Cyclopentenones constitute an important family of organic compounds present as key structural units in a huge number of natural products and biologically active compounds, such as prostanoids, jasmonoids, rethrolones, and methylenomycins. [1] In addition, they are also valuable intermediates in synthesis since they can easily undergo Michael-type additions, conjugated 1,4-additions, or cycloaddition reactions. [2] Among the different approaches presently available for the construction of the cyclopentenone ring, [3] the acid-promoted cyclization of divinyl ketones A is one of the most employed (Nazarov reaction; Scheme 1). [4,5] Both protic and Lewis acids, including transition-metal complexes, can be used to promote this process, which is classically formulated as a 4p-electron conrotatory cyclization of the transient pentadienyl cation B (Scheme 1).Successful application of this method in the total synthesis of several natural products and metabolites, such as ( AE )-roseophilin, [6a] nakiterpiosin, [6b] ( AE )-merrilactone A, [6c] ( AE )-terpestacin, [6d] or (+)-fusicoauritone, [6e] has been described, confirming its synthetic utility. However, as recently stated by Frontier and co-workers, [7] the accessibility of divinyl ketones with the desired substitution pattern is one of the most critical factors determining the applicability of the Nazarov cyclization process in synthesis. Herein, we describe a novel synthetic route to cyclopentenones, starting from readily available 1-ethynylcycloalkanols and aldehydes, which can be efficiently transformed into bicyclic fused cyclopentenones in a one-pot reaction, avoiding the use of the corresponding divinyl ketones as starting materials.Over the last few years, we have demonstrated that the combined use of the 16-electron allyl-ruthenium(II) precatalyst [Ru(h 3 -2-C 3 H 4 Me)(CO)(dppf)][SbF 6 ] (1) [8] and trifluoroacetic acid (TFA) opens new opportunities for the selective transformation of readily available propargylic alcohols into more valuable organic compounds. [9] Thus, taking advantage of the ability shown by the catalytic system 1/TFA to promote the Rupe rearrangement [10] of 1-ethynylcycloalkanols C into methylenones D, [9a] we envisaged that performing the catalytic reactions in the presence of an aldehyde would result in an aldol-type condensation to afford divinylketones E in situ (Scheme 2). Then, by action of the Brønsted acid TFA or the Lewis acidic Ru species derived from 1, Nazarov cyclization of E would occur, leading to the bicyclic fused cyclopentenones F. [11,12] Preliminary experiments were performed using 1-ethynylcyclooctanol (2) and benzaldehyde as model substrates (Scheme 3). Thus, using the same conditions previously employed by us to promote CÀC couplings of tertiary aromatic alkynols with enolizable ketones (i.e.[2]/[benzaldehyde]/[TFA]/[1] ratio = 20:200:2:1; T = 75 8C; sealed tube; without solvent), [9b] total consumption of 2 was detected after 10 h. However, GC/ MS and NMR spectroscopic analyses of the crude reaction mixture...
