The condensation of acyl chloride (I) with hexahydropyridazine (II) gives the corresponding amide (III), which is debenzylated with H2 over Pd/C and cyclized by means of SOCl2 to yield the pyridazinodiazepinedione (IV). Selective reduction of (IV) with borane in THF affords the pyridazinodiazepinone (V), which is treated with hydrazine to provide (1S,9S)-9-amino-10-oxoperhydropyridazino[1,2-a]azepine-1-carboxylic acid tert-butyl ester (VI). Finally, this compound is condensed with 4-phenyl-2(R)-(trifluoromethanesulfonyloxy)butyric acid ethyl ester (VII) and selectively hydrolyzed to afford the target carboxylic acid.