The present invention relates to processes of preparing PD-1/PD-L1 inhibitor (R)-1-((7-cyano-2-(3′-(3-(((R)-3-hydroxypyrrolidin-1-yl)methyl)-1,7-naphthyridin-8-ylamino)-2,2′-dimethylbiphenyl-3-yl)benzo[d]oxazol-5-yl)methyl)pyrrolidine-3-carboxylic acid, or salts thereof, related synthetic intermediates, and salts of the intermediates, where the PD-1/PD-L1 inhibitor is useful in the treatment of various diseases including infectious diseases and cancer.