Search for a simpler synthetic model system for intramolecular 1,3-dipolar cycloaddition to the 5,6-double bond of a pyrimidine nucleoside

In search for a simpler model system for the study of intramolecular thermal reactions between the base and 5'-functionalized sugar moiety in nucleosides, 1-(3-azidopropyl)uracil (2), 1-(4-azidobutyl) pyrimidines (12 and 13) and 1-(5-azidopentyl)-uracil (14) was synthesized through the corresponding ω-benzoyloxy-(6,7 and 8) and ω-hydroxyalkyl-pyrimidines (9,10 and 11). Heating 2 gave 1,N⁶-trimethylene-6-aminouracil (4), while heating 12 and 13 gave N¹-C₆ cleaved addition products. 15 and 16, respectively. 15 was regiospecifically transformed to 1,2,3-triazole derivatives, 17,18 and 19. Heating 1-(4-azidobutyl)-5-bromouracil (20) yielded 3,9-tetramethylene-8-azaxanthine (22). 9 with NBA gave 1,0⁶-tetramethylene-5-bromo-6-hydroxy-5,6-dihydrouracil (24) and the 5-brominated analog of 9 (25). The 4-functionalized butyl side chain proved to serve as a substitute for the 5'-functionalized sugar moiety in pyrimidine ribonucleosides.