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[2+3]-Cycloaddition Route to Pyrazole Boronic Esters from Sydnones and AlkynylboronatesContributor(s):Victor Snieckus, Jignesh J. Patel
University of Sheffield and Syngenta, Bracknell, UK
A Sydnone Cycloaddition Route to Pyrazole Boronic Esters
Angew. Chem. Int. Ed. 2007, 46: 8656-8658
23 January 2008 (online)
cycloaddition - N-aryl sydnones - pyrazole boronic esters - alkynylboronates
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.