Abstract:
2-(1,3-Dithietan-2-ylidene)-2-[N-(subsittuted)carbamoyl]acetate esters and 2-(1,3-dithiolan-2-ylidene)-2-[N-(substituted) carbamoyl]acetate esters, having potent therapeutic and prophylactic effect for hepatic disorders, are provided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to new dithioketene derivatives which exhibit excellent therapeutic and prophylactic effect for hepatic diseases and pharmaceutical compositions containing these compounds at a dose suitable for daily administration. 
     2. Description Of The Prior Art 
     Dialkyl 2-(1,3-dithietan-2-ylidene)malonates and the preparation thereof are disclosed in Ger. Offen. 2,625,220, 2,316,921 and Japan 74 39,260. 
     Dialkyl 2-(1,3-dithiolan-2-ylidene)malonates are disclosed in Japan Kokai 75 100,062, Japan 75 39,666 and 74 39,260. 
     Dithiolane derivatives, having a ketone functional group in the side chain, are disclosed in Japan Kokai Tokyo Koho JP 59 27,887. 
     Dithietane derivatives containing two ketone functional groups in the side chain are disclosed in Japan Kokai Tokyo Koho JP 62 158, 274 and Eur. Pat. Appl. EP 234,480. 
     Also, reference can be made in U.S. Ser. No. 206,659, which was filed by the same applicant of the present invention. 
     SUMMARY OF THE INVENTION 
     In brief, the present invention relates to compounds having less toxicity and improved pharmacological potency of the general formula (I) ##STR1## in which R 1  is a lower alkyl; R 2  is a hydrogen, lower alkyl, benzyl, phenyl, 4-hydroxyphenyl, halogenophenyl, 4-trifluoromethylphenyl, 2-thiazolyl, 4-methyl-2-thiazolyl, 4-phenyl-2-thiazolyl 2-pyridyl or 1,3,4-thiadiazol-2-yl; and n is 1 or 2. 
     Accordingly, it is an object of the present invention to provide novel dithioketene derivatives which possess excellent therapeutic and prophylactic effects for hepatic disorders. 
     Another object of the present invention is to provide pharmaceutical compositions containing said compounds at a dose suitable for daily administration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides compounds of the general formula (I) ##STR2## in which R 1  is a straight or branched alkyl group having 1 to 5 carbon atoms; R 2  is hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms, benzyl, phenyl, 4-hydroxyphenyl, halogenophenyl, 4-trifluoromethylphenyl, 2-thiazolyl, 4-methyl-2-thiazolyl, 4-phenyl-2-thiazolyl, 2-pyridyl or 1,3,4-thiadiazol-2-yl; and n is 1 or 2. 
     Two particularly preferred classes of formula I are formed when R 1  is methyl, ethyl, isopropyl or sec-butyl and n is 1 or 2. But in one class R 2  is hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms, and in the other class R 2  is phenyl, a substituted phenyl or a heteroaromatic ring. 
     The compounds according to the invention can be administered orally. They will, in general, be associated with a pharmaceutically acceptable carrier or diluent to provide a pharmaceutical dosage form. 
     The pharmaceutical composition can most conveniently be in the form of capsules or tablets, which may be in the form of slow releasing formulations. The composition may also take the form of a dragee or syrup. 
     The convenient daily dose would be of the order of 100 mg to 1.0 g per day and the form of dosage units contains from 50 mg to 200 mg. 
     The compounds of the present invention may be prepared from the reaction of an acid anhydride of formula (II) and an amine of formula (III); ##STR3## in which R 1 , R 2  and n have the same meanings given previously and R 3  is tert-butyl, a lower alkoxy or benzyloxy. 
     The reaction may be carried out by allowing the acid anhydride (II) and the amine (III) to stand in a solvent such as methylene chloride, acetonitrile or alcohol. The reaction with a weakly basic heteroaromatic amine (e.g. 2-aminopyridine, 2-aminothiazole, or 2-amino-1,3,4-thiadiazole) can be carried out by heating the reaction mixture at 50°-100° C. in a solvent such as acetonitrile or N,N-dimethylformamide. 
