Xanthine69-89-631 % similar 4,5-Diamino-2,6-dimercaptopyrimidine31295-41-731 % similar 6-Aminouracil873-83-631 % similar 5-Chlorouracil1820-81-130 % similar 5-Bromouracil51-20-730 % similar Thymine65-71-430 % similar 5-Iodouracil696-07-130 % similar