Synthesis of β-Pyridyl α-Amino Acids: Conformationally Sensitive Charge Transfer-Based Fluorophores.

Unnatural α-amino acids with charge transfer-based poly aromatic side chains have been designed as conformationally sensitive fluorophores. These were prepared using a hetero-Diels-Alder reaction and a Knoevenagel-Stobbe process to generate a biaryl pyridyl unit, followed by iron-catalyzed bromination and a Suzuki-Miyaura cross-coupling reaction to complete the triaryl system. A photophysical study led to the discovery of a p -methoxy analogue which exhibited viscosity-sensitive fluorescence in which emission could be controlled between twisted and planar conformations.