Development of a Commercial Manufacturing Route to 2‑Fluoroadenine, The Key Unnatural Nucleobase of Islatravir

We report the practical synthesis of a key fragment of islatravir (MK-8591), a novel nucleoside reverse transcriptase translocation inhibitor (NRTTI) currently under investigation for treatment and pre-exposure prophylaxis (PrEP) against HIV infection. The fragment, the unnatural nucleobase 2-fluoroadenine, is incorporated into MK-8591 via a biocatalytic aldol-glycosylation cascade, which imposes stringent requirements for its synthesis and isolation. Presented herein is the development work leading to a practical, scalable route from guanine, featuring a dual fluorination approach to a novel 9-THP-2,6-difluoropurine intermediate that enables a mild, highly selective, direct amination. This one-pot fluorination/amination sequence utilizes a direct isolation to deliver high purity 9-THP-2-fluoroadenine, which features ideal properties with respect to reactivity, solubility, and crystallinity. An acid-catalyzed liberation of 2-fluoroadenine in aqueous buffer delivers the appropriate purity profile to facilitate the enzymatic cascade to access MK-8591.