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 transesterification with release of methanol. The characterization of the products of these reactions by exhaustive mass spectrometric analysis is discussed.