to differences in the activation barriers for the step following ylide formation. If the ortho-methoxy group undergoes oxonium ylide formation, a 1,4-methyl shift can then generate an undesired product. The oxonium ylide formed from the other ortho-alkoxy group is primed to undergo a [2,3] sigmatropic rearrangement to yield the desired compound. Pirrung and coworkers reported complete selectivity for the desired product over the product resulting from a 1,4-methyl shift. This result suggests that oxonium ylide formation is reversible, but that the subsequent step is irreversible. The symmetry-allowed [2,3] sigmatropic rearrangement must follow a pathway that is lower in activation energy than the 1,4-methyl shift, explaining the exclusive formation of the desired product. Synthesis of (+)-allocyathin B2 A potential Curtin-Hammett scenario was also encountered during the enantioselective total synthesis of (+)-allocyathin B2 by the Trost group. The pivotal step in the synthesis was a Ru-catalyzed diastereoselective cycloisomerization. The reaction could result in the formation of two possible double bond isomers. The reaction provided good selectivity for the desired isomer, with results consistent with a Curtin-Hammett scenario. Initial oxidative cycloruthenation and beta-hydride elimination produce a vinyl-ruthenium hydride. Hydride insertion allows for facile alkene isomerization. It is unlikely that the reaction outcome mirrors the stability of the intermediates, as the large CpRu group experiences unfavorable steric interactions with the nearby isopropyl group. Instead, a Curtin–Hammett situation applies, in which the isomer favored in equilibrium does not lead to the major product. Reductive elimination is favored from the more reactive, less stable intermediate, as strain relief is maximized in the transition state. This produces the desired double bond isomer. See also Transition state theory Chemical kinetics Gibbs free energy References External links https://web.archive.org/web/20111005191716/http://www.joe-harrity.staff.shef.ac.uk/meetings/CurtinHammettreview.pdf https://web.archive.org/web/20120402124752/http://evans.harvard.edu/pdf/smnr_2009_WZOREK_JOSEPH.pdf Chemical kinetics Physical organic chemistry