Chiral Calix[3]pyrrole Derivatives: Synthesis, Racemization Kinetics, and Ring Expansion to Calix[9]‐ and Calix[12]pyrrole Analogues
Yuya Inaba, Jian Yang, Yu Kakibayashi, Tomoki Yoneda, Yuki Ide, Yuh Hijikata, Jenny Pirillo, Ranajit Saha, Jonathan L. Sessler, Yasuhide Inokuma
Abstract
Chiral pyrrolic macrocycles continue to attract interest. However, their molecular design remains challenging. Here, we report a calixpyrrole-based chiral macrocyclic system, calix[1]furan[1]pyrrole[1]thiophene (1), synthesized from an oligoketone. Macrocycle 1 adopts a partial cone conformation in the solid state, and undergoes racemization via ring inversion. Molecular dynamics simulations revealed that inversion of the thiophene is the rate determining step. Pyrrole N-methylation suppressed racemization and permitted chiral resolution. Enantioselective N-methylation also occurred in the presence of a chiral ammonium salt, although the stereoselectivity is modest. A unique feature of 1 is that it acts as a useful synthetic precursor to yield several calix[n]furan[n]pyrrole[n]thiophene products (n=2-4), including a calix[12]pyrrole analogue that to our knowledge constitutes the largest calix[n]pyrrole-like species to be structurally characterized.