Synthesis optimization of AAR antagonists: shortening the route to accelerate discovery.

Adenosine A receptor (AAR) antagonists have emerged as promising candidates for cancer immunotherapy. In our previous work, we developed benzo[4, 5]imidazo[1, 2‑a]pyrazin-1-amine-3-amide derivatives, including compound 12o, which demonstrated promising pharmacodynamic properties but suffered from poor metabolic stability, with a microsomal half-life of approximately 30 min. To enable structural optimization of 12o and improve lead compound identification, we sought to streamline its lengthy 10-step synthetic route, which had hindered further analog development. Guided by step-economy principles, we developed a new 7-step synthesis of the central carboxylic acid intermediate, achieving a 33% increase in overall yield and reducing the E-factor by approximately four-fold. Key improvements included a one-step transformation of an acyl-protected hydroxymethyl group into a carboxylic acid using 1-hydroxycyclohexyl phenyl ketone (1-HCPK) and the replacement of the Boc protecting group with a more efficient PMB group. These modifications significantly enhanced the synthetic efficiency and sustainability of the route, providing a more robust platform for the development of stable and potent AAR antagonists.