Heterocyclic compounds are important in pharmaceutical applications, and researchers at the California Institute of Technology
(Caltech) (US) recently reported on an advance in the synthesis of such compounds. They reported on their work in the enantioselective
construction of quaternary N-heterocycles by palladium-catalysed decarboxylative allylic alkylation of lactams (3).
"We think it's going to be a highly enabling reaction, not only for preparing complex natural products, but also for making
pharmaceutical substances that include components that were previously very challenging to make," said Brian Stoltz, professor
of chemistry at Caltech, in a university press release. "This has suddenly made them quite easy to make, and it should allow
medicinal chemists to access levels of complexity they couldn't previously access.
Specifically, the researchers reported on the highly enantioselective palladiumcatalysed decarboxylative allylic alkylation
of lactams to form 3,3-disubstituted pyrrolidinones, piperidinones, caprolactams and structurally related lactams. The researchers
assert that the synthesis provides a new approach for the asymmetric synthesis of such structures, an important development
given the prevalence of quaternary N-heterocycles in biologically active alkaloids and pharmaceutical agents. The researchers
reported that the catalysis provided enantiopure quaternary lactams that intercept synthetic intermediates previously used
in the synthesis of the Aspidosperma alkaloids, quebrachamine and rhazinilam, but that were previously produced by chiral
auxiliary approaches or as racemic mixtures (3).
Advances in biocatalysis
Codexis, a biocatalysis company, reported last month on its work with Merck & Co. for developing an enzyme-based production
method for a key intermediate in the production of boceprevir, the API in Merck's Victrelis, a drug to treat hepatitis C.
The companies reported on a chemoenzymatic process for manufacturing the boceprevir bicyclic proline intermediate based on
amine oxidasecatalysed desymmetrisation. The key structural feature in boceprevir is the bicyclic proline moiety, which during
development stages, was produced by a classical resolution. As the drug candidate advanced, Codexis and Schering–Plough (now
Merck) jointly developed a chemoenzymatic asymmetric synthesis where the net reaction was an oxidative Strecker reaction.
The key part of the reaction sequence is an enzymatic oxidative desymmetrisation of a prochiral amine substrate (4).
According to Codexis, the new method increased chemical intermediate yield 150% over the previous process. It also reduced
raw material use by 60%, water use by 61% and overall process waste by 63%. Codexis used its proprietary CodeEvolver directed
evolution technology to develop the custom enzyme for use in the commercialscale manufacturing of the boceprevir intermediate.