Synthesis of Classical, Three-Carbon-Bridged 5-Substituted Furo[2,3-d]pyrimidine and 6-Substituted Pyrrolo[2,3-d]pyrimidine Analogues as Antifolates

Bridge homologation of the previously reported classical two-carbon-bridged antifolates, a 5-substituted 2,4-diaminofuro[2,3- d ]pyrimidine ( 1 ) [which is a 6-regioisomer of LY231514 (Alimta)] and a 6-subsituted 2-amino-4-oxopyrrolo[2,3- d ]pyrimidine, afforded the three-carbon-bridged antifolates analogues 4 and 5, with enhanced inhibitory activity against tumor cells in culture (EC 50 values in the 10 -8 −10 -7 M range or less). These two analogues were synthesized via a 10-step synthetic sequence starting from methyl 4-bromobenzoate ( 14 ), which was elaborated to the α-chloromethyl ketone ( 8 ) followed by condensation with 2,6-diamino-pyrimidin-4-one ( 7 ) to afford the substituted furo[2,3- d ]pyrimidine 9 and the pyrrolo[2,3- d ]pyrimidine 10 . Subsequent coupling of each regioisomer with diethyl- l -glutamate followed by saponification afforded 4 and 5 . The biological results indicate that elongation of the C8−C9 bridge of the classical 5-substituted 2,4-diaminofuro[2,3- d ]pyrimidine and 6-substituted 2-amino-4-oxopyrrolo[2,3- d ]pyrimidine are highly conducive to antitumor activity in vitro, despite a lack of increase in inhibitory activity against the target enzymes. This supports our original hypothesis that truncation of the B-ring of a highly potent 6−6 ring system to a 6−5 ring system can be compensated by bridge homologation to restore the overall length of the molecule.