     In an alternative procedure, the compounds of the present invention can be prepared from the reaction of an active ester of formula (IV) with an amine of formula (III); ##STR4## wherein R 1  and n are the same as defined previously and R 4  is 1-benzotriazolyl, N-succinimidoyl or N-phthalimidoyl. 
     The reaction can be carried out in a solvent such as acetonitrile, methylene chloride or N,N-dimethylformamide at 10°-100° C. 
     The reaction with alkyl amines or aniline derivatives can readily take place at a lower temperature (10°-30° C.), whereas the reaction with weakly basic heteroaromatic amines requires a high temperature (50°-100° C.). 
     In another process, the compounds of formula (I) can be prepared by reacting a monoacid of formula (V) with phosphorous pentachloride to yield an acid chloride of the monoacid which is very unstable. An amine of formula (III) is then added to the acid chloride of monoacid in the presence of pyridine or triethylamine, ##STR5## wherein R 1  and n are the same as defined previously. This reaction can be conducted in the presence of a solvent such as methylene chloride or acetonitrile at a temperature from 0° C. to -15° C. 
     In another alternative process, the compounds of formula (I) can be prepared by reacting the compounds of formula (VI) with the compounds of formula (VII). ##STR6## wherein R 1 , R 2  and n are the same as defined previously; and X is a halogen. 
     The compound of formula (VI) can be obtained by the reaction of carbon disulfide with the compound of formula (VIII) in the presence of sodium hydroxide at 15°-50° C. in an organic solvent, e.g. N,N-dimethylformamide or acetone, ##STR7## wherein R 1  and R 2  are the same as defined previously. 
     In order to obtain the objective compounds of formula (I), the acid chloride of formula (II) and the active ester of formula (IV) are used as an intermediate in the present invention. These intermediates are novel ones, and also constitute the present invention. These intermediates may be made by the processes described below. 
     Acid anhydrides of formula (II), isolated in a stable form, may be prepared by reacting the monoacid of formula (V) with the acid chloride of R 3  COOH, wherein R 3  is the same as defined previously, in the presence of an organic base, e.g. triethyl amine or pyridine, at 0°-30° C. 
     The active ester of formula (IV) can be prepared by reacting the monoacid of formula (V) with 1-hydroxybenzotriazole, N-hydroxysuccinimide or N-hydroxyphthalimide in the presence of a dehydrating agent such as N, N&#39;-dicyclohexylcarbodiimide. 
     The monoacid of formula (V) can be prepared by hydrolyzing one ester group of formula (IX) using the corresponding alcohol as a solvent at ambient temperature; ##STR8## wherein R 1  and n are the same as defined previously. 
     The starting material of formula (VIII) may be prepared by the reaction of alkyl malonyl chloride with the corresponding amine of formula (III). 
     The compounds of formula (IX) can be prepared by similar methods described in the aforementioned literatures. 
     In order to make the present invention more fully understood, the following Experiments and Examples are given. 
     The descriptions of experiments explain the protective and therapeutic effects of the compounds. 
     Examples are provided to illustrate the preparation of the compounds of formula (I), the starting materials and the related intermediates. 
     EXAMPLE 1 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate 
     Diisopropyl 2-(1,3-dithietan-2-yilidene)malonate (10 g) was added to a solution of potassium hydroxide (3.0 g) in isopropyl alcohol (300 ml). The mixture was stirred at room temperature for 12 hrs, after which water (500 ml) was added to dissolve the solid. The aqueous solution was acidified with acetic acid and the solid formed was filtered. The filter cake was purified with methylene chloride to afford the titled compound as a white solid. (6.6 g, 79%). 
     m. p.: 123°-124° C. 
       1  H--NMR(CDCl 3 ) δ: 12.30(bs,1H), 5.11(m,1H), 4.00(s,2H), 1.27(d,6H). 
     IR (KBr) cm -1  : 1712, 1627. 
     EXAMPLE 2 
     Ethyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate 
     Diethyl 2-(1,3-dithietan-2-ylidene)malonate was treated with a solution of potassium hydroxide in ethanol. The resultant solid was isolated and purified by the same method as in Example 1 to afford the titled compound as a white solid. (51%). 
     m. p.: 166°-167° C. 
