Add a new entry to the list of reactions that bulky proton donors can catalyze: asymmetric additions to carbon-carbon double bonds (Nature, DOI: 10.1038/nature09723). Researchers typically turn to chiral proton-donating Brønsted acids to functionalize carbonyl or other polar groups. Instead, F. Dean Toste and coworkers at the University of California, Berkeley, used a dithiophosphoric acid to turn dienes into chiral five-membered ring pyrrolidines and tetrahydrocarbazoles. The acid’s chiral counterion controls the reaction’s selectivity by covalently attaching to the diene, and it is ultimately ejected once the chiral five-membered ring forms. Metal catalysts can accomplish this type of reaction, but the UC Berkeley study provides proof of concept that acids help make chiral products from alkenes through a pathway that is distinct from the ones metal catalysts take, Toste explains. This pathway appears to avoid carbocation intermediates that typify textbook descriptions of acid reactions with double bonds, he adds. The UC Berkeley team is now working to apply the chemistry to a range of substrates.