as is described above , the procedure of the inventive method for the preparation of a β - thiolactam compound is very simple and efficient because the synthetic reaction can proceed only by uniformly mixing the two kinds of the starting materials to give the desired compound in a high yield . one of the starting reactants is an isothiocyanate compound represented by the above given general formula ( ii ). in this general formula , r is a monovalent hydrocarbon group including alkyl and aryl groups . the alkyl group is exemplified by straight - chain , branched and cyclic alkyl groups having 1 to 70 carbon atoms such as methyl , ethyl , n - propyl , isopropyl , n - butyl , isobutyl , sec - butyl , tert - butyl , pentyl , hexyl , octyl , decyl , cyclopentyl and cyclohexyl groups . aryl - substituted alkyl groups such as benzyl and 2 - phenylethyl groups are also suitable . the aryl group is exemplified by those having 6 to 10 carbon atoms such as phenyl , tolyl , xylyl , naphthyl , ethylphenyl , n - propylphenyl and isopropylphenyl groups . the aryl group can be substituted on the aromatic ring by a substituent including lower alkyl groups , lower alkoxy groups and halogen atoms as exemplified by methoxyphenyl , ethoxyphenyl , n - propoxyphenyl , isopropoxyphenyl , fluorophenyl , chlorophenyl , bromophenyl and iodophenyl groups . particular examples of the isothiocyanate ester compound represented by the general formula ( ii ) and suitable as the starting reactant in the inventive method include : methyl isothiocyanate , ethyl isothiocyanate , phenyl isothiocyanate , tolyl isothiocyanate , xylyl isothiocyanate , methoxyphenyl isothiocyanate , chlorophenyl isothiocyanate and benzyl isothiocyanate . the reaction of the isothiocyanate ester compound of the general formula ( ii ) and 2 , 3 - dihydrofuran proceeds in a molar ratio of 1 : 1 of these reactants but it is usually preferable that 2 , 3 - dihydrofuran is used in a large excess over the isothiocyanate compound so that the excess amount of 2 , 3 - dihydrofuran serves as a diluent of the reaction mixture although it is optional that 2 , 3 - dihydrofuran is used in an amount of about 2 moles per mole of the isothiocyanate compound and the reaction mixture is diluted with a conventional inert organic solvent , which should preferably have relatively low polarity , such as aliphatic , alicyclic and aromatic hydrocarbon solvents . preferable solvents include , for example , hexane , heptane , octane , decane , cyclohexane , cyclodecane , tetrahydronaphthalene , decahydronaphthalene , benzene , toluene , xylene , ethylbenzene and the like . the reaction between the isothiocyanate compound and 2 , 3 - dihydrofuran proceeds , though not essential , by bringing the reaction mixture under a superatmospheric pressure which should preferably be as high as possible even at such a temperature at which the reaction does not proceed under normal pressure . the pressure should desirably be at least 2000 atmospheres or , preferably , in the range from 2000 to 10000 atmospheres . the reaction temperature is not particularly limitative and the reaction can proceed even at room temperature although the reaction can be accelerated by increasing the temperature while an excessively high temperature is disadvantageous due to increase in the side reactions . in consideration of the balance between the reaction velocity and side reactions , the reaction temperature should be in the range from 0 ° to 200 ° c . or , preferably , from 20 ° to 150 ° c . the reaction time naturally depends on various factors such as the kinds of the isothiocyanate compound and solvent , pressure , reaction temperature and the like but the reaction is usually complete within 1 to 50 hours . in this way , the desired β - thiolactam compound can be obtained in a high yield of , for example , 50 % or even higher of the theoretical value though dependent on various conditions . in the following , the method of the present invention is described in more detail by way of examples . a mixture consisting of 0 . 