Studies on Fluorinated Pyrimidines. IV. Stereochemistry of 6-Alkoxy-5-fluoro-5, 6-dihydrouracils and 5-Alkoxycarbonyl-5-fluoro-6-substituted-5, 6-dihydrouracils

Both cis (6) and trans (7) isomers of 6-alkoxy-5-fluoro-5, 6-dihydrouracils were prepared by catalytic hydrogenolysis of t-6-alkoxy-r-5-halogeno-5-fluoro-5, 6-dihydrouracils (4 and 5). The structures were estimated by comparing coupling constants (JH6F) in the proton magnetic resonance (PMR) spectra and confirmed by a single-crystal X-ray analysis of cis-6-ethoxy-5-fluoro-5, 6-dihydrouracil (6a). The stereochemistry of 6-alkoxy-5-alkoxycarbonyl-5-fluoro-5, 6-dihydrouracils (11-13) was clarified similarly on the basis of the PMR data. The chemical behavior of these compounds in acidic and basic media was examined.