       1  H--NMR(DMSO--d 6 ) δ: 4.21(q,2H), 4.14(s,2H), 1.08(t,3H). 
     IR (KBr) cm -1  : 1701, 1637. 
     EXAMPLE 3 
     Sec-butyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate 
     Di-sec-butyl 2-(1,3-dithietan-2-ylidiene)malonate was treated with a solution of potassium hydroxide in sec-butanol. The resultant solid was isolated and purified by the same method as in Example 1 to afford the titled compound as a white solid. (41%). 
     m. p.: 80°-82° C. 
       1  H--NMR(CDCl 3 ) δ: 12.28(bs,1H), 5.04(m,1H), 4.04(s,2H), 1.77(m,2H), 1.28(d,3H), 0.91(t,3H). 
     IR (KBr) cm -1  : 1712, 1627. 
     EXAMPLE 4 
     Isopropyl 2-(1,3-dithiolan-2-ylidene)-2-carboxyacetate 
     Diisopropyl 2-(1,3-dithiolan-2-ylidene)malonate was treated with a solution of potassium hydroxide in isopropyl alcohol. The resultant solid was isolated and purified by the same method as in Example 1 to afford the titled compound as a white solid. (64%). 
     m. p.: 104°-106° C. 
       1  H--NMR(CDCl 3 ) δ: 13.25(bs,1H), 5.22(m,1H), 3.39(s,4H), 1.11(d,6H). 
     IR (KBr) cm -1  : 1711, 1606. 
     EXAMPLE 5 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate 
     Triethylamine (5.18 g) was added to a solution of isopropyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate (10 g) in methylene chloride (100 ml). The mixture was cooled to 0° C., to which ethyl chloroformate (5.60 g) was added dropwise and then stirred for an hour. The precipitate was removed by filtration. The filtrate was washed with water, dried and evaporated to afford the titled compound as a pale yellow oil. (10 g, 81%). 
       1  H--NMR(CDCl 3 ) δ: 5.06(m,1H), 4.25(q,2H), 4.05(s,2H), 1.20(t,3H), 1.15(d,6H). 
     IR (KBr) cm -1  : 1783, 1706, 1654. 
     EXAMPLE 6 
     Ethyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate 
     Following the procedure of Example 5, the titled compound was obtained as a white solid (99%) from ethyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate. 
     m.p.: 38°-39° C. 
       1  H--NMR(CDCl 3 ) δ: 4.25(2q&#39;s,4H), 4.05(s,2H), 1.32(2t&#39;s,6H). 
     IR (KBr) cm -1  : 1767, 1707. 
     EXAMPLE 7 
     Sec-butyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate 
     Following the procedure of Example 5, the titled compound was obtained as a yellow oil (97%) from sec-butyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate. 
       1  H--NMR(CDCl 3 ) δ: 5.34(m,1H), 4.32(q,2H), 4.08(s,2H), 1.72(m,2H), 1.45(t,3H), 1.34(d,3H), 0.94(t,3H). 
     IR (KBr) cm -1  : 1783, 1705, 1652. 
     EXAMPLE 8 
     Isopropyl 2-(1,3-dithiolan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate 
     Following the procedure of Example 5, the titled compound was obtained as a yellow oil (97%) from isopropyl 2-(1,3-dithiolan-2-ylidene)-2-carboxyacetate. 
       1  H--NMR(CDCl 3 ) δ: 5.13(m,1H), 4.26(q,2H), 3.42(s,4H), 1.72(t,3H), 1.23(d,6H). 
     IR (KBr) cm -1  : 1786, 1691. 
     EXAMPLE 9 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[(1-benzotriazolyl)oxycarbonyl] acetate 
     N,N&#39;-Dicyclohexylcarbodiimide (7.1 g) was added to a mixture of isopropyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate (10 g) and 1-hydroxybenzotriazole(4.61 g) in methylene chloride(100 ml). The mixture was stirred at room temperature for 4 hrs, after which the precipitate was removed by filtration. The filtrate was concentrated to afford the titled compound as a white solid. (15 g, 96%). 
     m.p.: 146°-148° C. 