68 g ( 5 . 0 mmoles ) of phenyl isothiocyanate and 1 . 76 g ( 25 mmoles ) of 2 , 3 - dihydrofuran was sealed in a teflon tube and heated in an autoclave at 130 ° c . for 20 hours under a pressure of 10 atmospheres to effect the reaction between the reactants . after completion of the reaction time , the reaction mixture taken out of the autoclave and freed from unreacted 2 , 3 - dihydrofuran by distillation under reduced pressure was subjected to column chromatography through a silica gel column using a 1 : 1 by volume mixture of hexane and ethyl acetate as the developer to obtain 0 . 54 g of a purified product having a melting point of 59 ° c . which could be identified from the identification data shown below to be 7 - phenyl - 2 - oxa - 7 - azabicyclo [ 3 . 2 . 0 ] heptan - 6 - thione which is a compound of the general formula ( i ) with r being a phenyl group . the above mentioned yield of this product compound corresponds to 52 % of the theoretical value . mass spectrometry ( m / z ): 205 ( m + ) ( c 11 h 11 nos ═ 205 ) 1 h -- nmr : ( δ , ppm ) 8 . 02 - 7 . 98 ( m , 2h ), 7 . 40 - 7 . 34 ( m , 2h ), 7 . 25 - 7 . 19 ( m , 1h ), 6 . 2 ( d , j = 3 hz , 1h ), 4 . 27 ( t , j = 9 hz , 1h ), 3 . 95 - 3 . 86 ( m , 1h ) , 3 . 72 ( dd , j = 3 , 7 hz , 1h ) , 2 . 28 ( dd , j = 5 , 13 hz , 1h ), 1 . 97 - 1 . 83 ( m , 1h ) 13 c -- nmr : ( δ , ppm ) 198 . 73 , 137 . 74 , 128 . 97 , 2c , 126 . 17 , 117 . 56 , 2c , 92 . 05 , 67 . 52 , 58 . 04 , 27 . 11 ______________________________________ c h n s______________________________________calculated for c . sub . 11 h . sub . 11 nos 64 . 36 5 . 40 6 . 83 15 . 62found 64 . 37 5 . 39 6 . 85 15 . 16______________________________________ the experimental procedure was substantially the same as in example 1 excepting replacement of 0 . 68 g of the phenyl isothiocyanate with 0 . 41 g ( 5 . 6 mmoles ) of methyl isothiocyanate to give 0 . 14 g of a reaction product which could be identified to be 7 - methyl - 2 - oxa - 7 - azabicyclo [ 3 . 2 . 0 ] heptan - 6 - thione which is a compound of the general formula ( i ) with r being a methyl group . the yield of the product was 17 % of the theoretical value . mass spectrometry ( m / z ): 143 ( m + ) ( c 6 h 9 nos ═ 143 ) 13 c -- nmr : ( δ , ppm ) 203 . 59 , 93 . 24 , 67 . 45 , 58 . 58 , 29 . 06 , 26 . 46 the experimental procedure was substantially the same as in example 1 excepting replacement of 0 . 68 g of the phenyl isocyanate with 0 . 85 g ( 5 . 0 mmoles ) of 4 - chlorophenyl isothiocyanate to give 0 . 47 g of a reaction product which could be identified to be 7 -( 4 - chlorophenyl )- 2 - oxa - 7 - azabicyclo [ 3 . 2 . 0 ]- heptan - 6 - thione which is a compound of the general formula ( i ) with r being a 4 - chlorophenyl group . the yield of the product was 39 % of the theoretical value . 13 c -- nmr : ( δ , ppm ) 198 . 92 , 136 . 30 , 131 . 18 , 129 . 14 , 2c , 118 . 92 , 2c , 92 . 27 , 67 . 69 , 58 . 38 , 27 . 22 the experimental procedure was substantially the same as in example 1 excepting replacement of 0 . 68 g of the phenyl isothiocyanate with 0 . 75 g ( 5 . 0 mmoles ) of p - tolyl isothiocyanate to give 0 . 19 g of a reaction product which could be identified to be 7 -( 4 - methylphenyl )- 2 - oxa - 7azabicyclo [ 3 . 