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  • Title: Toward safer processes for C-C biaryl bond construction: catalytic direct C-H arylation and tin-free radical coupling in the synthesis of pyrazolophenanthridines.
    Author: Hernández S, Moreno I, SanMartin R, Gómez G, Herrero MT, Domínguez E.
    Journal: J Org Chem; 2010 Jan 15; 75(2):434-41. PubMed ID: 20000343.
    Abstract:
    A series of pyrazolo[1,5-f]phenanthridine derivatives has been efficiently synthesized by a short, straightforward sequence. A tandem amine-exchange/heterocyclization of enaminones was successfully applied to the regioselective preparation of 1,5-diarylpyrazole intermediates with structure resemblance to relevant nonsteroidal anti-inflammatory drugs such as celecoxib or tepoxalin. The final key step, cyclization by intramolecular biaryl bond formation, was accomplished by two alternative methodologies: radical coupling and catalytic direct arylation via C-H activation. The scope and limitations of the two methodologies have been explored and their complementariness has been established. In addition, polymer-supported heterogeneous catalysts have been compared with homogeneous analogues. In the radical process, toxic tin derivatives have been avoided in order to employ environmentally safer protocols.
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