Contradictory Diels-Alder Reactions and Stereoselective “One-pot” Synthesis of Building Blocks for Antibiotics

To date, antibiotics are still the most effective weapon for treatment of bacterial infec-tions in clinic. By looking into the molecular structures of antibiotics closely, most of them have a core structure that is referred to as “Polypropionates” or “Skipped Pol-yols” with multiple consecutive carbon chiral centers. The chemical expression of this core structure is “-*CH(OH)-*CH(CH3)-*CH(OH)-*CH(CH3)-”. For each chiral center, it has two possible 3D orientations, therefore, for a structure with four chiral centers, there exists 16 possible stereoisomers (2 x 2 x 2 x 2), yet, more than likely, only one out of these 16 structures processes the power against bacteria. Therefore, it has become a challenge for chemists to find novel ways to synthesize such core structures in lab.

Dr. Shang-Cheng Hung’s team has made the record breakthrough of this quest within the organic chemistry domain by successfully synthesizing batches of “Polypropio-nates” using one-pot method lately. In the article published in Nature Communica-tions, they reported how they develop a novel and straightforward methodology to conduct three steps, [4+2] cycloaddition, acidic hydrolysis, and oxygen insertion, re-peatedly, and successfully derived the core structure in one shot. Their way of mak-ing a cyclic molecule first, and then break the ring by oxidation to yield the linear compound, make it possible to control 4 chiral centers at the same time, and deliver the dynamic multi alcohol base elements. This innovative approach makes it possible to develop the basic elements for a variety of antibiotics effectively.

When the team was working on the reaction of ‘[4+2] cycloaddition’, they were puz-zled by a phenomenon that absolutely contradicted to what every learned chemist would predict the facial addition of a diene with a dienophile. ‘[4+2] Cycloaddition’ is commonly referred to as the Diels-Alder Reaction, and by the book, due to the ‘Secondary Orbital Interaction’, the resulting product should be an inwardly turning “endo-Addition.” However, Hung’s team has discovered that when the -carbon of the dienophiles with various electron-withdrawing groups has a substituent, regardless the conditions are direct heating or catalyzed by Lewis acids, the outcome can be an exo-addition. By overturning the set rule in the text books of organic chemistry, they have provided a much wider scope for scientific explorations down the road. This revolutionary finding was published in Scientific Reports back in late 2016.

Both studies, funded by the Ministry of Science and Technology and Academia Sinica, were mainly carried out by Dr. Guo-Ming Ho. He is currently doing postdoctoral re-search in the Technion-Israel Institute of Technology. The research publication, enti-tled “Stereoselective one-pot synthesis of polypropionates”, can be read online at the Nature Communications website:https://www.nature.com/articles/s41467-017-00787-y
The anti-Diels-Alder report “Unconventional exo selectivity in thermal nor-mal-electron-demand Diels-Alder reactions” can be read online at the Scientific Re-ports website: (https://www.nature.com/articles/srep35147)。

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