Ring-Opening of Aziridines Using Tethered Hemiketals

We present the first examples of aziridine ring-opening reactions by pendant hemiketal tethers. The reactions proceed smoothly at room temperature with catalytic quantities of the Lewis acid triphenylcarbenium tetrafluoroborate. The hemiketal tether is derived from the reaction solvent, acetone, in most cases. Other ketone solvents, including cyclopentanone and cyclobutanone, are also compatible. We hypothesize that the Lewis acid catalyzes the addition of the solvent to the alcohol of the aziridine substrate, forming a hemiketal tether, which then attacks the aziridine in an S_N2 manner. Given that mixtures of products are formed in many cases, an alternate mechanism involving an intermolecular attack by the carbonyl solvent followed by ketalization may also be operative. A variety of functional groups are compatible, and the products are useful for further transformations.