The key intermediate 9-(2,3,5,-tri-O-acetyl-beta-D-arabinofuranosyl)purine-6-carbonitrile (7) was synthesized in four steps from 9-beta-D-arabinofuranosylpurine-6-thione (3) via 6-(methylsulfonyl)-9-(2,3,5-tri-O-acetyl-beta-D-arabinofuranosyl)purine (6). Reaction of compound 7 with methanolic ammonia provided the rearranged compound 4-amino-8-(beta-D-arabinofuranosylamino)pyrimido[5,4-d]pyrimidine (8). Treatment of 7 with ammonium hydroxide and hydrogen peroxide provided 9-beta-D-arabinofuranosylpurine-6-carboxamide (9). Compound 7 was also treated with sodium hydrosulfide to yield 9-beta-D-arabinofuranosylpurine-6-thiocarboxamide (10). Similarly, 9-(2-deoxy-3,5-di-O-acetyl-beta-D-erythro-pentofuranosyl)purine 6-carbonitrile (17) was prepared from 6-chloro-9-(2-deoxy-beta-D-erythro-pentofluranosyl)purine (11) via 9-(2-deoxy-beta-D-erythro-pentofuranosyl)purine-6-thione. Compound 17 was converted into 4-amino-8-[(2-deoxy-beta-D-erythro-pentofuranosyl)amino]pyrimido[5,4-d]pyrimidi ne (18) and 9-(2-deoxy-beta-D-erythro-pentofuranosyl)purine-6-carboxamide (20), respectively. Compound 2 showed immunosuppressive activity and also inhibited the growth of L-1210 leukemia in mice. Arabinonucleoside analogues 8-10 were inactive when tested against RNA and DNA viruses in cell culture.