1, 5, 6, 8-Tetradeoxy-8-diazo-~-~-erythro-7-octulofuranosylthymine (20) and 5, 6, 8-trideoxy- 8-diazo-@-~-ribo-7-octulofuranosyluracil (31), pyrimidine nucleosides containing the reactive diazomethyl ketone function, were prepared from thymidine and uridine via their 5 '- aldehydes 2 and 22 by the Wittig reaction with (carbalkoxy-methy1ene) triphenylphosphoranes followed by modification of the side chains of 3, 4, and 23 and ...