The total synthesis of clitocine [6-amino-5-nitro-4-(beta-D-ribofuranosylamino)pyrimidine] (1), a nucleoside recently isolated from the mushroom Clitocybe inversa, has been accomplished. Glycosylation of 4,6-diamino-5-nitropyrimidine (4) with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose afforded the protected nucleoside 6-amino-5-nitro-4-[(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl) amino]pyrimidine (5) in good yield exclusively as the beta-anomer. Deprotection of 5 with NaOMe/MeOH gave 1 as an 11.5:1 mixture of the beta- and alpha-anomers, respectively. Recrystallization from MeOH, followed by chromatography, afforded 1 containing less than 1% of its alpha-anomer. X-ray crystal data revealed a planar aglycon moiety in clitocine with each oxygen atom of the nitro group intramolecularly hydrogen bonded to the hydrogen atoms of the two adjacent amino functions. Clitocine inhibited L1210 cells in vitro with an ID50 of 3 X 10(-8) M. Clitocine was also found to be a substrate and inhibitor of adenosine kinase with a Ki value of 3 X 10(-6) M.