Nucleosides. Part 153. Synthesis of 1-Methyl-5-(3-azido-2,3-dideoxy-. beta.-D-erythro-pentofuranosyl)uracil and 1-Methyl-5-(3-azido-2,3- dideoxy-2-fluoro-β-D-arabinofuranosyl)uracil. The C-Nucleoside Isostere of 3′-Azido-3′-deoxythymi