54 UNITY AND DIVERSITY IN BIOCHEMISTRY 



(k) Alloxazine Derivatives 

 The double heterocyclic ring of lumazine, 



/N. NH 



HC ' C 2 CO 



HC « C 3 NH 



^ N ^ ^ CO 

 condensed with a benzene ring forms the alloxazine nucleus 



H H 



HC7 C C 2 CO 



HC6 /^ rl^ ' 



HC 6 c C 3 NH 



CH 



N 



this has been known for over half a century and the method of synthesis by 

 condensation of an aromatic o-diamine with alloxan (Kuhling's sythesis) 

 has also been known for a considerable time. 



NH 



H 



N 



/ \ 



' OC CO 



-t- I I 



alloxazine + 2 H2O 



NHj OC NH 



\ / 



C 



o 



It was the great achievement of Richard Kuhn and his fellow workers 

 and then of Karrer and co-workers to show that a very important series of 

 natural substances contained a tautomeric form of alloxazine, isollaoxazine 

 or flavin, a substance which is unstable in the free state. 



~ H H 



CNN 



HC7 C C 2 CO 



I 11 I I 



HC 6 C C 3 NH 



C N C 



H O 



Isoalloxazine or Flavin (unstable) 



