Synfacts 2008(2): 0137-0137  
DOI: 10.1055/s-2007-992465
Synthesis of Heterocycles
© Georg Thieme Verlag Stuttgart · New York

[2+3]-Cycloaddition Route to Pyrazole Boronic Esters from Sydnones and Alkynylboronates

Contributor(s):Victor Snieckus, Jignesh J. Patel
D. L. Browne, M. D. Helm, A. Plant, J. P. A. Harrity*
University of Sheffield and Syngenta, Bracknell, UK
A Sydnone Cycloaddition Route to Pyrazole Boronic Esters
Angew. Chem. Int. Ed.  2007,  46:  8656-8658  
Further Information

Publication History

Publication Date:
23 January 2008 (online)


Reported is a regiospecific synthesis of pyrazole boronic esters from alkynylboro­nates and N-alkyl or -aryl sydnones by cycloaddition-retro-cycloaddition reaction. The regio-chemistry of the decarboxylative [2+3] cycloaddition is governed by the R2 group of the alkynyl­boronate; thus pyrazole B predominates when R2 = aryl, alkyl, TMS while C is the major product for terminal alkyne. As expected, electron-rich sydnones A were found to be slow-reacting (22-24 h) while more electron-deficient sydnones A showed significantly faster reactions (2-4 h). The cycloaddition is highly effective for sydnones D affording fully substituted pyrazoles E with complete regioselectivity. To complete the presumed reason for this study, pyrazole B was subjected to Suzuki cross-coupling conditions to form F in good yields.


Pyrazoles represent a very important class of heterocycles which are found in pharmaceutical and agrochemical substances (C. Lamberth Heterocycles 2007, 71, 1467). Although the present report appears to be the first example of alkynylboronate [2+3] cycloaddition with sydnones for the direct preparation of functionalized pyrazole boronic esters, a similar report of cyclo-addition of sydnone with alkynylstannanes and alkynylsilanes has recently appeared (A. M. González-Nogal et al. Tetrahedron 2007, 63, 224). As reported herein, an alternate route rather than a metallation-boronation sequence for the synthesis of frequently unstable heterocyclic boronic acids and their derivatives is welcome.