Carbon-13 nuclear magnetic resonance spectra of some mesoionic xanthine analogs

Natural abundance ¹³C NMR chemical shifts have been experimentally determined for a series of mesoionic thiazolo3,2-a]pyrimidine-5,7-diones. The spectral data are compared with those of related mesoionic dihydrothiazolo3,2-a]pyrimidine-5,7-diones and mesoionic 1,3,4-thiadiazolo3,2-a]pyrimidine-5,7-diones. Resonable correlation between the observed ¹³C NMR chemical shifts and CNDO/2 total charge densities have been obtained for the different carbon atoms of 8-methylthiazolo3,2-a]pyrimidine-5,7-dione.