Chiroptical Switching by Stereoselective Photochemical Sigmatropic Rearrangements: Experimental Realization of...

The stereochemical outcomes of pericyclic reactions provide direct insight into the geometry and directionality of concerted bond reorganization. Although torquoselectivity has emerged as a defining concept for electrocyclizations, an analogous principle for sigmatropic rearrangements on a single cyclic π system has remained unknown. We report that a homoannulene-based photoswitch undergoes an auxiliary-controlled, stereoselective photochemical [1,11]-sigmatropic rearrangement, thereby establishing selectivity over directional migration on a single cyclic π framework. We coin the term calderoselectivity for such an unreported stereoselective sigmatropic rearrangement. The switching process is reversible, proceeds to photostationary mixtures of homoannulene and iso-homoannulene isomers, and furnishes diastereomeric products in a 56:44 ratio; combined chiral HPLC, NMR, ECD, and DFT studies assign the major directionality of acyl migration in pro-R directionality. This study provides the first experimental realization of calderoselectivity and reveals a direct analogue in sigmatropic rearrangements to torquoselectivity with implications for both mechanistic physical organic chemistry and chiroptical switch design.