Cyclopentanone as a cation-stabilizing electron-pair donor in the calcium-catalyzed intermolecular carbohydroxylation of alkynes.

Although they have been used as reactivity-controlling additives in cationic polymerizations for decades, Lewis basic "electron pair donor" (ED) compounds were never used for the stabilization of cationic intermediates in transformations of small molecules. As such an ED, cyclopentanone proved highly efficient for the stabilization of allyl and vinyl cations in combination with our calcium-based catalyst system. Therefore, the first general transition-metal-free intermolecular carbohydroxylation of alkynes with allyl and propargyl alcohols was realized.