the kojic acid derivatives ( i ) of the present invention may be prepared by the process shown in the following reaction scheme : ## str3 ## wherein , r has the same meaning defined as above . the compounds of the formula ( iv ) may include , for example , sodium salts of 3 , 4 - dihydroxycinnamic acid and of 4 - hydroxy - 3 - methoxycinnamic acid . the compounds of the formula ( i ) may include but are not limited to , 2 -( 3 , 4 - dihydroxycinnamoyl ) oxymethyl - 5 - hydroxy - 4h - pyran - 4 - one and 2 -( 4 - hydroxy - 3 - methoxy cinnamoyl ) oxymethyl - 5 - hydroxy - 4h - pyran - 4 - one . the novel kojic acid derivatives of the formula ( i ) according to the present invention may be prepared , as shown in the above reaction scheme , by reacting kojic acid of the formula ( ii ) with thionyl chloride ( soc12 ) in a solvent such as chloroform to give 2 - chloromethyl - 5 - hydroxy - 4h - pyran - 4 - one (&# 34 ; chlorokojic acid &# 34 ;) of the formula ( iii ) and reacting the compound of the formula ( iv ) with the chlorokojic acid in a solvent such as n , n - dimethylformamide to give the present compound ( i ). the compound of the formula ( i ) according to the present invention is a kojic acid derivative in which the 2 - hydroxymethyl group is substituted with hydroxycinnamic acid ; it exhibits a 20 fold higher activity than that of known kojic acid derivatives in terms of ic 50 ( concentration of kojic acid to inhibit the enzyme activity by 50 %)( see experimental example 1 below ). particularly , kojic acid substituted at the 2 - position with dihydroxycinnamic acid , for example 3 , 4 - dihydroxy cinnamic acid , is capable of strongly inhibiting tyrosinase as well as can effectively scavenging harmful radicals . in general , radicals formed in the living body are known to cause a skin aging and a radical - scavenger can retard the skin aging ( japan fragrance journal , june , 1990 , pp 39 - 46 ). known scavengers of harmful radicals in the living body include the anti - oxidants , vitamin e ( tocopherol ), hydroquinone , and superoxide dismutase ( sod ). the 2 - dihydroxycinnamic acid substituted kojic acids ( i ) show an equivalent activity to that of these known scavengers ( see experimental example 2 ). further , it has been confirmed that the present compounds ( i ) have only slight said effects on human skin as demonstrated in a skin safety experiment ( experimental example 3 ). accordingly , the present compounds ( i ) can be incorporated into cosmetic compositions or topical dermatological medicaments for whitening the skin , preventing skin tanning or pigmentation . the present invention will be illustrated hereinafter in more detail by way of non - limiting examples . examples 1 and 2 show the preparation and identification of the compounds ( i ); experimental example 1 compares the tyrosinase - inhibiting activity of the compounds ( i ) and of kojic acid ; experimental example 2 compares the radical - scavenging activity of the compounds ( i ) and of conventional anti - oxidants ; and experimental example 3 shows a skin safety test of these compounds ( i ). the method of preparing the intermediate compound ( ii ) is illustrated in reference example . 50 g ( 351 . 8 mmol ) of kojic acid was dissolved in 250 ml of n , n - dimethylformamide . the resulting solution was cooled in an ice bath of 10 ° c . and 30 ml ( 411 . 3 mmol ) of thionyl chloride was added dropwise thereto . the mixture was stirred at room temperature for 2 hours and 2000 ml of ice water was added . the precipitate was filtered and dissolved into 1000 ml of ethyl acetate . the reaction product was dried over magnesium sulfate , decolored with active charcoal and filtered . the filtrate was concentrated and hexane was added to give crystals , which were then dried under vacuum to give 37 . 