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Nucleophilic catalysts for a 1,6 addition/Nazarov cyclization/elimination sequence were evaluated for their ability to induce enantioselectivity in the electrocyclization step. Of the tertiary amines examined, it was found that a cinchona alkaloid derivative was able to generate substituted 5-hydroxy γ-methylene cyclopentenones with excellent enantioselectivity. The study results suggest that successful cyclization depends upon the ability of the dienyl diketone substrate to readily adopt an s - cis conformation. [ABSTRACT FROM AUTHOR]

Abstract: The scope and utility of the metal-catalyzed rearrangement of propargyl acetates first reported by Rautenstrauch were expanded. Treatment of a series of appropriate acetate substrates with Pd(II)- and Hg(II)-catalysts afforded synthetically useful fused 5,6-bicyclic-1,4-cyclopentadienyl acetates and 2-cyclopentenones. It was found that the substituents at the terminal alkynyl and alkenyl positions of the acetate substrate had a significant impact on the outcome of the reaction. [Copyright &y& Elsevier]

Furans and benzofurans of type 1 were dearomatized via the [2,3]-Still–Wittig rearrangement. Enol ethers 2 could be isolated or isomerized to the corresponding furans 3. The substitution pattern at the homofuranylic position had a strong influence on reaction behavior. Benzofurans rearranged with the greatest efficiency, and employment of a 3-substituted benzofuran (1; R′=CH3) allowed the creation of a quaternary carbon center. [Copyright &y& Elsevier]

Abstract: The dicationic Ir(III) complex [IrMe(CO)(dppe)(DIB)](BAr4 f)2 where dppe=bis(diphenylphosphino)ethane and DIB=o-diiodobenzene possesses adjacent labile sites and is found to be a very active catalyst for the Nazarov cyclization. 31P NMR spectroscopy provides evidence for substrate–catalyst binding by chelation, and this is found to be the resting state of the system during catalysis. The efficiency of the cyclization is attributed to the electrophilicity of the Ir(III) complex and substrate activation via O,O′-chelation which employs two substrate carbonyl groups or one carbonyl and an ether function, and encourages the s-trans/s-trans conformation required for cyclization. When two point binding occurs through an oxygen atom and one of the vinyl groups, the s-trans/s-trans conformation is not achieved, and cyclization is not observed. In one case, monodentate binding of substrate occurs, and the rate of cyclization is significantly slower than when O,O′-chelation is possible. The viability of O,O′-chelation is shown by the crystal structure determination of a model substrate–catalyst complex. [Copyright &y& Elsevier]

Synthetic efforts toward the rapid assembly of the AB ring system of the tetrapetalones is described. Key to this work was the use of [3+2] cycloaddition/oxidative extrusion methodology to furnish functionalized aryl enones. The Nazarov cyclization of these substrates was examined, and optimized to generate the AB ring carbon skeleton. Then, Pd-catalyzed cross-coupling were conducted, and conditions were identified that enabled installatiion of the requisite C14–N bond. [ABSTRACT FROM AUTHOR]

Abstract: A mild, convenient reaction sequence for the synthesis of Nazarov cyclization substrates is described. The [3+2] dipolar cycloaddition of a nitrone and an electron-deficient alkyne gives an isolable isoxazoline intermediate, which upon oxidation undergoes stereoselective extrusion of nitrosomethane to give aryl vinyl or divinyl ketones. [Copyright &y& Elsevier]