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. 2010 Sep 17;49(39):7096-100.
doi: 10.1002/anie.201003469.

Enantioselective addition of boronates to chromene acetals catalyzed by a chiral Brønsted acid/Lewis acid system

Philip N Moquist  1 , Tomohiro Kodama, Scott E Schaus
Affiliations

Enantioselective addition of boronates to chromene acetals catalyzed by a chiral Brønsted acid/Lewis acid system

Philip N Moquist et al. Angew Chem Int Ed Engl. .

Abstract

Chiral α,β-dihydroxy carboxylic acids catalyze the enantioselective addition of alkenyl- and aryl boronates to chromene acetals. The optimal carboxylic acid is a tartaric acid amide, easily synthesized via a 3-step procedure. The reaction is enhanced by the addition of Lanthanide triflate salts such as cerium(IV)-and ytterbium(III) triflate. The chiral Brønsted acid and metal Lewis acid may be used in as low as 5 mol % relative to acetal substrate. Optimization of the reaction conditions can lead to yields >70% and enantiomeric ratios as high as 99:1. Spectroscopic and kinetic mechanistic studies demonstrate an exchange process leading to a reactive dioxoborolane intermediate leading to enantioselective addition to the pyrylium generated from the chromene acetal.

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Figures

Figure 1
Figure 1
Natural product benzopyrans
Figure 3
Figure 3
Proposed catalytic cycle.
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