       1  H--NMR(CDCl 3 ) δ: 8.27-7.03(m,4H), 5.18(m,1H), 4.03(s,2H), 1.27(d,6H). 
     IR (KBr) cm -1  : 1709, 1655. 
     EXAMPLE 10 
     Ethyl 2-(1,3-dithietan-2-ylidene)-2-[(1-benzotriazolyl) oxycarbonyl] acetate 
     Following the procedure of Example 9, the titled compound was obtained as a white solid (89%) from ethyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate. 
     m.p.: 168°-169° C. 
       1  H--NMR(CDCl 3 ) δ: 8.27-7.06(m,4H), 4.43(q,2H), 4.11(s,2H), 1.37(t,3H). 
     IR (KBr) cm -1  : 1728, 1693. 
     EXAMPLE 11 
     Sec-butyl 2-(1,3-dithietan-2-ylidene)-2-[(1-benzotriazolyl) oxocarbonyl]acetate 
     Following the procedure of Example 9, the titled compound was obtained as a white solid (93%) from sec-butyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate. 
     m.p.: 143°-144° C. 
       1  H--NMR(CDCl 3 ) δ: 8.29-7.27(m,4H), 5.02(m,1H), 4.09(s,2H), 1.70(m,2H), 1.31(d,3H), 0.92(t,3H). 
     IR (KBr) cm -1  : 1712, 1663. 
     EXAMPLE 12 
     Isopropyl 2-(1,3-dithiolan-2-ylidene)-2-[(1-benzotriazolyl)oxycarbonyl] acetate 
     Following the procedure of Example 9, the titled compound was obtained as a white solid (97%) from isopropyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate. 
     m.p.: 88°-89° C. 
       1  H--NMR(CDCl 3 ) δ: 8.28-7.15(m,4H), 5.24(m,1H), 2.31(s,4H), 1.38(d,6H). 
     IR (KBr) cm -1  : 1720, 1687. 
     EXAMPLE 13 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-carbamoylacetate 
     A solution of isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate (10 g) in methanolic ammonia (60 ml) was stirred at room temperature for 4 hrs. The mixture was concentrated and purified with n-hexane and ethyl ether to give the titled compound as a white solid. (5.2 g, 68%) 
     m.p.: 128°-129° C. 
       1  H--NMR(DMSO--d 6 ) δ: 7.68(bs,1H), 7.30(bs,1H), 5.13(m,1H), 4.03(s,2H), 1.37(d,6H). 
     IR (KBr) cm -1  : 1675, 1647. 
     EXAMPLE 14 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-(N-ethylcarbamoyl)acetate. 
     Following the procedure of Example 13, the titled compound was obtained as a white solid (72%) from a solution of isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate in methanolic ethylamine. 
     m.p.: 65°-67° C. 
       1  H--NMR(DMSO--d 6 ) δ: 8.42(bs,1H), 5.25(m,1H), 3.97(s,2H), 3.37(q,2H), 1.29(d,6H), 1.08(t,3H). 
     IR (KBr) cm -1  : 1683, 1613. 
     EXAMPLE 15 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-(N-benzylcarbamoyl)acetate 
     The mixture of isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate (15 g) and benzylamine (6.4 ml) in methylene chloride (150 ml) was stirred at room temperature for 4 hrs. The reaction mixture was then washed with dil-HCl and water to remove benzylamine. The solvent was evaporated and the residue was purified with ethyl acetate to give the titled compound as a white solid. (12.2 g, 77%). 
     m.p.: 93°-94° C. 
       1  H--NMR(CDCl 3 ) δ: 8.82(bs,1H), 7.52(m,5H), 5.06(m,1H), 4.48(dd,2H), 3.96(s,2H), 1.28(d,6H). 
     IR (KBr) cm -1  : 1669, 1615. 
