Bronsted acid-assisted chiral Lewis acids BLAs

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

Yamamoto and co-workers have introduced a conceptually interesting series of catalysts that incorporate an acidic proton into the active catalyst. Termed Bronsted acid-assisted chiral Lewis acid (BLA), catalyst 14 selectively catalyzes a number of diene-aldehyde cycloadditions reactions (Scheme 16) [67]. While extremely selective for the substrates shown, no aldehydes lacking an a-substitu-ent were reported to be effective in this reaction. This feature was addressed in... 

Since the first report of chiral oxazaborolidine-based Bronsted acid-assisted chiral Lewis acid (BLA) for enantioselective Diels-Alder reactions by Corey and coworkers... 

Bronsted acid assisted chiral Lewis acid catalysts (BLA) are designed for the asymmetric Diels-Alder reaction [11]. Both BLA (17) [11] and (18) [12] were employed in... 

Ishihara and Yamamoto et al. have found that chiral boron ate complexes prepared from chiral tetraols and BHj THF or B(OR)j give new catalysts [Bronsted acid-assist-ed chiral Lewis acids (BLA)] in enantioselective synthesis, which achieve selectivity... 

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