reactions were observed: 3-acetyl-5,6-dihydro-2-methyl-1,4-oxathiin o n treatment with hydrazine gives 4-(2-hydroxyethylthi0)-3,5-dimethylpyrazole, instead of the hydrazone. The 1,4-oxathiins, N-(5,6-dihydro-2-methyl-1,4-oxathiin-3-ylcarbony1)-N'-phenylurea and 5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxylic acid hydrazide rearrange to give 5-(2-hydroxyethy1thio)-6-methyl-I-phenylracil and 4-(2-hydroxyethy1thio)-3-methyl-2-pyrazolin-5-one, respectively. Finally, treatment of 5,6-dihydro-2-methyl-1,4-oxathiin-3-carbonyl chloride with 2-aminopyridine and with 2-aminopyrimidine affords 4H-3-(2-hydroxyethy1thio)-2-methylpyrido[l,2-nlpyrimidin-4-one and 4H-3-(2-hydroxyethylthio)-2-methylpyrimido[l,2-a]pyrimidin-4-one, respectively, a s the predominant products.