     EXAMPLE 16 
     Methyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-fluorophenyl)carbamoyl]acetate 
     A mixture of methyl 2-[N-(4-fluorophenyl)carbamoyl]acetate (10 g), carbon disulfide (3.2 ml) and acetone (200 ml) was cooled to 5°-10° C., to which 50% aqueous sodium hydroxide (4.2 ml) was added dropwise. The mixture was stirred at same temperature for 2 hrs, then dibromomethane (4 ml) was added dropwise and the reaction mixture was stirred at room temperature for 1 hr and refluxed for 2 hrs. The solid was removed by filtration and the filtrate was poured onto ice-water. The solid formed was filtered, and dissolved in methylene chloride. The solution was washed with 10% aqueous sodium hydroxide solution and water, then dried. The solvent was evaporated and the residue was purified with ethyl acetate and ethyl ether to give the titled compound as a white solid. (8.5 g, 60%). 
     m.p.: 141°-142° C. 
       1  H--NMR(CF 3  COOD) δ: 7.42-7.03(m,4H), 4.15(s,2H), 3.98(s,3H). 
     IR (KBr) cm -1  : 1685, 1622. 
     EXAMPLE 17 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-chlorophenyl)carbamoyl]acetate 
     A solution of isopropyl 2-(1,3-dithietan-2-ylidene)-2-carboxyacetate (10 g) in methylene chloride (100 ml) was cooled to -10° C. and thereto phosphorous pentachloride (8.9 g) was added portionwise and then stirred for 1 hr. To the reaction mixture pyridine (13.8 ml) and 4-chloroaniline (5.45 g) were added and stirred for 2 hrs. The mixture was washed with dil-HCl, 10% aqueous sodium hydroxide solution and water and then evaporated. The residue was purified with ethyl acetate to afford the titled compound as a white solid. (8.9 g, 61%). 
     m.p.: 164°-165° C. 
       1  H--NMR(CF 3  COOD) δ: 7.37(m,4H), 5.21(m,1H), 4.12(s,2H), 1.45(d,6H). 
     IR (KBr) cm -1  : 1678. 
     EXAMPLE 18 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-(N-phenylcarbamoyl)acetate 
     A solution of isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate (10 g) and aniline (3.6 ml) in methylene chloride (100 ml) was stirred at room temperature for 8 hrs. The reaction mixture was washed with 10% aqueous sodium hydroxide solution, dil-HCl and water and then evaporated to dryness. The residue solid was purified with ethyl acetate to give the titled compound as a white solid. (7.9 g, 78%). 
     m.p.: 115°-116° C. 
       1  H--NMR(CDCl 3 ): 10.52(bs,1H), 7.85-6.75(m,5H), 5.16(m,1H), 3.99(s,2H), 1.34(s,6H). 
     IR (KBr) cm -1  : 1671. 
     EXAMPLE 19 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-trifluoromethylphenyl)carbamoyl]acetate 
     Following the procedure of Example 18, the titled compound was prepared as a white solid (67%) from isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and 4-aminobenzotrifluoride. 
     m. p.: 151°-152° C. 
       1  H--NMR(CDCl 3 ) δ: 10.78(bs,1H), 7.68(m,4H), 5.40(m,1H), 4.02(s,2H), 1.34(d,6H). 
     IR (KBr) cm -1  : 1686, 1623. 
     EXAMPLE 20 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-hydroxyphenyl)carbamoyl]acetate 
     Following the procedure of Example 18, the titled compound was prepared as a white solid (66%) from isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and p-aminophenol. 
     m. p.: 175°-176° C. 
       1  H--NMR(DMSO--d 6 ) δ: 10.20(s,1H), 9.15(s,1H), 7.40-6.73(dd,4H), 5.13(m,1H), 4.21(s,2H), 1.42(d,6H). 
     IR (KBr) cm -1  : 1675, 1606. 
     EXAMPLE 21 
     Isopropyl 2-(1,3-dithiolan-2-ylidene)-2-[N-(4-trifluoromethylphenyl)carbamoyl]acetate 
     Following the procedure of Example 18, the titled compound was obtained as a white solid (80%) from isopropyl 2-(1,3-dithiolan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and 4-aminobenzotrifluoride. 
     m. p. 139°-140° C. 
       1  H--NMR(CDCl 3 ) δ: 10.90(bs,1H), 7.82-7.65(dd,4H), 5.24(m,1H), 3.40(s,4H), 1.42(d,2H). 
     IR (KBr) cm -1  : 1679, 1631. 
