SYNTHESIS OF 1-(3-AZIDO-2,3-DIDEOXY-β-D-ribo-HEXOFURANOSYL)-, 1-(2,3-DIDEOXY-β-D-erythro-HEXOFURANOSYL)- AND 1-(2,3-DIDEOXY-β-D-erythro-HEX-2-ENOFURANOSYL)PYRIMIDINE NUCLEOSIDES