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.

Die Hydrolyse des Anilids (I) führt zu der Enolform des Ringspa1tungs‐Pro‐ dukts (II) (die mit 2,4‐Dinitrophenylhydrazon das Hydrazon der Acetylform gibt, beim Erhitzen mit Natronlauge deacetyliert und beim Wasserentzug mit Benzyl recyclisiert wird).