FYRIMIDINES. 



The cyclic ureides derived from urea and acids containing three 

 carbon atoms (p. 280) are heterocyclic compounds. Those cyclic 

 ureides which are derived from urea and the unsaturated acids, acrylic 

 acid, methylacrylic acid, crotonic acid, form the group of compounds 

 termed the pyrimidines. They have the structure : 



1 N c 6 



I I 



2 C C 5 



I y 



3 N-C 4 



To this group belong the three compounds, thymine, uracil and 

 cytosine which are constituents of nucleic acid : 



N = CH HN CO N = C . NH 2 HN CO 



II- II I ! II 



HC CH OC C.CHj OC CH OC CH 



II II I II | II I II 



N CH HN CH HN CH HN CH 



Pyrimidine. Thymine, or Cytosine, or Uracil, or 



5-methyluracil, 6-amino- 2, 6-dioxy- 



or 2-oxy- pyrimidine. 



5-methyl, 2, 6- pyrimidine. 

 dioxypyrimidine. 



The ring contained in these compounds is a portion of the ring 

 structure of the purines and it was first supposed that cytosine and uracil 

 were decomposition products of adenine and guanine, but it has been 

 definitely proved that these compounds are not secondary products 

 ancl that they are part of the molecule of nucleic acid (p. 299). 



Preparation. 



These three compounds have been prepared by synthesis and their con- 

 stitution established (see W. Jones' monograph on Nucleic Acid). They are 

 more readily prepared from nucleic acid. Plant nucleic acid yields uracil and 

 cytosine ; animal nucleic acid yields thymine and cytosine. TKe following 

 method is given by W. Jones : 



50 gm. of nucleic acid are hydrolysed by heating in an autoclave with 

 250 c.c. of 25 per cent, sulphuric acid for 5 hours at 150-160. The solution 

 is diluted with water to 1000 c.c. and hot saturated baryta is added in excess 

 to remove phosphoric and sulphuric acids. The excess of baryta is removed 

 with carbon dioxide. The yellow solution is evaporated to about 400 c.c. and 

 faintly acidified with nitric acid. The purines, which are precipitated, are 

 filtered off. Silver nitrate is added to precipitate the remainder of the purines. 

 To the clear yellow filtrate silver nitrate is added .in small portions until a 

 test portion gives a yellow coloration with a drop of baryta on a watch glass. 

 The solution is made faintly alkaline with baryta and the precipitate of pyrimi- 

 dine silver compounds is filtered off. The subsequent treatment depends on 

 whether thymine and cytosine are being prepared from animal nucleic acid or 

 uracil and cytosine from plant nucleic acid. 



282 



