Pummerer-type Cyclization of Arnstein Tripeptide Analogs Induced by O-Silylated Ketene Acetals: Studies of Penicillin Biosynthesis
摘要:
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.
The first successful biomimeticconversion of an Arnstein tripeptide analogue into the cis-β-lactam via a silicon-induced Pummerer-type rearrangement is described.