     EXAMPLE 22 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(2-thiazolyl)carbamoyl]acetate 
     The solution of isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate (10 g) and 2-aminothiazole (4.0 g) in methylene chloride (100 ml) was stirred at room temperature for 24 hrs. The reaction mixture was washed with 10% aqueous sodium hydroxide solution and water and then evaporated. The residue was chromatographed on silica gel column using methylene chloride as an eluent to afford the titled compound as a white solid (6.5 g, 63%). 
     m. p.: 174°-175° C. 
       1  H--NMR(CDCl 3  /DMSO--d 6 ) δ: 11.70(bs,1H), 7.24(dd,2H), 5.29(m,1H), 4.19(s,2H), 1.29(d,6H). 
     IR (KBr) cm -1  : 1675, 1621. 
     EXAMPLE 23 
     Ethyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-methyl-2-thiazolyl)carbamoyl]acetate 
     Following the procedure of Example 22, the titled compound was obtained as a white solid (39%) from ethyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and 2-amino-4-methylthiazole. 
     m. p.: 218°-219° C. 
       1  H--NMR(CDCl 3 ) δ: 11.50(bs,1H), 6.45(s,1H), 4.39(q,2H), 4.02(s,2H), 2.34(s,3H), 1.33(t,3H). 
     IR (KBr) cm -1  : 1685. 
     EXAMPLE 24 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-methyl-2-thiazolyl)carbamoyl]acetate 
     Following the procedure of Example 22, the titled compound was obtained as a white solid (46%) from isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and 2-amino-4-methylthiazole. 
     m. p.: 197°-198° C. 
       1  H--NMR(CDCl 3 ) δ: 11.62(bs,1H), 6.44(2,1H), 5.16(m,1H), 4.06(s,2H), 2.35(s,3H), 1.43(d,6H). 
     IR (KBr) cm -1  : 1678, 1627. 
     EXAMPLE 25 
     Sec-butyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-methyl-2-thiazolyl)carbamoyl]acetate 
     Following the procedure of Example 22, the titled compound was prepared as a white solid (67%) from sec-butyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and 2-amino-4-methylthiazole. 
     m.p.: 153°-154° C. 
       1  H--NMR(DMSO--d 6 ) δ: 11.67(bs,1H), 6.47(s,1H), 5.01(m,1H), 4.07(s,2H), 2.35(s,3H), 1.49(m,2H), 1.29(d,3H), 0.93(t,3H), 
     IR (KBr) cm -1  : 1675, 1624, 
     EXAMPLE 26 
     Isopropyl 2-(1,3-dithiolan-2-ylidene)-2-[N-(4-methyl-2-thiazolyl)carbamoyl]acetate 
     Following the procedure of Example 22, the titled compound was prepared as a white solid (41%) from isopropyl 2-(1,3-dithiolan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and 2-amino-4-methylthiazole. 
     m.p.: 161°-162° C. 
       1  H--NMR(CDCl 3 ) δ: 11.85(bs,1H), 6.64(s,1H), 5.24(m,1H), 3.37(s,4H), 2.34(s,3H), 1.42(d,6H), 
     IR (KBr) cm -1  : 1668, 1623. 
     EXAMPLE 27 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(4-phenyl-2-thiazolyl)carbamoyl]acetate 
     A solution of isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate (10 g) and 2-amino-4-phenylthiazole (6.2 g) in N,N-dimethylformamide (100 ml) was stirred at 60° C. for 24 hrs. The reaction mixture was poured onto ice-water (500 ml) and the solid formed was filtered. 
     The filter cake was dissolved in methylene chloride, and the solution was washed with 10% aqueous sodium hydroxide solution and water and then evaporated to dryness. The residue was chromatographed on silica gel column using methylene chloride as an eluent to afford the titled compound as a white solid. (6.9 g, 54%). 
     m.p.: 225°-226° C. 
       1  H--NMR(CF 3  COOD) δ: 7.99-7.44(m,5H), 7.26(s,1H), 5.25(m,1H), 4.30(s,2H), 1.44(d,6H). 
     IR (KBr) cm -1  : 1673. 