5 g ( 66 . 83 %) of the desired chlorokojic acid as a yellow solid . 0 . 81 g ( 4 . 5 mmol ) of 3 , 4 - dihydroxycinnamic acid and 0 . 18 g ( 4 . 5 mmol ) of sodium hydroxide were dissolved into 40 ml of methanol . residues obtained after distillating in methanol dissolved into 70 ml of n , n - dimethylformamide and 0 . 65 g ( 4 . 05 mmol ) of chlorokojic acid prepared in the reference example was added thereto . the resulting mixture was heated with stirring for 2 hours in an oil bath of 110 ° c . after distillating the solvent , the residue was dissolved in 300 ml of ethyl acetate . the ethyl acetate solution was washed with 5 % hydrochloric acid and distilled water , dried over magnesium sulfate , decolored with active charcoal and filtered . the filtrate was concentrated to give 0 . 66 g ( 48 . 8 %) of 2 -( 3 , 4 - dihydroxycinnamoyl ) oxymethyl - 5 - hydroxy - 4h - pyran - 4 - one as a solid . mp . : 186 °- 189 ° c . tlc ( in acetic acid : ethyl ether = 25 : 1 ) rf = 0 . 35 ir ( kbr pellet ) : 3421 , 1686 , 1652 cm - 1 1 h - nmr ( dmso - d 6 , δ ) : 5 . 05 ( s , 2h ), 6 . 34 ( d , 1h ), 6 . 49 ( s , 1h ) 6 . 74 - 7 . 17 ( m , 3h ), 7 . 56 ( d , 1h ), 8 . 10 ( s , 1h ) 9 . 15 ( s , 1h ), 9 . 23 ( s , 1h ), 9 . 66 ( s , 1h ) 0 . 87 g ( 4 . 5 mmol ) of 4 - hydroxy - 3 - methoxycinnamic acid and 0 . 18 g ( 4 . 5 mmol ) of sodium hydroxide were dissolved into 40 ml of methanol . residue obtained after distillating methanol was dissolved in 70 ml of n , n - dimethylformamide and 0 . 65 g ( 4 . 05 mmol ) of chlorokojic acid prepared in the reference example was added thereto . the resulting mixture was heated with stirring for 2 hours in an oil bath of 110 ° c . after distillating the solvent , the residue was dissolved in 300 ml of ethyl acetate . the ethyl acetate solution was washed with 5 % hydrochloric acid and distilled water , dried over magnesium sulfate , decolored with active charcoal and filtered . the filtrate was concentrated to give 0 . 6g ( 41 . 9 %) of 2 -( 4 - hydroxy - 3 - methoxy cinnamoyl ) oxymethyl - 5 - hydroxy - 4h - pyran - 4 - one as a solid . mp . : 182 °- 183 ° c ., tlc ( in acetic acid : ethyl ether = 25 : 1 ) rf = 0 . 45 ir ( kbr pellet ) : 3474 , 1711 , 1649 cm - 1 1 h - nmr ( dmso - d 6 , δ ) : 3 . 81 ( s , 3h ), 5 . 06 ( s , 2h ), 6 . 51 ( s , 1h ) 6 . 56 ) d , 1h ), 7 . 12 - 7 . 36 ( m , 3h ) 7 . 63 ( d , 1h ), 8 . 10 ( s , 1h ), 9 . 25 ( s , 1h ) 9 . 66 ( s , 1h ) in order to examine the tyrosinase - inhibiting activity , harmful radical - scavenging activity and skin safety of the present kojic acid derivatives , the following tests were carried out using the compounds of examples 1 and 2 . tyrosinase - inhibiting activity of the compounds of examples 1 and 2 were compared with that of kojic acid as follows : tyrosinase , which was extracted from mushroom , was purchased from sigma . tyrosine and a substrate were dissolved in distilled water at a concentration of 0 . 3 mg / ml and 1 . 0 ml of the resulting solution was placed into a test tube . 1 . 0 ml of potassium sulfate buffer ( 0 . 1m , ph6 . 8 ), 0 . 9 ml of stepwise dilutions of compounds of examples 1 and 2 or kojic acid were added thereto . the resulting mixture was reacted at 37 ° c . in an incubator for 10 minutes . ethanol and water were used to dilute the test compounds . as a control , 0 . 9 ml of a 1 : 1 mixture of ethanol and water was used . 0 . 