[1,3], [3,3], and [3,5] Sigmatropic Rearrangements of Esters Are Pseudopericyclic

Allowed by orbital symmetry rules, the [3,5] sigmatropic ester rearrangement of ester 1 even has a lower barrier than a competing [3,3] rearrangement, according to ab initio calculations. This surprising result is a consequence of the lack of orbital overlap in the pseudopericyclic transition state.