Synthesis and biological evaluation of certain 3-beta-D-ribofuranosyl-1,2,4-triazolo[4,3-b)pyridazines related to formycin prepared via ring closure of pyridazine precursors

All three amino-substituted 3-beta-D-ribofuranosyl-1,2,4-triazolo4,3-b]pyridazines (5, 19, and 20) structurally related to formycin A were prepared and tested for their antitumor and antiviral activity in cell culture. Dehydrative coupling of 4-amino-5-chloro-3-hydrazinopyridazine (7) with 3,4,6-tri-O-benzoyl-2,5-anhydro-D-allonic acid (6) in the presence of DCC and subsequent thermal ring closure of the reaction product (8) provided 8-amino-7-chloro-3-(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl)- triazolo4,3-b]pyridazine (9). Dehalogenation of 9, followed by debenzoylation, gave the formycin congener 8-amino-3-beta-D-ribofuranosyl-1,2,4- triazolo4,3-b]pyridazine (5). Similar condensation of 5-amino-4-chloro-3-hydrazinopyridazine (13) with 6 and dehalogenation of the cyclized product (16), followed by debenzoylation, gave the isomeric 7-amino-3-beta-D-ribofuranosyl-1,2,4- triazolo4,3-b]pyridazine (19). DCC-mediated coupling of 6 with 6-chloro-3-hydrazinopyridazine (12), followed by ammonolysis of the cyclized product (21) with liquid NH3, provided a convenient route to 6-amino-3-beta-D-ribofuranosyl-1,2,4-triazolo4,3-b]pyridazine (20). The structural assignment of 5 was made by single-crystal X-ray diffraction analysis. Compounds 5, 19, 20, and certain deprotected nucleoside intermediates were evaluated against L1210, WI-L2, and CCRF-CEM tumor cell lines, as well as against DNA and RNA viruses in culture. These compounds did not exhibit any significant antitumor or antiviral activity in vitro.