2 . 0 ]- heptan - 6 - thione which is a compound of the general formula ( i ) with r being a p - tolyl group . the yield of the product was 17 % of the theoretical value . 13 c -- nmr : ( δ , ppm ) 198 . 08 , 135 . 43 , 129 . 49 , 2c , 117 . 67 , 2c , 114 . 81 , 92 . 10 , 67 . 54 , 57 . 99 , 27 . 13 , 21 . 18 the experimental procedure was substantially the same as in example 1 excepting replacement of 0 . 68 g of the phenyl isothiocyanate with 0 . 85 g ( 5 . 1 mmoles ) of 4 - methoxyphenyl isothiocyanate to give 0 . 33 g of a reaction product which could be identified to be 7 -( 4 - methoxyphenyl )- 2 - oxa - 7 - azabicyclo [ 3 . 2 . 0 ]- heptan - 6 - thione which is a compound of the general formula ( i ) with r being a 4 - methoxyphenyl group . the yield of the product was 27 % of the theoretical value . 13 c -- nmr : ( δ , ppm ) 197 . 18 , 757 . 62 , 137 . 36 , 779 . 33 , 2c , 114 . 10 , 2c , 92 . 25 , 67 . 57 , 57 . 95 , 55 . 49 , 27 . 05 a mixture consisting of 0 . 68 g ( 5 . 0 mmoles ) of phenyl isothiocyanate and 7 . 76 g ( 25 mmoles ) of 2 , 3 - dihydrofuran was sealed in a teflon tube which was heated in a high - pressure reactor at 100 ° c . for 20 hours under a pressure of 2000 atmospheres to effect the reaction . after the above mentioned reaction time , the reaction mixture was taken out of the reactor and subjected to a quantitative analysis by the glc method with hexadecane as an internal standard to find that the desired product , i . e . 7 - phenyl - 2 - oxa - 7 - azabicyclo [ 3 . 2 . 0 ]- heptan - 6 - thione , was formed in a yield of 55 % based on the theoretical value . to examine the effect of the pressure on the reaction , the same synthetic procedure as above was repeated except that the pressure was decreased to 6 atmospheres to find that the yield of the desired product was only 17 % of the theoretical value . a mixture consisting of 1 . 37 g ( 10 mmoles ) of phenyl isothiocyanate and 3 . 51 g ( 50 mmoles ) of 2 , 3 - dihydrofuran was sealed in a teflon tube which was heated in a high - pressure reactor at 40 ° c . for 20 hours under a pressure of 8000 atmospheres to effect the reaction . after the above mentioned reaction time , the reaction mixture was taken out of the reactor and analyzed in the same manner as in example 6 to find that the yield of the desired product , i . e . 7 - phenyl - 2 - oxa - 7 - azabicyclo [ 3 . 2 . 0 ] heptan - 6 - thione , was 29 % of the theoretical value . to examine the effect of the pressure on the reaction , the same synthetic procedure as above was repeated except that the teflon tube containing the reaction mixture was heated at 40 ° c . for 20 hours under normal pressure . the results of the reaction was that the desired product could not be obtained at all . a mixture consisting of 0 . 68 g ( 5 . 0 mmoles ) of phenyl isothiocyanate , 0 . 71 g ( 10 mmoles ) of 2 , 3 - dihydrofuran and 1 . 5 ml of toluene was sealed in a teflon tube which was heated in a high - pressure reactor at 100 ° c . for 20 hours under a pressure of 4000 atmospheres to effect the reaction . after the above mentioned reaction time , the reaction mixture was taken out of the reactor and analyzed in the same manner as in example 6 to find that the yield of the desired product , i . e . 7 - phenyl - 2 - oxa - 7 - azabicyclo -[ 3 . 2 . 0 ] heptan - 6 - thione , was 48 % of the theoretical value . to examine the effect of the presence of toluene on the reaction , the same synthetic procedure as above was repeated excepting omission of toluene in the reaction mixture to find that the yield of the desired product was only 8 % of the theoretical value .