Inverse electron demand diels‐alder reactions of psoralens. Synthesis and mass spectra of novel pyridazinocoumarins

Diels‐Alder reactions between the furan double bond of 8‐methoxypsoralen and 1,2,4,5‐tetrazine or 3,6‐bistrifluoromethyl‐1,2,4,5‐tetrazine were accompanied by the release of diatomic nitrogen and the opening of the furan ring to leave a 6‐pyridazinocoumarin. When 3,6‐bis(methoxycarbonyl)‐1,2,4,5‐tetrazine was used as diene with 8‐methoxy‐, 5‐methoxy‐ or 8‐hydroxypsoralen as substrate a previously unknown heterocyclic framework was created. Formation of the fourth ring was accompanied by conversion of the furan ring to a pyrone, presumably by intramolecular transestenfication with release of methanol. The characterization of the products of these reactions by exhaustive mass spectrometric analysis is discussed.