English: Prepn: M. A. Ondetti, D. W. Cushman, DE 2703828; eidem, US 4046889 and US 4105776 (1977, 1977, 1978 all to Squibb). Design and synthesis: M. A. Ondetti et al., Science 196, 441 (1977); D. W. Cushman et al., Biochemistry 16, 5484 (1977). Improved synthesis: D. H. Nam et al., J. Pharm. Sci. 73, 1843 (1984).