CHEMICAL CONSTITUTION 



Since the methylimino group was not present in riboflavine the hy- 

 droxylated side-chain must be attached to the methylene group. This 

 was supported by the observation of R. Kuhn and F. Bar ^ that the 

 photolysis of riboflavine was closely simulated by the behaviour of 

 2-tetrahydroxybutyl-quinoxaline, which on irradiation lost its 

 hydro xylated side-chain. Just as lumiflavine had the same absorption 

 spectrum as riboflavine so the product, quinoxaline, had the same 

 absorption spectrum as its tetrahydroxybutyl derivative. 



Isoalloxazines 



Next, Kuhn and Weygand^ synthesised 9-methyl-isoalloxazine, 

 m.p. 392° C. by boiling N-methyl-o-phenylene diamine with alloxan in 

 hydrochloric acid solution ; it had properties closely resembling those 

 of Imniflavine. Thus, with sodium dithionite, it yielded a leuco- 

 derivative that was reconverted to a fluorescent substance on exposure 

 to air. Its absorption spectrum was similar to that of riboflavine and 

 it gave urea when boiled with baryta solution ; the product was 

 i-methyl-2-keto-i ; 2-dihydroquinoxaline-3-carboxylic acid. In their 

 next papers, Kuhn et al. described the preparation of 3 : g-dimethyl- 

 isoalloxazine from N-methyl-o-phenylene diamine and methyl alloxan,^® 

 and the synthesis of lumiflavine itself, m.p. 330° C, from 2-methyl- 

 amino-4 : 5-dimethylaniline and alloxan, ^^ thereby proving it to be 

 6:7: 9-trimethyl-isoalloxazine (II). 



The complete synthesis was described in a subsequent paper by 

 R. Kuhn and K. Reinemund ^^ and is similar to that used for ribo- 

 flavine (see page 140). 



R. Kuhn and H. Rudy ^^ isolated another degradation product of 

 riboflavine resulting from the treatment of lumiflavine with alkali, 

 and identified it as 6 : 7-dimethyl-alloxazine, which they synthesised 

 from 3 : 4-dimethyl-o-phenylene diamine and alloxan. Both the 

 synthetic substance and the product from riboflavine yielded 1:3:6:7- 

 tetramethyl-alloxazine on treatment with diazomethane. 



A photolytic product was also obtained by Karrer et al.,^^ by 

 exposure of a fiavine solution to sunlight with access of air. Lumi- 

 chrome, as they called the substance, had a characteristic absorption 

 spectrum, gave a yellowish-green fluorescence in alkaline solution and 

 a sky-blue fluorescence in aqueous or alcoholic solution, and was 

 identified as 6 : 7-dimethyl-alloxazine. Karrer et al. also obtained 

 lumiflavine, and showed that it was the 9-methyl derivative of lumi- 

 chrome. They postulated that riboflavine was an N-tetrahydroxy- 

 amyl-isoalloxazine, since the side-chain split off in the formation of 

 lumichrome gave a strong positive pentose reaction. 



137 



