case and an S = 5 species in the latter, preventing the decarboxylation in the smaller ring system. An anti-bredt molecule is one that is found to exist and be stable (within certain parameters) despite this rule. A recent (2006) example of such a molecule is 2-quinuclidonium tetrafluoroborate. Bridgehead double bonds can be found in some natural products, discussed in a review by Mak, Pouwer and Williams, and an older review by Shea looked at bridgehead alkenes more generally. References Physical organic chemistry Stereochemistry