a b s t r a c tThe cyclization of an alkene-bearing cyclopentanone to a [2.2.1]-norcamphor ring system is described. The reaction is catalyzed by a combination of rhodium and Brønsted acid. Control experiments indicate that both are needed for acceptable yield. Control experiments with bulky base additives show that rhodium promotes alkene isomerization, likely the first step of this cascade reaction, and that rhodium alone does not promote cyclization. Cyclization is promoted by Brønsted acid in a Prins-type cyclization and carbocation rearrangement process. Trace Brønsted acid present in commercial samples of Rh(cod) 2 OTf is likely responsible for the observed reaction. Indeed, the norcamphor product can be obtained simply with strong acid, presumably initiated by acid-promoted alkene isomerization. Since our initial motivation for this work was the development of rhodium catalysts for the activation of C-C bonds adjacent to ketones, this communication serves to identify other, perhaps less obvious, pathways for the reactions of unsaturated ketone compounds by the action of rhodium catalysts.Ó 2015 Elsevier Ltd. All rights reserved.Carbon-carbon sigma (C-C) bond activation is a growing area of research for organic synthesis. 1 Our group has developed alkene carboacylation reactions based on the insertion of rhodium adjacent to unstrained ketones. 2,3 The general mechanistic strategy is to couple the activation of the C-C bond between a carbonyl carbon and the a-carbon with the insertion of an alkene. 4,5 In our prior work, we placed a nitrogen five atoms away from the carbonyl carbon to provide a chelate for C-C activation and prevent decarbonylation prior to an alkene engaging the activated bond in a productive capturing event. 6,7 Others have developed related alkene carboacylation reactions (sometimes termed 'cut-andsew') without the judiciously placed heteroatom within the substrate, but this work is largely limited to strained ring ketones for starting materials, most often cyclobutanones. [8][9][10][11][12] Our goal at the outset of this work was to investigate similar chemistry of cyclopentanones, which have substantially less ring strain than the corresponding cyclobutanones. Herein, we report an unexpected cyclization encountered in our study. We show that a combination of alkene isomerization and a likely acid-mediated Prins-type cyclization and semi-pinacol rearrangement can account for the formation of this unexpected product. Our purpose in communicating this work is to demonstrate the need for an appropriate control experiment to rule out classic reaction pathways when developing C-C bond activation approaches to alkene carboacylation reactions.We designed cyclopentanone 1 with the notion that rhodium might reversibly insert adjacent to the carbonyl. The tethered alkene could then coordinate to the metal center to promote carboacylation potentially generating the bridged compound 2 via alkene carboacylation (Scheme 1). This mechanistic thinking is in line with the documented cyclobutanone carboac...