Chiral proton donor reagents: Tin tetrachloride-coordinated optically active binaphthol derivatives

The Lewis acid-assisted chiral Bronsted acids (chiral LBAs), which are prepared from tin tetrachloride and optically active binaphthol derivatives, are highly effective chiral proton donor reagents for enantioselective protonation and biornimetic polyene cyclization. These chiral LBAs can directly protonate various silyl enol ethers and ketene disilyl acetals to give the corresponding alpha-aryl or alpha-halo ketones and alpha-arylcarboxylic acids, respectively, with high enantiomeric excess (up to 98% ee). A catalytic version of enantioselective protonation was also achieved using stoichiometric amounts of 2,6-dimethylphenol and catalytic amounts of monomethyl ether of optically active binaphthol in the presence of tin tetrachloride. The biornimetic cyclization of simple isoprenoids to polycyclic isoprenoids using chiral LBA is also described. This is the first example of a chiral Bronsted acid-induced enantioselective ene cyclization in synthetic chemistry. Geranyl phenyl ethers, o-geranylphenols, and homogeranylphenol derivatives were directly cyclized in the presence of (R)binaphthol derivatives and tin tetrachloride (up to 90% ee). Compounds bearing a farnesyl group could also be cyclized under the same conditions to give the natural products (-)-ambrox(R) and (-)chromazonarol, and (-)-tetracyclic polyprenoids of sedimentary origin. These chiral LBAs recognize the prochiral face of a trisubstitured terminal olefin and site selectively generate carbocations on the substrates. (C) 2002 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.