1 ml of tyrosine solution ( 2 , 500 unit / ml ) was added and the reaction mixture was reacted at 37 ° c . in an incubator for 10 minutes . the test tube was quickly cooled in an ice water bath to stop the reaction , and absorbance was measured at 475 nm . the tyrosinase - inhibiting activities of the compounds of examples 1 and 2 and kojic acid were calculated by the following equation : ## equ1 ## ic 50 , i . e ., the concentration of test compound which inhibits tyrosinase by 50 % was calculated by serial dilution for each test compound . table 1______________________________________test compounds ic . sub . 50 ( μm ) ______________________________________kojic acid 80compound in example 1 4compound in example 2 4______________________________________ as can be seen from the results in table 1 , the compounds of examples 1 and 2 show a 20 fold higher activity of tyrosinase inhibition than that of kojic acid . harmful radical - scavenging activities of the compounds of examples 1 and 2 were compared with those of conventional anti - oxidants using a dpph ( diphenylpicrylhydrazyl ) radical - scavenging test as follows : dpph was dissolved into ethanol to a concentration of 100 μm . a series of dilutions of compounds of examples 1 and 2 , vitamin c , vitamin e or hydroquinone were prepared . ethanol and water were used to dilute the test compounds . as a control , a 1 : 1 mixture of ethanol and water was used . the series of dilutions of each test compound were mixed with 1 , 900 μl of dpph solution and the dilution solvent was added to 2 . 0 ml . the resulting mixture was reacted in an incubator at 37 ° c . for 30 minutes . absorbance was measured at 515 nm . the radical - scavenging activities of the compounds of examples 1 and 2 and conventional anti - oxidants were calculated by the following equation : ## equ2 ## ic 50 , i . e ., the concentration of test compound which scavenge the radical , dpph by 50 % was calculated by the serial dilution method for each test compound . table 2______________________________________test compounds ic . sub . 50______________________________________compound in example 1 12 . 3 μmcompound in example 2 70 . 0 μmvitamin c 29 . 2 μmvitamin e 34 . 8 μmhydroquinone 25 . 4 μm______________________________________ as can be seen from the results in table 2 , the compound of example 1 , i . e ., 2 -( 3 , 4 - dihydroxycinnamoyl )- substituted kojic acid showed a strong activity as a radical scavenger . in order to examine the safety of the compounds of examples 1 and 2 , a human body patch test was carried out as follows : the brachium of thirty ( 30 ) healthy volunteers ( 15 females and 15 males ) was thoroughly washed with 70 % ethanol and applied with a plaster containing 20 μl of a conventional creamy preparation incorporating 0 . 2 wt . % of one of the compounds of examples 1 and 2 . the plaster was thoroughly bonded to the brachium using an adhesive tape . 48 hours later , the tape and chamber were removed , and the adherent site of the arm was wiped with a gauze to remove any remaining preparation . the site was observed for redness ( erythma ), swelling , and papuol . 48 hours later , the site was examined again . table 3______________________________________test preparations 48 hours later 96 hours later______________________________________control 0 / 30 0 / 30containing compd in ex . 1 1 / 30 0 / 30containing compd in ex . 2 1 / 30 0 / 30______________________________________ the results are shown in terms of the number of positive subjects / the number of total subjects . as can be seen from the results in table 3 , although the preparations containing the compounds of examples 1 and 2 show a weak positive response after 48 hours , these responses disappeared after 96 hours . accordingly , the compounds of examples 1 and 2 are safe to human skin .
kojic acid derivatives of the following formula ## str1 ## where r is a hydrogen atom or methyl group , as radical scavengers and strong inhibitors of tyrosinase , an enzyme involved in a melanin formation .