A practical synthesis of the purine nucleoside phosphorylase (PNP) inhibitor BCX-4208 (1) was accomplished in three telescoped steps. Mannich condensation of the 4-benzyloxy-9-deazahypoxanthine with (3R,4R)-3-hydroxy-4-(hydroxymethyl)pyrrolidine and formaldehyde followed by removal of the protecting group and crystallization furnished the desired product as a hydrochloride salt in 85% overall yield and 99.8% purity. A scalable synthesis of 9-deazahypoxanthine is also reported.

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Forodesine HCl is being investigated as a potential therapeutic target for the control of T-cell proliferation. During our ongoing process development work on forodesine HCl several novel compounds were identified as possible impurities in the process. Herein we present the synthesis of three novel compounds (2-4).

BCX-4208, a novel inhibitor of the enzyme purine nucleoside phosphorylase, mimics the charged ribosyl oxocarbenium ion formed during the transition state of the enzyme-catalyzed C-N bond cleavage of nucleosides. A slow-onset, tight-binding inhibitor with a Ki(*) of 16 +/- 1.4 pM, BCX-4208 is one of the most potent inhibitors known for the enzyme. In support of our BCX-4208 clinical program, a mass spectrometric assay has been developed that required labeled BCX-4208 as an internal standard. The synthesis of [(2)H](2)-BCX-4208 and [(13)C]-BCX-4208 is described in this report.