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The synthesis of isoindazoles bearing alpha-ketoester and alpha-hydroxyester groups via the coarctate cyclization of ester-terminated azo-ene-yne precursors is described. Whereas previous studies on isoindazole formation have shown the reaction to proceed through a kinetic coarctate pathway, functionalization of the terminal acetylene with a methyl ester sufficiently stabilizes the carbene intermediate to make the coarctate cyclization the thermodynamic pathway. Density functional theory (DFT) computations reveal ca. 8-9 kcal mol(-1) lower energy transition states for the coarctate pathway compared to the parent system.

journal of organic chemistry

Synthesis of α-Ketoester- and α-Hydroxyester-Substituted Isoindazoles via the Thermodynamic Coarctate Cyclization of Ester-Terminated Azo-Ene-Yne Systems