Stereoselective Synthesis of Axially Chiral Allenes and Styrenes via Chiral Phosphoric Acid Catalysis: An Overview

Chiral allenes and styrenes are essential components in fields like medicinal chemistry, materials science, and organic synthesis. They serve a crucial role as chiral ligands and catalysts in asymmetric synthesis. Over the past decade, there has been a significant advancement in the development of practical methods utilizing organocatalytic strategies for the synthesis of chiral allenes and styrenes. It is noteworthy that despite extensive studies on the formation of allenes and styrenes, existing reviews on their construction confined to a specific organocatalysis, called chiral phosphoric acid catalysis, are less documented. This review aims to explore different conceptual approaches based on the electrophilic species involved in the reaction to produce stereoselective chiral allenes and styrenes, providing insights into recent advancements in the field. Emphasis is placed on works published since 2017, with detailed discussions on reaction mechanisms and examples from recent literature.

Organocatalytic formal [3 + 3] cyclization of α-(6-indolyl) propargylic alcohols

With the aid of acetic acid, a 1,10-conjugate addition-mediated formal [3 + 3] cyclization of alkynyl indole imine methides formed in situ from α-(6-indolyl) propargylic alcohols with 1,3-dicarbonyl compounds such as 4-hydroxycoumarins and cyclohexane-1,3-diones was developed, which provided robust access to a wide range of pyranocoumarin and pyran derivatives containing an indole skeleton with high efficiency under mild conditions.

Enantioselective Friedel-Crafts Alkylation Reaction of Pyrroles with N-Unprotected Alkynyl Trifluoromethyl Ketimines

Developed herein is an enantioselective Friedel-Crafts alkylation reaction of N-unprotected alkynyl trifluoromethyl ketimines with pyrroles catalyzed by chiral phosphoric acid to furnish chiral primary α-trifluoromethyl-α-(2-pyrrolyl)propargylamines with high enantioselectivity. Transformation of the alkynyl group of the adducts afforded optically active α-trifluoromethylated amines bearing various substituents such as alkyl, alkenyl, enyne, and triazole without loss of optical purity.

Catalytic Asymmetric Allylic Substitution/Isomerization with Central Chirality Transposition

We have developed a catalytic asymmetric allylic substitution/isomerization process with central chirality transposition. This process takes advantage of the ambident reactivity of the 2-indole imine methide generated in situ from racemic tertiary indolylmethanols. The use of a suitable chiral phosphoric acid catalyst and an ortho-directing group allowed regioselective formation a C-C bond at the 3 position but enantiocontrolled construction of a stereogenic center at the 2-benzylic position.