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Cycloaddition of diphenylnitrilimine to coumarins the synthesis of 3a 9b dihydro 4 oxo 1h benzopyrano 4 3 c pyrazole derivatives



Cycloaddition of diphenylnitrilimine to coumarins the synthesis of 3a 9b dihydro 4 oxo 1h benzopyrano 4 3 c pyrazole derivatives



Tetrahedron 41(10): 1877-1884



The cycloaddition of diphenylnitrilimine to a series of coumarins occurs with the same regioselectivity in all cases, regardless of the nature of the substituent present, to yield the corresponding 3a, 9b -dihydro-4-oxo-1H-benzopyrano [4,3-c]pyrazoles 4a -f. Dehydrogenation of 4a yields 4-oxo-1H-benzopyrano [4,3-c]-pyrazole 6. In ethanolic sodium ethoxide solution, the reaction between coumarin and diphenylnitrilimine precursor, namely N-phenylbenzohydrazidoyl chloride 1, affords no cycloadducts, but yields o-(.beta.-ethoxycarbonylvinyl)phenyl N-phenylbenzohydrazidate 10. The regiochemistry of the cycloadducts 4a -f and the reaction sequence leading to 10 are outlined. The structures of the products were established by spectroscopic methods and independent syntheses wherever possible.

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Accession: 005079654

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DOI: 10.1016/s0040-4020(01)96550-0


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