Studies on the biosynthesis of β-lactam antibiotics. Part II. Synthesis, and incorporation into penicillin G, of (2RS,2′RS,3R,3′R)-[3,3′-3H2]cystine and (2RS,2′RS,3S,3′S)-[3,3′-3H2]cystine

Abstract

n-Butyl glycidate with sodium phenylmethanethiolate gave 3-benzylthio-2-hydroxypropanoic acid (70%). In contrast, ethyl 2,3-epoxybutyrate underwent α-attack by phenylmethanethiolate yielding, after methylation, methyl 2-benzylthio-3-hydroxybutyrate (25%) and ethyl (benzylthio)acetate (70%). n-Butyl[cis-2,3-²H₂]acrylate (synthesized by reduction with deuterium of n-butyl 9,10-dihydro-9,10-ethenoanthracene-11-carboxylate, followed by thermolysis of the [²H₂]adduct) was epoxidised stereospecifically with trifluoroperacetic acid to yield n-butyl[cis-2,3-²H₂]glycidate, which with phenylmethanethiolate gave, stereospecifically, DL-3-benzylthio-2-hydroxy[cis-2,3-²H₂]propanoic acid. The analogous tritium compound, obtained similarly, was resolved via the brucine and quinine salts to afford the (2S,3S)-acid (7c) and the (2R,3R)-enantiomer (14). These were converted into the title compounds by bromination, treatment with ammonia to give the corresponding S-benzyl-cysteines, equilibration of the α-tritium atoms with acetic anhydride–acetic acid followed by acidic hydrolysis, and finally reduction with sodium–liquid ammonia and oxidation in air of the resultant (3R)- and (3S)-[3-³H]-cysteines.

The title compounds, mixed with [3,3′-¹⁴C]cystine, were incubated with Penicillium chrysogenum, and the resultant benzylpenicillin was isolated as the N-ethylpiperidine salt. The ³H : ¹⁴C ratio of the salt indicated loss of the 3-pro-S and retention of the 3-pro-R hydrogen atom of cystine in the biosynthesis of penicillin. Thus the β-lactam ring is formed with retention of configuration at the β-carbon atom of cysteine.