SYNTHESIS OF 1-(3-AZIDO-2,3-DIDEOXY-4-C-HYDROXYMETHYL-α-L-threo-PENTOFURANOSYL)THYMINE, 1-(2,3-DIDEOXY-4-C-HYDROXYMETHYL-α-L-glycero-PENTOFURANOSYL)THYMINE AND 1-(2,3-DIDEOXY-4-C-HYDROXYMETHYL-α-L-glycero-PENT-2-ENOFURANOSYL)THYMINE