This application relates to processes and intermediates for the preparation of the PD-1/PD-L1 inhibitor (R)-1-((7-cyano-2-(3′-((2-(difluoromethyl)-7-((3-hydroxypyrrolidin-1-yl)methyl)pyrido[3,2-d]pyrimidin-4-yl)amino)-2,2′-dimethyl-[1,1′-biphenyl]-3-yl)benzo[d]oxazol-5-yl)methyl)piperidine-4-carboxylic acid, and salts and crystalline forms thereof, where the PD-1/PD-L1 inhibitor and solid forms and salt forms thereof are useful in the treatment of various diseases including infectious diseases and cancer.