The formation of 18 can be rationalized by an unprecedented [1,11] sigmatropic rearrangement from 15 to 18, which is suprafacially allowed under photochemical conditions in accordance with the Woodward-Hoffman rules . UV/vis spectroscopy indicates a clean rearrangement, supported by the HPLC traces and the isosbestic points observed (Fig. and). Integration of the peak areas of the UV/vis-HPLC traces revealed a ratio 15/18 of 30:70 at the PSS (see Supplementary Fig. ). This photochemical reaction was found to be independent of solvent polarity and could not only be observed in cyclohexane but equally rapid in MeCN, supportive of a non-polar transition state typically observed for pericyclic reactions (see Supplementary Fig. ).