     EXAMPLE 28 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(1,3,4-thiadiazol-2-yl)carbamoyl]acetate 
     Following the procedure of Example 27, the titled compound (38%) was obtained from isopropyl 2-(1,3-dithietan-2-ylidene)-2-(ethoxycarboxycarbonyl)acetate and 2-amino-1,3,4-thiadiazole. 
     m.p.: 212°-213° C. 
       1  H--NMR(DMSO--d 6 ) δ: 9.16(s,1H), 5.20(m,1H), 4.26(s,2H), 1.33(d,6H), 
     IR (KBr) cm -1  : 1676, 1625. 
     EXAMPLE 29 
     Isopropyl 2-(1,3-dithietan-2-ylidene)-2-[N-(2-pyridyl)carbamoyl]acetate 
     Following the procedure of Example 27, the titled compound (77%) was obtained from isopropyl 2-(1,3-dithietan-2-ylidene)-2-[(1-benzotriazolyl)oxycarbonyl]acetate and 2-aminopyridine. 
     m.p.: 162°-163° C. 
       1  H--NMR(CDCl 3 ) δ: 11.10(s,1H), 8.72-6.93(m,4H), 5.21(m,1H), 3.99(s,2H), 1.37(d,6H), 
     IR (KBr) cm -1  : 1676, 1631, 
     EXAMPLE 30 
     Ethyl 2-(1,3-dithietan-2-ylidene)-2-[N-(2-pyridyl)carbamoyl]acetate 
     Following the procedure of Example 27, the titled compound (51%) was prepared from ethyl 2-(1,3-dithietan-2-ylidene)-2-[(1-benzotriazolyl)oxycarbonyl]acetate and 2-aminopyridine. 
     m.p.: 181°-183° C. 
       1  H--NMR(CDCl 3 ) δ: 10.97(bs,1H), 8.52-6.72(m,4H), 4.40(q,2H), 4.01(s,2H), 1.34(t,3H), 
     IR (KBr) cm -1  : 1676, 1628, 
     EXAMPLE 31 
     Sec-butyl 2-(1,3-dithietan-2-ylidene)-2-[N-(2-pyridyl)carbamoyl]acetate 
     Following the procedure of Example 27, the titled compound (76%) was obtained from sec-butyl 2-(1,3-dithietan-2-ylidene)-2-[(1-benzotriazolyl) oxycarbonyl]acetate and 2-aminopyridine. 
     m.p.: 118°-119° C. 
       1  H--NMR(CDCl 3 ) δ: 11.03(bs,1H), 8.53-6.74(m,4H), 5.02(m,1H), 4.05(s,2H), 1.51(m,2H), 1.31(d,3H), 0.95(t,3H), 
     IR (KBr) cm -1  : 1671, 1632. 
     EXAMPLE 32 
     Isopropyl 2-(1,3-dithiolan-2-ylidene)-2-[N-(2-pyridyl)carbamoyl]acetate 
     Following the procedure of Example 27, the titled compound was obtained as a white solid (53%) from isopropyl 2-(1,3-dithiolan-2-ylidene)-2-[(1-benzotriazolyl)oxycarbonyl]acetate and 2-aminopyridine. 
     m.p.: 121°-122° C. 
       1  H--NMR(DMSO--d 6 ) δ: 10.61(bs,1H), 8.59-6.79(m,4H), 5.02(m,1H), 3.37(bs,4H), 1.22(d,6H), 
     IR (KBr) cm -1  : 1666, 1629, 
     EXPERIMENT 1 
     Protective Effect Against Acute Hepatic Damage Induced by Carbon Tetrachloride 
     Principle 
     Carbon tetrachloride (CCl 4 ) is a well-known hepatotoxic agent, thus widely used to produce experimental animal models for screening the potential drugs acting upon hepatic diseases. In this experiment test compounds were administered to mice prior to treatment with CCl 4 . 24 Hours after the CCl 4  treatment, the animals were sacrificed and serum alanine aminotransferase (ALT) levels were determined. The hepatoprotective effect of a test compound was evaluated by the suppressive action against the increase of serum ALT induced by CCl 4 . 
