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

ABSTRACT 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.