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Crystal structure, 1H and 13C nmr spectroscopy and pKa measurements of 1,2,6-thiadiazin-3,5(2H,6H)-dione derivatives related with phenylbutazone are reported. An anomalous variation in the pKa values of compounds 1,2, and 3 has been observed. Antiinflammatory, analgesic, and antipyretic activities of these compounds have been evaluated.

The title compounds (Ia)‐(Ic), (II) and (III) are obtained by nitration of the corresponding thiadiazine 1,1‐dioxides (no yields given for (Ib), (IIb) and (IIIc)).

A comparative study of hydrophobicity in biological active pyrazolone derivatives 1 and 3 and related 1,2,64hiadiazinone 1,l -dioxides 2 and 4 using HPLC (RPC) technique has been carried out. The capacity factors (K) and retention index (I) of the compounds studied have been measured in five different elution conditions (mobile phase in which the percentage of methanol varies). A correlation between hydrophobic parameters of series 1 and 2 has been established.

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