FUBIN BASES. 339 



wise all known. One tetra-methyl uric acid is possible and is 

 known but in addition an isomer exists in methoxy-caffein, 



Xanthin. — The successful synthesis of xanthin by Fischer (1897) 

 has shown that this interesting base is 2-6-di-oxypurin and as such 

 possesses the formula originally deduced by Medicus. A considera- 

 tion of the purin formula will show that three di-oxypurins are 

 possible. Xanthin therefore has two isomers, 6-8 and 2-8 dioxy- 

 purins, the latter of which is as yet unknown although methyl de- 

 rivatives have been prepared. 



H2!f— CO HN— CO HN— CO 



C— NH NH,.C 0— OT NH,.C 0—: 



OC 



in 



-I 



II r ' 



HN— C— N N— O-N N- 



Xanthin. Guahih. KpionAHiH os 



7-Mkthyl QnANiN. 



The methyl derivatives of xanthin are of great interest inasmuch 

 as they make up the greater part of the natural purin bases. By 

 replacing a hydrogen atom in either one of the three imido groups 

 1, 3, or 7, in xanthin by a methyl group a mono-methyl xanthin 

 resiJts. AH three of the possible mono-methyl xanthins are known. 

 The 1-methyl xanthin was discovered in urine by Kruger and Salo- 

 mon in 1897, but as yet it has not been synthesized. Heteroxanthin, 

 which is 7-methyl xanthin (Kruger and Salomon 1895), was likewise 

 isolated from urine by Salomon in 1884. The 3-methyl xanthin has 

 been isolated from the urine after administration of large doses of 

 oaffein. 



OH, 



HN-npo 



OC Q— ]s 



HN— O— N 



1-Metuyl Xanthin. d-METHTL Xanthin. Heteroxanthin ob 



7-Mkthyi, Xanthin. 



Only three di-methyl xanthins are possible and these have been 

 known to exist in nature for many years. They are theobromin, the 

 active principle of theobroma cacao ; theophyllin, which exists in tea, 

 and paraxanthin which occurs in the urine. The relationship which 

 exists between these three isomers is readily seen from the formulse, 



CH, 



:,.N-CO CH,.N— CO HN-CO 



OC C— NH OC C— N.CHj OC O-N.CH, 



in I II CH ' " -'- 



■IX-i 



-H HN— 0-N CH3. 



Theophyllin or Paraxanthin or Tbeobbouin or 



1-8 Di-MSTHYL Xanthin. 1-7 Di-uethyl Xanthin. 3-7 Di-methyl Xanthin. 



