We report the first biomimetic conversion of Arnstein tripeptide analogues (1a and 1b) into cis beta-lactams (2a and 2b) using 0-silylated ketene acetal (3) involving asymmetric induction from the sulfoxide sulfur to the alpha-carbon. The peptide 1 was treated with 3 at room temperature in the presence of a catalytic amount of ZnI2 in MeCN to give cis-2, trans-2, and alpha-siloxysulfide (7). Reaction of R-1 with 3 gave cis-2 predominantly, and S-1 gave a mixture of cis-2 and trans-2. High cis selectivity was obtained by the use of a large volume of solvent and was strongly influenced by the absolute stereochemistry of the sulfoxide, the cysteinyl amino group, and the volume of solvent. The cis beta-lactams (2a,b) were obtained preferentially from R-1a,b. These chemical transformations strongly support Baldwin’s mechanism which involves the initial formation of the cis beta-lactam by the Pummerer-type cyclization of the Arnstein tripeptide in penicillin biosynthesis and provide useful information on the first key step in penicillin biosynthesis.