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Synthesis

Single-Enantiomer Dearomatization Tool Set For Chiral Action

by Stephen K. Ritter
March 9, 2015 | APPEARED IN VOLUME 93, ISSUE 10

If Spanish architect Antoni Gaudí had designed a transition-metal complex, it would have looked like [tris(pyrazolyl)borate]W(NO)[P(CH3)3](η2-benzene). As gaudy as this molecule appears, chemists have found it to be a versatile synthetic tool for binding aromatic molecules such as phenols, pyridines, pyrroles, and naphthalenes and dearomatizing them, a key step in building the core structures of natural products and other bioactive compounds. One drawback to the tungsten reagent is that it’s prepared as a racemic mixture, so the products obtained using it are racemic mixtures that must be separated before continuing. A research team led by W. Dean Harman of the University of Virginia has solved that problem by devising a method to isolate the (R) and (S) versions of the tungsten complex so they can be used in enantioselective dearomatizations (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b00490). The researchers first protonated the dimethoxybenzene analog of the tungsten complex with L- or D-dibenzoyltartaric acid, which enabled them to selectively precipitate a single diastereomeric salt of the complex. They then redissolved and deprotonated the complex and exchanged the dimethoxybenzene ligand for benzene to obtain the tungsten complex in its pure (R) or (S) form.

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