A Detailed Investigation of The Methylation Reaction of A 5-Bromopyrimidine Nucleoside

5-Bromo-2′, 2-anhydrouridine was methylated using NaH in DMSO and methyl iodide as methylating reagent. Applying two or more equivalents of the methylating reagent a Br to CH 3 substitution took place at C5 of the pyrimidine ring and besides the N, O-dimethyl-epoxide 9 the N-methyl-5-methyl-dianhydro compound 16 could be isolated 1 . Under drastic conditions a N, O, C-trimethyl-epoxide 15 was formed, the structure of which was determined by X-ray crystallography. Compounds 9 , 15 and 16 were formed in different ratios depending on the reaction conditions applied. Based on these facts a mechanism is suggested for this reaction.