One-Pot Synthesis of Helical Aromatics: Stereoselectivity, Stability against Racemization, and Assignment of Absolute Configuration Assisted by Experimental and Theoretical Circular Dichroism

Helical aromatics (1) were synthesized via one step in good quantity by solvent-free condensation
of N,N‘-p-phenylenediamine (2) and various carboxylic acids in the presence of a Lewis acid.
Microwave irradiation greatly facilitated the condensation reaction to furnish 1 with a 100%
diastereo- and a 50% enantioselectivity, when a chiral carboxylic acid was utilized. 1f, derived
from 2-methylglutaric acid, was quite stable, no racemization taking place even at 200 °C. The
assignment of the absolute configurations to the helical aromatics has been achieved by experimental
and theoretical CD spectra calculated by time-dependent density functional theory.