SYNTHESIS OF 1-(2,3-DIDEOXY-4-C-METHYL-β-D-glycero-PENT-2-ENOFURANOSYL)THYMINE, 1-(2,3-DIDEOXY-4-C-METHYL-β-D-glycero-PENTOFURANOSYL)THYMINE AND 1-(4-C-AZIDOMETHYL-2-DEOXY-β-D-threoPENTOFURANOSYL)THYMINE