A Tight Spot For Spiroacetals

By building a Brønsted acid with a compact chiral environment, chemists in Germany have developed an enantioselective route to spiroacetals (Nature,DOI: 10.1038/nature10932). The spiroacetal structural motif, in which the so-called spiro carbon forms the junction between two rings and makes single bonds to two oxygen atoms, appears in a number of natural products. For example olean, the sex pheromone of the olive fruit fly, features a spiroacetal. The stereochemistry of the spiro carbon can be critical. In the case of olean, the S enantiomer attracts female fruit flies while the R enantiomer beguiles males of the species. Benjamin List and Ilija Čorić, of the Max Planck Institute for Coal Research, reasoned they could accomplish an asymmetric synthesis of spiroacetals if they used a catalyst that confined the compounds’ oxocarbenium ion precursors in such a manner that they were only attacked on one side by pendant alcohol groups. The bulky chiral Brønsted acid catalyst they developed is based on an imidodiphosphoric acid and creates an enzymelike pocket in which the precursor can bind. Using this approach they were able to make a series of spiroacetals enantioselectively.