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

Significance

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.