Memory of chirality in the phthalimide photocyclization of adamantane dipeptides

Upon sensitized irradiation of N-phthalimido dipeptides containing l- or d-phenylalanine, enantioselective cyclization to diazamacrocycles takes place (ee >99%). The high optical purity of the products was explained by the memory of chirality effect. The chirality is preserved owing to the axial chirality of the biradical intermediate formed. An insight into the origin behind the enantioselective cyclization of the intermediate was given by density functional calculations. The very high enantiomeric excess in this type of photocyclization establishes it as a new method for the enantioselective synthesis of different large aza-heterocycles.