Highly Atroposelective Synthesis of Arylpyrroles by Catalytic Asymmetric Paal–Knorr Reaction

A general
and efficient method for accessing enantiomerically pure
arylpyrroles by utilizing the catalytic asymmetric Paal-Knorr reaction
has been developed for the first time. A wide range of axially chiral
arylpyrroles were obtained in high yields with good to excellent enantioselectivities.
The key to success is the use of the combined-acid catalytic system
involving a Lewis acid and a chiral phosphoric acid for achieving
effective enantiocontrol. Noteworthy is that an unexpected solvent-dependent
inversion of the enantioselectivity was observed in the above-mentioned
asymmetric reaction.