Chiral Proton Donor Reagents: Tin Tetrachloride—Coordinated Optically Active Binaphthol Derivatives

Abstract

The Lewis acid-assisted chiral Brønsted 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 biomimetic 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 biomimetic cyclization of simple isoprenoids to polycyclic isoprenoids using chiral LBA is also described. This is the first example of a chiral Brønsted 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 trisubstituted terminal olefin and site selectively generate carbocations on the substrates.

Aluminium(III) Trifluoromethanesulfonate as an Efficient Catalyst for the Intramolecular Hydroalkoxylation of Unactivated Olefins: Experimental and Theoretical Approaches

“…For example, the combination of SnCl 4 with an optically active binaphthol derivative increased the acidity of the OH proton in enantioselective protonation and enantioselective polyene cyclization. [24] To explain the high catalytic activity of AlA C H T U N G T R E N N U N G (OTf) 3 in the cycloisomerization of unsaturated alcohols, we propose a Lewis acid assisted Brønsted acid situation (LBA) in which the Lewis acid AlA C H T U N G T R E N N U N G (OTf) 3 strongly enhances the Brønsted acidity of the hydroxyl protons of unsaturated alcohols.…”

Biomimetic Cyclization of Small Terpenoids Promoted by Zeolite NaY: Tandem Formation of α‐Ambrinol from Geranyl Acetone

“…[1] The conformational immobility of the reacting substrates in the confined environment allows the cascade attack of the proximal double bonds to an intermediate carbocation, [2] proceeding in a highly stereoselective manner. Non-enzymatic cyclization of the acyclic terpenoids has been achieved by various means, such as catalysis by protic acids, [3] super acids [4] or Lewis acids, [5] via radical-initiated pathways, [6] and by photoinduced electron-transfer. [7] The acid-catalyzed cyclization of polyenes (e.g., by FSO 3 H) requires very low reaction temperatures (approximately À 80 8C), multimolar excess of the acid relative to the substrate, while toxic nitro compounds are preferentially used as solvents.…”

Development of a new Lewis base-tolerant chiral LBA and its application to catalytic asymmetric protonation reaction

“…The combined acid system is an excellent method to increase the reactivity of Brønsted acids 6. The coordination of a Brønsted acid to a Lewis acid activator increases its acidity, and thus increases its reactivity.…”