448 D. O. JORDAN 



which have been studied. Clews and Cochran' -^ have determined the struc- 

 O O NH2 NH2 



C C CH3 C C CHs 



/\ / \ / ^ \ ^ \ / 



HN CH HN C N CH N C 



I II I II I II I II 



C CH C CH C CH C CH 



^ \ / ^ \ / /\/ /\/ 



ON ON ON ON 



H H H H 



I II III IV 



ture of 2-amino-4-methyl-6-chloropyrimidine, 2-amino-4 , 6-dichloropyrimi- 

 dine, 4-amino-2,6-dichloropyrimidine, and 5-bromo-4,6-diaminopyrimi- 

 dine and their results are shown in Fig. 1. Clews and Cochran conclude 

 that the pyrimidine ring is planar, in agreement with the result of Schneider^ 

 obtained from dipole moment measurements. The C^ — N and C — C bond 

 distances in the pyrimidine ring of these derivatives correspond to approxi- 

 mately 50 % double bond character, as would be expected for a six-mem- 

 bered ring in which resonance of the benzene type is possible. Whether 

 resonance of the benzene type can occur in the amino- and hydroxypyrimi- 

 dines which are found in nucleic acids is dependent on whether these groups 

 exist in the amino or imino and the enol or keto forms, respectively. 



An amino group in position 6(4) of the pyrimidine nucleus may show 

 greatly reduced amino behavior owing to the possibility of a tautomeric 

 change converting the 6-aminopyrimidine (V) into the iminodihydropyrimi- 

 dine form (VI). The X-ray evidence shows that 4-amino-2,6-dichloro- 

 pyrimidine and 5-bromo-4 , 6-diaminopyrimidine^ are in the amino form 



NH2 NH 



I II 



C C 



/ \ / \ 



N CH HN CH 



II I I II 



HC CH HC CH 



\ Z' \ / 



N N 



V VI 



in the crystal and it is therefore probable that the structures of cytosine 

 and of 5-methylcytosine are analogous. 



' C. J. B. Clews and W. Cochran, Acta Cryst. 1, 4 (1948). 



2 C. J. B. Clews and W. Cochran, Ada Cryst. 2, 46 (1949). 



3 W. C. Schneider, J. Am. Chem. Soc. 70, 627 (1948). 



