The amides prepared from 5,6-dihydro-2-methyl-1,4-oxathiin-3-carbonyl chloride 2 and imidamides 4, carbamimidothioates 4, 2-benzimidazolamine 7, 2-benzothiazolamine 12, and 2-thiazolamine have been studied. The reaction of 2 with imidamides 4 and with carbamimidothioates 4 in the presence of base gave the amides 5, which when heated in toluene underwent the 1,4-oxathiin ring opening and rearrangement to produce 2-substituted-5-[(2-hydroxyethyl)thio]-6-methyl-4(1H)-pyrimidinones 6. Acylation of 2-benzimidazolamine 7 with 2 yielded the amide 10. This, in boiling ethanol, ring opened and rearranged to give the compound 11. The reaction of 2-benzothiazolamine 12 with 2 gave the rearrangement product 14 directly and neither of the possible amides 13 and 15 could be detected. The amide 15 was synthesized by an unambiguous route from 2-aminobenzenethiol and 5,6-dihydro-2-methyl-1,4-oxathiin-3-carbonyl isothiocyanate 3. It would not undergo ring opening and rearrangement when heated. Acylation of 2-thiazolamine with 2 yielded a mixture from which the amide 19 and the rearrangement ester 18 were separated. Structural assignments are based mainly on nmr, uv, and ir spectral evidence.