Photochemical locking and unlocking of an acyl nitroso dienophile in the Diels–Alder reaction

[Display omitted] Photochromic Diels–Alder cycloadducts consisting of acyl nitroso dienophiles, which are known nitroxyl (HNO) donors, and dithienyldienes are presented. The dithienylethene-type photochromic cycloadducts were found to exhibit reversible electrocyclic ring closing and ring opening reactions to ‘lock’ or ‘unlock’ the retro Diels–Alder reaction, respectively. The release of an acyl nitroso dienophile via a retro Diels–Alder reaction at 92°C was shown to occur only from the open or ‘unlocked’ form of a photochromic dithienylcyclopentene and not from the closed or ‘locked’ isomer.