We have reported an efficient gold(I)-catalyzed tandem cyclization of tert-butyl carbonate derivatives of hex-1-en-5-yn-3-ol where nucleophilic participation of the O-Boc group appears to intercept a carbocationic (or cyclopropyl carbene) Au intermediate. This novel protocol leads to densely functionalized cyclohexene-3,4-diol derivatives where 1,2- or 1,2,3-stereocenters are controlled in a highly diastereoselective fashion.
A comparative study on the Au(I)-catalyzed and IBr-promoted tandem cyclization of 1,5-enyne was reported. This study provides a meaningful mechanistic insight to the concerted nature of this tandem reaction and also provides interesting applications in the synthesis of 7/5-or 8/5-fused bicycles and biaryls.
A diastereoselective, metal-free tandem iodocarbonatecyclization of 1,5-enyne, emulating the reactivity of gold catalysis is described. The normal selectivity of iodonium reagents for alkene is reversed favoring alkyne activation in the presence of aryl alkynes.
Significance: Enantioselective radical reactions have attracted interest lately due to their ability to introduce various stable radical intermediates to electrophiles. This paper reports the radical conjugate addition to enones using a 1,4-chelating template as opposed to the more popular 1,5-chelate.Comment: Generally, asymmetric syntheses using radical intermediates are more difficult than other reactive species, such as simple nucleophiles. The use of this specific 1,4-chelating motif has previously been reported by C. Palomo et al. for other enantioselective C-C bond formation reactions, such as: Diels-Alder cycloadditions (J. Am. Chem. Soc. 2003, 125, 13942-13943), Friedel-Crafts alkylation (J. Am. Chem. Soc. 2005, 127, 4154-4155), and conjugate addition of carbamates (J. Am. Chem. Soc. 2004, 126, 9188-9189). Aluminium-salen complexes were also investigated, however, magnesium Ph-Box complexes showed the best selectivity. While the yields and enantioselectivities are not high, this paper demonstrates that investigation in this area could lead to improved results.
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