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Synlett 2009(14): 2328-2332  
DOI: 10.1055/s-0029-1217714
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

1,3-Dipolar Cycloaddition of Nitrile Imines with Functionalized Acetylenes: Regiocontrolled Sc(OTf)3-Catalyzed Synthesis of 4- and 5-Substituted Pyrazoles

Bianca Flavia Bonini, Mauro Comes Franchini*, Denis Gentili, Erika Locatelli, Alfredo Ricci
Dipartimento di Chimica Organica ‘A. Mangini’, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
Fax: +39(051)2093654; e-Mail: mauro.comesfranchini@unibo.it;
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Publication History

Received 6 May 2009
Publication Date:
31 July 2009 (online)

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Abstract

1,3-Dipolar cycloaddition of C-aryl-N-aryl- and C-carboxymethyl-N-aryl-nitrile imines with functionalized acetylenes have been studied. Regioisomeric mixtures have been obtained with the 5-substituted pyrazole as the major cycloadduct. Under scandium triflate catalysis a reversal in the regiochemistry was observed, especially in the case of C-carboxymethyl-N-aryl-nitrile imines.

Keywords

dipolar cycloadditions - regiochemistry - nitrile imines - pyrazoles - scandium triflate

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27

General Procedure for the 1,3-DC of 1 and 2 with with 3a-c and 5a-c To a solution of 1 or 2 (1 mmol) and the precursor of the 1,3-dipoles (1 mmol) in 1,4-dioxane (4.4 mL) was added Ag2CO3 (0.69 g, 2.5 mmol) and catalytic Sc(OTf)3 (0.1 mmol) as specified in Tables  [¹] and  [²] . The reaction mixture was stirred at 80 ˚C for 18 h. The reaction was then filtered on Celite and the solvent was removed under vacuum. Purification was carried out as reported in the Supporting Information. Characterization for all the compounds is available as Supporting Information.