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Lab-made catalysts increase chiral yields.

When chemists make new molecules, they often produce mixtures that include several versions of a product. Each version, or isomer, contains the same atoms connected in much the same way. But in one isomer, a chemical side group might point up, for example, while in another, that same side group might point down. This property, called chirality, can greatly affect how the molecule functions; it can even turn a useful medication into a harmful one.

So, taking a lesson from nature -- catalytic enzymes produce specific isomers -- scientists have developed ever more selective reactions.

Using a lab-made catalyst formed by an osmium compound and a modified plant alkaloid, organic chemists have produced complex chiral molecules from nonchiral starting materials. This catalyst can oxidize squalene, a 30-carbon molecule that liver enzymes convert into hormones, says K. Barry Sharpless of Scripps Research Institute in La Jolla, Calif.

The multistep reaction produces surprisingly high yields of just one isomer of the final molecule -- not a mix of the 36 possible isomers -- Sharpless and his colleagues report in the Jan. 1 SCIENCE.

During the 1980s, Sharpless discovered that he could use this catalyst to control the addition of two hydroxyl side groups to olefins -- molecules with carbon atoms connected by double bonds. Depending on the version of the alkaloid used, the catalytic reaction causes hydroxyl side groups to attach either above or below the plane of olefin's carbon atoms.

The work with squalene "extends [the use of the catalyst] to multiple double bonds," says Charles M. Zepp, a chemist with Sepracor, Inc., in Marlborough, Mass. "It's a much more general chiral synthesis than almost anything."

The catalyst causes each of squalene's six double bonds to break so that each doubly bonded carbon atom can then take on a hydroxyl. The side group attaches in a particular orientation, depending on the isomer of the alkaloid used.

Sepracor plans to use this process, called asymmetric catalysis, to produce starting compounds for pharmaceutical manufacturers. Sharpless thinks it will also provide shortcuts to new drugs, including the anticancer drug taxol (SN: 4/18/92, p.244).
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Author:Pennisi, Elizabeth
Publication:Science News
Article Type:Brief Article
Date:Jan 2, 1993
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