Pyridazino-psoralen

JOC6503>, and new pyridazino-psoralens 15 were prepared via a furan ring expansion reaction <05T4805>. The reaction of 3-acetylcoumarins with alloxan followed by NH2NH2 easily produced 3-(2-oxo-2//-chromen-3-yl)-6//,8//-pyrimido[4,5-c]pyridazine-5,7-diones <05JHC1223>. Furano- and pyrano[2,3-c]pyridazines 17 and 18a,b as well as substituted quinolines were conveniently prepared from pyridazinone 16 and vinyl- and allyltriphenyl-phosphonium salts <05HAC56>. 

The reaction of benzodifuran 113 with 3,6-dimethoxycarbonyl-l,2,4,5-tetrazine 114 proved to be an efficient way to make pyridazino-psoralen-based aromatic polycycles such as 115 through a reaction sequence illustrated in Scheme 57 <2005T4805>. 

Dimethyl l,2,4,5-tetrazine-3,6-dicarboxylate continues to be an important reagent in the inverse Diels-Alder reaction, and has been used in the synthesis of ningalin D <05JA10767>, dihydrodiol and diol epoxide of phthalazine <05T1545> and novel pyridazino-psoralen derivatives <05T4805>. 

In 2003, Gonzales-Gomez described a similar transformation for the decoration of the pyridazino[4,3-/z]psoralen skeleton [96]. Furocoumarins are of pharmacological interest due to their capacity to link covalently to DNA and other macromolecules upon irradiation with long-wavelength UV light. 

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