     Method 
     The experimental animals were divided into the normal, the CCl 4  treated and the test compound treated groups. Each group consisted of 8 mice with body weight of 20-25 grams. The test compounds were suspended in 0.2% sodium carboxymethylcellulose (CMC--Na) solution and administered orally at the dose of 50 mg/125 ml/kg body weight. Only the vehicle 0.2% CMC--Na solution, was administered to the normal and to the CCl 4  treated group instead of the test compound suspension. 
     6 Hours after the drug administration, the CCl 4  solution in olive oil was administered orally to the CCl 4  treated-group and the test compound treated group at the dose of 50 ul/25 ml olive oil/kg body weight. The normal group was administered with olive oil only at the same dose. 24 Hours after the CCl 4  administration, blood samples were collected from the orbital sinus of the animals and the sera were monitored using an automatic blood analyzer (Gilford, SBA 300). The hepatoprotective effect of the test compounds was expressed by the suppressive percentage against the increase of serum ALT level induced by CCl 4  calculated by the following formula; ##EQU1## 
     Results 
     The test results are shown in Table I. 
     
                       TABLE I______________________________________(Protective effect of test compound againstincrease of serum ALT activity induced by CCl.sub.4)Compound      Inhibitory (%)______________________________________Example 13     28Example 15     97Example 18    100Example 19    100Example 21     98Example 22    100Example 24    100Example 26     98Example 27     11Example 28    100Example 29    100Example 32     85______________________________________ 
    
     EXPERIMENT 2 
     Determination of 50% Effective Dose (ED 50 ) of Test Compounds 
     Principle 
     The various doses of test compounds were administered to mice, followed by the treatment with CCl 4  after 6 hours. 24 Hours after the CCl 4  treatment, the serum ALT level of the animals were determined. The hepatoprotective effect of the test compounds and the doses were plotted to form doseresponse curves, from which the ED 50  doses of the test compounds were estimated. 
     Method 
     The experimental animals were divided into the normal, the CCl 4  treated and the test compound treated groups. Each group consisted of 8 mice with body weight of 20-25 grams. The test compounds were suspended in 0.2% CMC--Na solution and administered orally at a dose of 50 mg, 25 mg, 12.5 mg or 6.25 mg/125 ml/kg body weight. Only the vehicle, 0.2% CMC--Na solution, was administered to the normal and to the CCl 4  treated group instead of the test compound suspension. 6 Hours later, the CCl 4  solution in olive oil was administered orally to the CCl 4  treated and the test compound treated groups at the dose of 50 ul/25 ml oilve oil/kg body weight. The norm group was administered olive oil only at the same dose. 24 Hours after the CCl 4  administration, blood samples were collected from the orbital sinuses of the animals and the sera were obtained by centrifugation. The serum ALT activities were monitored and the hepatoprotective effect of the test compounds were calculated by the same method as described in experiment 1. The percentages were plotted against the corresponding doses of the test compounds to make dose-response curves, from which the 50% effective dose of test compound was estimated. 
     Results 
     The 50% effective doses (ED 50 ) of the test compounds are shown in Table II. 
     
                       TABLE II______________________________________(ED.sub.50 of test compound)          ED.sub.50  Compound          (mg/kg)______________________________________  Example 13          &gt;50  Example 15          10  Example 18          7  Example 19          6.25  Example 21          &lt;50  Example 22          &lt;6.25  Example 24          &lt;6.25  Example 26          &lt;50  Example 27          &gt;50  Example 28          &lt;6.25  Example 29          10  Example 32          &lt;50______________________________________ 
    
     EXPERIMENT 3 
     Acute Toxicity of Test Compounds 
     Each test compound was suspended in 0.2% CMC--Na solution in various concentrations to make test suspensions. The test suspensions were administered orally to male ICR mice (10 in each group) at various doses. The number of dead mice was counted for 14 days and the value of median lethal dose (LD 50 , g/kg) was calculated by the Hitchifield-Wilcoxon method. 
     The results are shown in Table III. 
     
                       TABLE III______________________________________(LD.sub.50 of Test Compound)          ED.sub.50  Compound          (mg/kg)______________________________________  Example 13          &lt;5,000  Example 22          &gt;5,000  Example 24          &gt;5,000  Example 28          &gt;5,000  Example 29          &gt;5,000______________________________________