Abstract: A method for producing an optically active amino acid or derivative thereof having a high optical purity from an optically active amino acid comprising optical isomers or derivative thereof, which comprises any one of processes (A), (B), and (C), wherein the process (A) comprises the steps: (1) previously preparing an optically active amino acid or derivative thereof having an optical purity higher than a convergent value of a mutual solubility of the optical isomers and (2) crystallizing the optically active amino acid or the derivative thereof that exists in excess, said convergent value being a ratio of the desired optical isomer in the optical isomers dissolved in a mother liquor in which crystals of a racemate and an optically active compound coexist at equilibrium (the optical purity in a mother liquor). The processes (B) and (C) are described in the specification.

Abstract: Novel arylthiadiazole derivatives and salts thereof useful for preventing and treating human viral infection and a novel virucide which contains the arylthiadiazole derivative or a salt thereof are provided. N,N-dimethyl ?3-(3-(amino-2,6-dichlorophenyl)-1,2,5-thiadiazol-4-yl! carbamate or its salt, and virucide containing the same as an effective component.

Abstract: Disclosed is (a) 1,2,5-thiadiazole derivatives represented by the following formulae [I] or [III]: ##STR1## wherein X and Y are the same or different and each represents a hydrogen atom, a halogen atom, an alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 4 carbon atoms, a nitro group, a trifluoromethyl group, a carboxyl group, an alkoxycarbonyl group having from 2 to 5 carbon atoms, a naphthyl group or a tetrahydronaphthyl group; and A represents an alkyl group having from 1 to 6 carbon atoms, --C(.dbd.O)OR.sup.1, --C(.dbd.O)NR.sup.2 R.sup.3, a phenyl group which may be substituted by at least one of an alkyl group having from 1 to 6 carbon atoms and an alkoxy group having from 1 to 4 carbon atoms, --CHR.sup.4 C(.dbd.O)R.sup.5 or --C(.dbd.O)R.sup.6, in which R.sup.1 represents an alkyl group having from 1 to 6 carbon atoms, R.sup.2 and R.sup.3 are the same or different and each represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms, R.sup.