To a solution of 1-methylcyclohexanecarboxylic acid (32.0 mg, 0.222 mmol) in DCM (1.5 mL) was added one drop of DMF (cat.), followed by oxalyl chloride (37.0 mg, 0.228 mmol), while at ambient temperature and under a constant stream of nitrogen. Upon cessation of gas evolution, the reaction mixture was cooled to 0° C. prior to the introduction of triethylamine (34.0 mg, 0.333 mmol) as a solution in DCM (2.0 mL), and then N-(3,5-difluorobenzyl)-2-{[(4S)-3-methyl-2,5-dioxo-1′,3′-dihydrospiro[imidazolidine-4,2′-inden]-5′-yl]amino}-2-oxoethanaminium chloride (50.0 mg, 0.111 mmol, Intermediate 1). After 1 hour the reaction mixture was applied directly to a silica gel column for purification, eluting with DCM:MeOH—99:1 to 95:5. Product containing fractions were pooled and concentrated in vacuo to give the title compound. MS: m/z=561 (M+23(Na+)). HRMS: m/z=539.2477; calculated m/z=539.2464 for C29H33N4O4F2.
