Stereoselective Synthesis of an Immunomodulator (+)-Conagenin Using Dirhodium(II)-Catalyzed C-H Amination and Chelation-Controlled Reductions as Key Steps
The first totalsynthesis of (+)-conagenin, a novel immunomodulator produced by Streptomyces roseosporus, has been achieved in highly stereo- and enantioselective manner. The carboxylic acid moiety was synthesized starting with asymmetric aldol reaction of propiophenone with acetaldehyde followed by in situ syn-selective NaBH4 reduction. The amino acid moiety was synthesized based upon Et2AlCl catalyzed
Asymmetric Catalytic Enantio- and Diastereoselective Boron Conjugate Addition Reactions of α-Functionalized α,β-Unsaturated Carbonyl Substrates
作者:Jian-Bo Xie、Siqi Lin、Shuo Qiao、Guigen Li
DOI:10.1021/acs.orglett.6b01998
日期:2016.8.5
established for the asymmetric boron conjugate addition of B2pin2 onto α-functionalized (involving C, N, O, and Cl) α,β-unsaturated carbonyls under mild, neutral conditions involving Cu[(S)-(R)-ppfa]Cl, AgNTf2, and alcohols. The dual additives of AgNTf2 and alcohols were found to play crucial roles for achieving high catalytic activity and enantio- and diastereoselectivity (up to 98% ee and 70:1 dr).
Stereoselective synthesis of an immunomodulator (+)-conagenin was achieved. Both amine and carboxylic acid moieties were prepared from commercially available optically active methyl 3-hydroxy-2-methylpropanoate using dirhodium(II)-catalyzed C–H amination and chelation-controlled reductions as key steps. In addition, demethyl analogs of conagenin were synthesized using similar procedures.
An efficient totalsynthesis of (+)-conagenin was achieved. The right fragment of conagenin, α-methylserine containing a quaternary stereocenter attached to nitrogen, was synthesized using allyl cyanate-to-isocyanate rearrangement. The left fragment, 2,4-dihydroxy-3-methylpentanoic acid, has three contiguous stereogenic centers, which was efficiently constructed by enantioselective monoreduction of
Stereo- and Regioselectivity of Reactions of Siliranes with Aldehydes and Related Substrates
作者:Paul M. Bodnar、Wylie S. Palmer、Brian H. Ridgway、Jared T. Shaw、Jacqueline H. Smitrovich、K. A. Woerpel
DOI:10.1021/jo970263k
日期:1997.7.1
Siliranes undergo stereoselective and regioselective insertions of benzaldehyde to provide oxasi-lacyclopentane products. The thermal reaction (>100 degrees C) leads to more decomposition and side products, whereas the catalyzed variant (t-BuOK, (25 degrees C) proceeds more cleanly with a high degree of inversion (>95%). Treatment of siliranes with enolizable aldehydes leads to silyl enol ethers. The reaction of a silirane at high temperatures with an imine leads to reductive dimerization, presumably by way of intermediate-free silylene. The mechanism for the catalyzed insertion of benzaldehyde is discussed.