The action of o-aminothiophenol hydrochloride on the copper salt of the enolic form of methyl pentafluorobenzoylpyruvate leads to the formation of 3-pentafluorobenzoylmethylene-2,3-dihydro-4H-1,4benzothiazin-2-one (1) [1]. The reaction of 3-p-toluoyl-1,2-dihydro-4H-pyrrolo[5,1-c][1,4]benzoxazine-1,2,4trione with o-aminothiophenol gives the o-hydroxyphenylamide of 2,4-dioxo-3-(3-oxo-2,3-dihydro-4H-1,4benzothiazin-2-ylidene)-4-p-tolylbutanoic acid (2) [2]. Prior to our investigations [3, 4], no other acylmethylene derivatives of 1,4-benzothiazinones had been reported. We also note that the report on the preparation of 2-aroylmethylene-2,3-dihydro-4H-1,4-benzothiazin-3-ones (3) in the reaction of aroylpyruvic acids or 5-aryl2,3-dihydro-2,3-furandiones with o-aminothiophenol [5] is unreliable. According to our results, this reaction leads to the formation of acyclic enaminoacids, amides, cyclic O,S-hemiacetals, and 1,4-benzothiazine-2,3-dione [3, 6]. We have obtained 3-phenacylidene-2,3-dihydro-4H-1,4-benzothiazin-2-one (4) upon brief heating of a mixture of 1,6-diphenyl-3,4-dihydroxy-2,4-hexadiene-1,6-dione (5) with o-aminothiophenol in acetic acid at reflux. An equilibrium of two tautomers was found for 4 in DMSO solution featuring the predominant enamino form A with an NH-chelate ring and –N–H···O=C< intramolecular hydrogen bond and 26% imino form B. The action of o-aminothiophenol on the ethyl ester of 2-carbamoyl-3,4-dihydroxy-6-oxo-6-phenyl-2,4hexadienoic acid (6) gave 2-phenacylidene-2,3-dihydro-4H-1,4-benzothiazin-3-one (3a), which is regioisomeric to 4. The yield of 3a was 64%.