6-Chloro-9-[2',3'-dideoxy-3'-C-(hydroxymethyl)-.alpha.-D-erythro-pentofuranosyl]purine & 6-Chloro-9-[2',3'-dideoxy-3'-C-(hydroxymethyl)-.beta.-D-erythro-pentofuranosyl]purine 6-Cl Purine derivatives AIDS-044742 AIDS044742