Overcoming dangerous antibiotic-resistant bacteria

replace an amide (carbonyl, RC=O linked to an amine) with an ester (a carbonyl, RC=O linked to an oxygen, O).

This one atom change changes the entire game and renders vancomycin ineffective. Until now.

The release notes that, like magnets, molecular interactions can be attractive (oppositely charged) or repulsive (identically charged). What chemists in the Boger lab have done is made this key interaction no longer repulsive, but attractive.

So now the new vancomycin analogue can grab hold of the mutant peptidoglycan, and again prevent the bacteria from making the cell wall and killing the resistant bacteria. What is so remarkable about the design, however, is that the redesigned antibiotic maintains its ability to bind the wild type peptidoglycan as well.

Changing the properties of a key amide at the core of the natural products structure required a new synthetic strategy that only the most talented chemists could achieve in the lab. The preparation of the entire structure took a great deal of time and a fresh approach.

The new compound has an amidine (an iminium, RC=NH+ linked to a nitrogen, N) instead of an amide at a key position buried in the interior of the natural product. However, to install such a functional group, the chemical properties of the amide carbonyl were not useful, as the natural product has seven of them.

Instead, the group relied on the chemical properties of sulfur (S), oxygen’s downstairs neighbor in the periodic table, to install the desired nitrogen. To do this, a second analogue was prepared in which the key amide was chemically altered to a thioamide. “The thioamide allowed us to make any modification at the residue 4 amide that we would like to make, such as the amidine, but we could also make the methylene analogue,” said Boger citing work published in another paper (B. Crowley and D. L. Boger, Journal of the American Chemical Society 128: 2885-92). “And there are other modifications that we are making at the present time that we haven’t disclosed. We are just getting to that work.”

The most fundamental finding in the synthesis was that the installation of the amidine could be done in the last step, as a single-step conversion, on the fully unprotected thioamide analogue. Providing an elegant and novel approach to the analogue, which contrasts other published multistep procedures. This chemical behavior was, as Boger said, “an astonishing result as there are no protecting groups and it is a single step reaction… in the end it was the simplest and most straightforward way to prepare the amidine.”

Although it is still at its early stages and there is much work ahead. Currently, the only route known to make the new antibiotic is the one published by Boger and his co-workers, which presently provides laboratory amounts of the compound. So Professor Boger now looks forward and will continue to investigate the “host of alternative approaches” for the preparation of the molecule “such as reengineered organisms to produce the material or semi-synthetic approaches to the analogue. That is going to be part of the next stage of the work.”

The work was funded by the U.S. National Institute of Health (CA041101) and the Skaggs Institute for Chemical Biology.