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A computational DFT examination (B3LYP/6-31G* and B3LYP/6-311+G**//B3LYP/6-31G*) of the thermal [5C + 2C] cycloaddditions of 6-acetoxy-3-pyranones and 3-silyloxy-4-pyrones with tethered alkenyl sulfoxides confirms that the high level of diastereofacial selectivity obtained is ultimately due to the preference of the alkenesulfinyl group to adopt a well-defined conformation in the transition state of the reaction. This conformation, which is different from that found in the ground state, is most probably dictated by dipolar interaction effects between the sulfoxide and the oxidopyrylium ylide intermediate. [ABSTRACT FROM AUTHOR]

Examines the ruthenium-catalyzed alkyne-alkene coupling and a prins-type cyclization. Use of 1-trimethylsilyl-1-alkyn-3-ols and allyl ethers; Application of aromatic system as the nucleophilic partner; Assembly of a homologous oxygen-bridged system.

A highly diastereo- and chemoselectiveintramolecular nickel-catalyzedcycloaddition of alkene- and alkyne-tethered alkynylidenecyclopropanesis reported. The method constitutes the first fully intramolecular[3 + 2 + 2] alkylidenecyclopropropane cycloaddition occurring viaa proximal cleavage of the cyclopropane and makes it possible to buildrelevant 6,7,5-tricyclic frameworks in a single-pot reaction. Importantly,the reaction outcome is highly dependent on the characteristics ofthe nickel ligands. [ABSTRACT FROM AUTHOR]

Gold(I) complexes featuring electron acceptor ligands such as phosphites and phosphoramidites catalyze the [4C+2C] intramolecular cycloaddition of allenedienes. The reaction is chemo- and stereoselective, and provides trans-fused bicyclic cycloadducts in good yields. Moreover, using novel chiral phosphoramidite-based gold catalysts it is possible to perform the reaction with excellent enantioselectivity. Experimental and theoretical data dismiss a cationic mechanism involving intermediate II and suggest that the formation of the [4C+2C] cycloadducts might arise from a 1,2-alkyl migration (ring contraction) in a cycloheptenyl Au-carbene intermediate (IV), itself arising from a [4C+3C] concerted cycloaddition of the allenediene. Therefore, these [4C+2C] allenediene cycloadditions and the previously reported [4C+3C] counterparts most likely share such cycloaddition step, differing in the final 1,2-migration step. [ABSTRACT FROM AUTHOR]

The article describes the [4+3] intramolecular cycloaddition of alkylidenecyclopropanes and dienes using a palladium catalyst. According to the authors, the cycloaddition reaction allows the synthesis of appealing, stereochemically rich 5,7-fused bicyclic systems from readily dienylidenecyclopropanes. They mention that the success of reaction is restricted to dienes blocked in a cisoid conformation and activated by the presence of electron-withdrawing substituents.

α,β-Unsaturated N,N-dialkyl hydrazones undergo a mild [2+ 2] cycloaddition to allenamideswhen treated with a suitable gold catalyst. The method, which representsthe first application of N,N-dialkylhydrazones in gold catalysis, is compatible with a wide variety ofsubstituents at the alkenyl moiety of the hydrazone component, proceedswith excellent levels of regio- and diastereoselectivity, and providesdensely substituted cyclobutanes with good to excellent yields. [ABSTRACT FROM AUTHOR]

The first highly enantioselective intermolecular (4 + 2) cycloaddition between allenes and dienes is reported. The reaction provides good yields of optically active cyclohexenes featuring diverse substitution patterns and up to three stereocenters. Key to the success of the process is the use of newly designed axially chiral N-heterocyclic carbene-gold catalysts. [ABSTRACT FROM AUTHOR]

We report a ruthenium-catalyzed (2 + 2) intramolecular cycloadclition of allenes and alkenes. We have found that the use of the ruthenium complex nH2Cl2(PiPr3)2., which has previously gone unnoticed in catalytic applications, is crucial for the observed reactivity. The reaction proceeds under mild conditions and is fully diastereoselective, providing a practical entry to a variety of bicydo[3.2.O]heptane skeletons featuring cyclobutane rings. [ABSTRACT FROM AUTHOR]