Regioselectivity studies of sydnone cycloaddition reactions of azine-substituted alkynes

The synthesis of azine-substituted pyrazoles by a sydnone cycloaddition strategy is described. Incorporation of a 3-pyridyl moiety at the sydnone N-atom has little effect on either reactivity or regioselectivity, however, 2-ethynyl-pyridine and -pyrimidine undergo cycloaddition with surprisingly poor levels of regiocontrol. A rationale for the observed regiochemical trends and potential routes for improving selectivities are discussed.