Direct Asymmetric Addition Of Naphthol To Acetals Catalyzed By Chiral (R)-BINOL-Derived Phosphoric Acid

Recently,chiral Br(?)nsted-acid catalysts have emerged as a new class of organocatalysts for a number of enantioselective carbon-carbon bond-forming reactions.More and more organic chemists pay attention to it because of being easy to handle and generally stable toward oxygen and water and friendly to environment.It was found that chiral phosphoric acid of(R)-BINOL derivatives are bifunctional catalysts bearing both Br(?)nsted-acidic site and Lewis-basic site and the 3,3’-substituents play a crucial role in attaining excellent enantioselectivity.So far,a variety of chiral phosphoric acid derivatives in catalytic asymmetric reactions have already reported with good enantioselectivity.Acetals are useful intermediates in organic synthesis and undergo coupling reactions with different nucleophiles via carbon-carbon bond formation.Successfully used nucleophiles include silyl enol ethers,vinyl ethers,and also common olefins as well as cyanide sources.However,the direct asymmetric catalysis of acetals to phenols are not reported.In this work,we synthesized a series of phosphoric acid catalysts 16a-16d and 21 from(R)-BINOL in high yields.Then the asymmetric addition reactions of naphthol to acetals which were catalyzed by 16a-16d and 21 were tested.The results showed that adducts which were obtained with the highest ee up to 71%and yield up to 68%in the presence of catalyst 21.To the best of our knowledge,we report the first catalytic enantioselective naphthol to acetal reaction.In addition,we also discussed the mechanism of this asymmetric reaction.we speculate that the bifunctional character of the phosphoric acid plays an important role.The bulkier substituted groups of the 3,3’-substituents and two hydrogen bonds from the phosphate anion through hydrogen bonding interaction with the oxocarbenium ion and the hydroxyl group provides a chiral environment make the reaction predominantly afford products.