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Total Synthesis of Swinholide A: An Exposition in Hydrogen-Mediated C–C Bond Formation
Shin, Inji,Hong, Suckchang,Krische, Michael J. American Chemical Society 2016 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.138 No.43
<P>Diverse hydrogen-mediated C–C couplings enable construction of the actin-binding marine polyketide swinholide A in only 15 steps (longest linear sequence), roughly half the steps required in two prior total syntheses. The redox-economy, chemo- and stereoselectivity embodied by this new class of C–C couplings are shown to evoke a step-change in efficiency.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2016/jacsat.2016.138.issue-43/jacs.6b10645/production/images/medium/ja-2016-10645f_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja6b10645'>ACS Electronic Supporting Info</A></P>
Kim, In Su,Han, Soo Bong,Krische, Michael J. American Chemical Society 2009 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.131 No.7
<P>Under the conditions of transfer hydrogenation employing an ortho-cyclometallated iridium catalyst generated in situ from [Ir(cod)Cl](2), 4-cyano-3-nitrobenzoic acid and the chiral phosphine ligand (S)-SEGPHOS, alpha-methyl allyl acetate couples to alcohols 1a-1j with complete levels of branched regioselectivity to furnish products of carbonyl crotylation 3a-3j, which are formed with good levels of anti-diastereoselectivity and exceptional levels of enantioselectivity. An identical set of optically enriched carbonyl crotylation products 3a-3j is accessible from the corresponding aldehydes 2a-2j under the same conditions, but employing isopropanol as the terminal reductant. Experiments aimed at probing the origins of stereoselection establish a matched mode of ionization for the (R)-acetate and the iridium catalyst modified by (S)-SEGPHOS, as well as reversible ionization of the allylic acetate with rapid pi-facial interconversion of the resulting pi-crotyl intermediate in advance of C-C bond formation. Additionally, rapid alcohol-aldehyde redox equilibration in advance of carbonyl addition is demonstrated. Thus, anti-diastereo- and enantioselective carbonyl crotylation from the alcohol or aldehyde oxidation level is achieved in the absence of any stoichiometric metallic reagents or stoichiometric metallic byproducts.</P>
Han, Soo Bong,Kim, In Su,Han, Hoon,Krische, Michael J. American Chemical Society 2009 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.131 No.20
<P>Enantioselective transfer hydrogenation of 1,1-dimethylallene 1a in the presence of aromatic, alpha,beta-unsaturated, or aliphatic aldehydes 2a-i mediated by 2-propanol and employing a cyclometalated iridium C,O-benzoate derived from allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS delivers reverse-prenylation products 4a-i in good to excellent isolated yields (65-96%) and enantioselectivities (87-93% ee). In the absence of 2-propanol, enantioselective carbonyl reverse prenylation is achieved directly from the alcohol oxidation level to furnish an equivalent set of adducts 4a-i in good to excellent isolated yields (68-94%) and enantioselectivities (86-91% ee). Competition and isotopic labeling experiments suggest rapid alcohol-aldehyde redox equilibration in advance of carbonyl addition along with capture of the kinetically formed pi-allyl complex at a higher rate than reversible beta-hydride elimination-hydrometalation. This protocol represents an alternative to the use of allylmetal reagents in enantioselective carbonyl reverse prenylation and represents the first use of allenes in enantioselective C-C bond-forming transfer hydrogenation.</P>
Persistent Paramagnons Deep in the Metallic Phase of<sub>Sr2−x</sub><sub>Lax</sub><sub>IrO4</sub>
Gretarsson, H.,Sung, N. H.,Porras, J.,Bertinshaw, J.,Dietl, C.,Bruin, Jan A. N.,Bangura, A. F.,Kim, Y. K.,Dinnebier, R.,Kim, Jungho,Al-Zein, A.,Moretti Sala, M.,Krisch, M.,Le Tacon, M.,Keimer, B.,Kim, American Physical Society 2016 Physical Review Letters Vol.117 No.10