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A Rh-catalyzed intramolecular [3+2+2] cycloaddition is reported. The cycloaddition affords synthetically relevant 5,7,5-fused tricyclic systems of type 2 from readily available dienyne precursors. The transformation takes place with moderate or good yields, high diastereoselectivity, and total chemoselectivity., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

An efficient tandem ring-closing dienyne metathesis of dienynes derived from cyclohex-2-enone affords the [5.3.1] carbon framework characteristic of taxanes in a single-step process. Further stereoselective functionalizations of the resulting [5.3.1] carbon framework lead to an advanced intermediate in a novel synthetic strategy for taxane analogs.

The mechanism of the palladium-catalysed [3+2] intramolecular cycloaddition of alkylidenecyclopropanes to alkynes has been computationally explored at DFT level. The energies of the reaction intermediates and transition states for different possible pathways have been calculated in a model system that involves the use of PH3 as a ligand. The results obtained suggest that the most favourable reaction pathway involves the initial C--C oxidative addition of the cyclopropane to a Pd0 complex to give an alkylidenepalladacyclobutane, which isomerises to a methylenepalladacyclobutane intermediate. Subsequent cyclisation by alkyne carbometallation, followed by reductive elimination affords the final product. An alternative mechanism consisting of a palladaene-type rearrangement is less probable in terms of Gibbs energy, but cannot be fully discarded because it is competitive if one considers electronic energies. For substrates that present an ester group at the terminal position of the triple bond we have found an alternative, more favourable mechanistic route that explains why the [3+2] cycloaddition of these types of systems does not lead to the expected cycloadducts.

A cascade dienyne ring-closing metathesis approach has been applied to the synthesis of the tetracyclic carbon framework of a new class of hybrid compounds-the taxosteroids-possessing carbon frameworks incorporating moieties characteristic of both taxanes (such as AB rings) and steroids (i.e., CD system and side chain). This tandem cyclization is highly stereoselective, allowing the one-step formation of the bicyclo[5.3.1]undecene system characteristic of taxol. In this work we describe the scope and limitations of such cyclizations.

[structure: see text] Callyberynes A and B, polyacetylenic hydrocarbons from Callyspongia sp., have been synthesized for the first time using highly convergent approaches based on optimized Cadiot-Chodkiewicz (Alami, Vasella) and sequential Sonogashira cross-coupling reactions.

Ruthenium-catalyzed coupling of readily accessible 1-trimethylsilyl-1-alkyn-3-ols to allyl ethyl ether followed by in situ ketalization and Lewis-acid induced cyclization affords 1,5-oxygen-bridged eight- and nine-membered carbocycles, products which are difficult to construct with previously published strategies.