169136-81-6Relevant articles and documents
4-Alkoxycarbonyl- and aminocarbonyl-substituted isoxazoles as masked acrylates and acrylamides in the asymmetric synthesis of Δ2- isoxazolines
Lee, Connie K. Y.,Herlt, Anthony J.,Simpson, Gregory W.,Willis, Anthony C.,Easton, Christopher J.
, p. 3221 - 3231 (2007/10/03)
4-Alkoxycarbonyl and aminocarbonyl-substituted isoxazoles undergo conjugate reduction to give Δ2-isoxazolines on treatment with sodium borohydride and sodium trifluoroacetoxyborohydride, respectively. They are also alkylated at C5 through sonication with secondary and tertiary alkyl iodides in the presence of zinc dust and copper(I) iodide. These reactions are analogous to those observed with acrylates and acrylamides. The behavior is characteristic of the 4-substituted isoxazoles but not the 5-substituted regioisomers. The reductions of 4,5-disubstituted isoxazoles and the C5 alkylations of 4-substituted isoxazoles generally afford trans-4,5-disubstituted isoxazolines. Incorporating chiral auxiliaries into the alkoxycarbonyl group maintains this relative stereoselectivity. It does not provide significant levels of asymmetric induction in the reductions, but the alkylations occur with good levels of stereocontrol at both C4 and C5. Because both enantiomers of the auxiliaries are available, this provides access to either enantiomer of the products, in 93 to ≥98% de. The methodology, therefore, provides a complementary approach to nitrile oxide cycloadditions to alkenes for the asymmetric synthesis of Δ2-isoxazolines.
Lewis acid catalyzed allylboration: Discovery, optimization, and application to the formation of stereogenic quaternary carbon centers
Kennedy, Jason W. J.,Hall, Dennis G.
, p. 4412 - 4428 (2007/10/03)
A full account of the development of the first catalytic manifold for the additions of allylboronates to aldehydes is described. The thermal additions (both diastereospecific and enantioselective) of 2-carboxyester 3,3-disubstituted allylboronates 1 to both aromatic and aliphatic aldehydes give biologically and synthetically important exo-methylene butyrolactones 2 containing a β-quaternary carbon center. Although the thermal reaction requires 14 d at room temperature to reach completion, the presence of certain metal salts allows for a 12-16 h reaction while preserving the dia-stereospecificity observed in the uncatalyzed process. Preliminary mechanistic studies on the origin of the catalytic effect are described as well as stereoselective transformations of lactones 2 into cyclic and acyclic stereotriads with potential usefulness as synthetic intermediates.
Novel isomerically pure tetrasubstituted allylboronates: Stereocontrolled synthesis of α-exomethylene γ-lactones as aldol-like adducts with a stereogenic quaternary carbon center
Kennedy, Jason W. J.,Hall, Dennis G.
, p. 898 - 899 (2007/10/03)
In spite of their inherent isomerization tendency and low reactivity, 1-alkoxycarbonyl vinylcopper(I) intermediates from the conjugate addition of organocuprates onto acetylenic esters were trapped with very high cis-addition selectivity with iodomethylboronic esters in the presence of HMPA. The resulting isomerically pure 3,3-disubstituted allylboronates react with aldehydes in a highly diastereo- and enantioselective manner, providing α-exomethylene γ-lactones with a stereogenic quaternary β-carbon center. Copyright
Diastereoselectivity in the intermolecular Pauson-Khand reaction of chiral 2-alkynoates
Fonquerna, Silvia,Moyano, Albert,Pericas, Miquel A.,Riera, Antoni
, p. 4239 - 4254 (2007/10/02)
Chiral 2-alkynoates have been readily prepared by condensation of several cyclohexyl- or camphor-based alcohols with 2-alkynoic acids or the corresponding acid chlorides. The hexacarbonyldicobalt complexes obtained therefrom reacted with olefins to afford the corresponding Pauson-Khand cyclopentenone adducts in good yields, with high regioselectivity, and with variable degrees of diastereoselectivity. In some instances, the degree of stereocontrol is superior to that observed for alkoxyacetylenes, so that the viability of an enantioselective approach to the intermolecular Pauson-Khand reaction of electron-deficient alkynes is demonstrated for the first time.
High Asymmetric Induction in Conjugate Additions of RCu*BF3 to Chiral Enoates
Opplzer, Wolfgang,Loeher, Heinz J.
, p. 2808 - 2811 (2007/10/02)
1,4-Additions of PhCu*BF3, n-BuCu*BF3 and MeCu*BF3 to the trans-8-phenylmenthyl enoates 1 proceeded with high chiral induction.Saponification of the resulting esters 2 gave the corresponding enantiomerically pure β-substituted alkanoic acids 3 and the recovered (-)-8-phenylmenthol in good overall yields.Analogous additions to the cis-crotonate 1 led preferentially to the acids 3 enantiomeric to those obtained from the trans-crotonate 1, although with lower selectivity.A stereochemical model is proposed consistent with the observed results (Scheme 2, Table).
