The reaction of 3-cyclopropyl propargylic carboxylates with Au(I) and Au(III) catalysts affords selectively 5-(E)-alkylidenecyclopentenyl acetates via [3,3]-sigmatropic rearrangement of the carboxylic moiety followed by cyclopropyl ring opening and cyclization. DFT calculations have been performed, supporting a two-step no-intermediate mechanism along the cyclization coordinate. The stereoselective formation of the exocyclic alkenes is kinetically controlled in the first of these events. Although stereospecific in nature through a gold-stabilized nonclassical carbocation, the chirality transfer in these cyclopentannulations is not complete. Computational and experimental evidence is provided for a Au-promoted cyclopropyl ring opening/epimerization/ring closure in both cis- and trans-cyclopropyl settings, which competes with the cyclization event, thus eroding the stereochemical information transfer. When tertiary acetates were used, products of both 1,2- and 1,3-acyloxy migration processes could be isolated, supporting the competitive coexistence of these two pathways along the reaction profile, as suggested also by DFT calculations.
Some of the most synthetically useful methods to construct molecular complexity include Diels−Alder, [1,3]-dipolar-and [m+n]-cycloadditions. In this context, the efficient generation of 1,n-dipoles plays a key role. Dipoles have been usually described as transient, difficult to harness species toward cycloaddition reactions. This review highlights the development of new methodologies for the efficient generation of these valuable intermediates by means of gold catalysis, and their application in the construction of small-medium size carbocycles. The mechanistic rationale underlying these transformations is also presented here.
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