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Synfacts 2010(8): 0880-0880  
DOI: 10.1055/s-0030-1257855
Synthesis of Heterocycles
© Georg Thieme Verlag Stuttgart ˙ New York

Synthesis of 2H-Indazoles via [3+2]-Dipolar Cycloaddition

Contributor(s): Victor Snieckus, Cédric Schneider
C. Wu, Y. Fang, R. C. Larock*, F. Shi*
Henan University, Kaifeng, P. R. of China and Iowa State University, Ames, USA
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Publication History

Publication Date:
22 July 2010 (online)

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Significance

Reported is an efficient and selective method for the synthesis of 2H-indazoles 3 generated by fluoride-mediated fragmentation of 1 and easily obtained sydnones 2 (see also: D. Azarifar, H. Ghasemnejad-Borsa Synthesis 2006, 1123). The sequence consists of [3+2]-­dipolar cycloaddition-decarboxylation. The optimized conditions involve use of an excess of TBAF instead of CsF, which is usually the most widely used fluoride ion source for generating benzynes. The scope of this reaction was investigated with symmetrical and unsymmetrical aryne precursors as well as a variety of C3-substituted N-phenylsydnones (R = aromatics with EDGs and EWGs, alkynyl, alkyl, vinyl, and heterocycles) providing the 2H-indazoles 3 in moderate to good yields. The use of an unsymmetrical benzyne precursor afforded a regioisomeric mixture.