External Reversal of Chirality Transfer in Photoswitches

The transfer of stereoinformation is at the heart of asymmetric reactions. By incorporating the natural monoterpene l‐menthone into the backbone of a diarylethene, we achieved efficient chirality transfer upon photocyclization, resulting in the preferred formation of one major closed isomer in a diastereomeric ratio (d.r.) of 85:15. More significantly, we were able to completely reverse the diastereomeric outcome of the ring closure simply by altering the chemical environment or the irradiation conditions. As a result, we could selectively accumulate the less favored minor closed isomer, with remarkable d.r. values of >99:1 and 74:26, respectively. Computations revealed that a stability inversion after photocyclization is the basis for the observed unprecedented control over diastereoselectivity. Which way to go? The monoterpene l‐menthone was incorporated into the backbone of a photochromic diarylethene. Chirality transfer upon photocyclization leads to preferred formation of one major stereoisomer. Complete reversal of the diastereomeric outcome was achieved by altering either the chemical environment or the irradiation wavelength, resulting in selective formation of the minor isomer.