Highly Enantioselective Organocatalytic Biginelli and Biginelli-Like Condensations: Reversal of the Stereochemistry by Tuning the 3,3′-Disubstituents of Phosphoric Acids

Abstract

Organocatalytic enantioselective Biginelli and Biginelli-like reactions by chiral phosphoric acids derived from 3,3'-disubstituted binaphthols have been investigated. The size of 3,3'-substituents of the catalysts is able to control the stereochemistry of the Biginelli reaction. By tuning the 3,3'-disubstituents of the phosphoric acids, the stereochemistry of the Biginelli reaction can be reversed. This organocatalytic Biginelli reaction by Brønsted acids 12b and 13 is applicable to a wide range of aldehydes and various beta-keto esters, providing a highly enantioselective method to access DHPMs. 3,3'-Di(triphenylsilyl) binaphthol-derived phosphoric acid afforded Biginelli-like reactions of a broad scope of aldehydes and enolizable ketones with benzylthiourea, giving structurally diverse dihydropyrimidinethiones with excellent optical purity. Theoretical calculations with the ONIOM method on the transition states of the stereogenic center forming step showed that the imine and enol were simultaneously activated by the bifunctional chiral phosphoric acid through formation of hydrogen bonds. The effect of the 3,3'-substituents in phosphoric acids on the stereochemistry of the Biginelli reaction was also theoretically rationalized. The current protocol has been applied to the synthesis of some pharmaceutically interesting compounds and intermediates, such as chiral thioureas, dihydropyrimidines, guanidines, and the precursor of (S)-l-771688.