Stereochemistry of tetrahydropyran-2-yl ions, radicals, and related species. Conjugative and inductive effects

Gregory, Allan R.; Malatesta, V.

doi:10.1021/jo01289a023

Cited by 26 publications

(8 citation statements)

References 11 publications

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“…Presumably the reaction proceeds by stepwise reduction of the phenylthio ethers to PhSLi and a modestly pyramidal axial 4-dioxyl radical. The axial radical is predicted to be much more stable than the equatorial radical. , Rapid reductive lithiation of the axial radical gives the axial alkyllithium. Reductive lithiation of 4-(phenylthio)-1,3-dioxanes reliably generates axial alkyllithium reagents with high stereoselectivity.…”

Section: Discussionmentioning

confidence: 99%

Synthesis, Equilibration, and Coupling of 4-Lithio-1,3-dioxanes: Synthons for syn- and anti-1,3-Diols

et al. 1999

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Configurationally defined alpha-alkoxylithium reagents were prepared by reductive lithiation of 4-(phenylthio)-1,3-dioxanes. A new and more general synthesis of 4-(phenylthio)-1,3-dioxanes has been developed on the basis of the reduction and in situ acetylation of 1,3-dioxan-4-ones. For each of the substitution patterns examined (23a-d), reductive lithiation gave the axial alkyllithium (24a-d) with 99:1 stereoselectivity. Equilibrations of these alkyllithium reagents were possible with unhindered substrates to give the equatorial alkyllithiums 26a and 26b with excellent stereoselectivities. The more hindered axial alkyllithium reagents (24c, 24d) did not equilibrate efficiently. The equilibrium between alkyllithium reagents 24c and 26c strongly favors the equatorial isomer 26c. The inefficient equilibration with this hindered substrate is attributed to a slow rate of equilibration rather than insufficient driving force. These alkyllithium reagents could be coupled with a variety of electrophiles with retention of configuration by direct addition, copper-mediated coupling, or transmetalation to the corresponding alkylzinc reagent followed by copper-mediated coupling.

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Section: Discussionmentioning

confidence: 99%

Synthesis, Equilibration, and Coupling of 4-Lithio-1,3-dioxanes: Synthons for syn- and anti-1,3-Diols

et al. 1999

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“…alkoxyakyl radicals, are pyramidal and that the non-bonded C-orbital prefers an orientation which allows conjugative delocali~ation~) with the n-type orbital on the 0-atom [22], i.e. a (pseudo)axial orientation in I-oxacycloalk-2-yl radicals [23] [24]. This is substantiated by a stereoelectronic preference for the radical abstraction of (pseudo)axial a -and P-H-atoms in conformationally biased cyclic ethers [21] [23] .…”

Section: Discussionmentioning

confidence: 99%

“…IR: 2990~, 2935m, 1745~, 1452w, 1382s, 13703, 1225~, 1166~, 1095~, 2.03 (s, CH3); 1.57 (s, CH3); 1.44 (s, CH3), 1.40 (s, 2 CH3); 1.35 (s, CH3); 1.32 (s, CH3); 1.30 (s, CH3); 1.25 (s, CH3). 13C-NMR: 169.21 (s); 112.37 (s); 109.54 (s); 108.86 (s); 96.43 (d); 81.30 (6); 80.32 (6); 80.15 (0; 79.72 (6) 24.91 (4); 24.56 (4); 24.23 (4); 20.83 (4). MS (70 eV): 544 (< l), 529 (29), 486 (I), 471 (2).…”

Section: 7-mentioning

confidence: 99%

Deoxy‐nitrosugars. 6th communication. Stereoelectronic control in the reductive denitration of tertiary nitro ethers. A synthesis of ‘C‐glycosides’

Baumberger

Vasella

1983

Helvetica Chimica Acta

127

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SummaryThe separate, radical denitration with Bu3SnH of the pyranose derivatives 3, 4, 9, and 10 gave in good yields exclusively the 'C-glycosides' 5 and 11, respectively (Scheme 1). Similar reduction of the cyclohexyl derivatives 15, 16, 19 and 20 gave 4:l mixtures of 17, 18, 21 and 22, respectively, always with predominant formation of an axial C,H-bond. In the furanose series a divergent behaviour was observed for the D-mannose-derived nitro ethers 25 and 27 and the D-ribose-derived nitro ethers 30 and 31, respectively, in that the former two gave isomerically homogeneous reduction products (26 and 28, respectively; Scheme 3) and the latter a 1 : 1 mixture of the diastereoisomers 32 and 33 (Scheme 4). The stereochemical results were explained on the basis of the stereoelectronic effect of the ring O-atom, the preferred conformation of the intermediate, pyramidal alkoxyalkyl radicals and steric effects in the trioxabicyclo [3.3.0]octane ring system. As far as it is known, these reactions follow a radical-chain mechanism and proceed via planar radical intermediates. As expected, the few stereochemically relevant reductions so far studied occur without stereospecificity [2] and with a variable diastereoselectivity [7] which is difficult to predict2).In the case of conformationally biased n-nitro ethers, however, the analogous reductive denitrations are expected to occur with a high degree of diastereoselectivi ty .

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“…The reactions of cationic, radical, and anionic intermediates at the 2-position of tetrahydropyrans usually show good stereoselectivity. The chair conformation of the six-membered ring constrains the carbon−oxygen bond to a staggered orientation in the ground state and results in significant anisotropy of reactive intermediates at the anomeric center . In the cyanohydrin coupling strategy, the relative configuration of the product is determined by the preferred conformation of the intermediate anomeric radical .…”

mentioning

confidence: 99%

Carbon−Carbon Bond Formation from Small- and Medium-Ring Lactol Acetates via Radical and Oxonium Ion Intermediates. Synthesis of (±)-Laurenan

Rychnovsky

Dahanukar

1996

J. Org. Chem.

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Stereochemistry of tetrahydropyran-2-yl ions, radicals, and related species. Conjugative and inductive effects

Cited by 26 publications

References 11 publications

Synthesis, Equilibration, and Coupling of 4-Lithio-1,3-dioxanes: Synthons for syn- and anti-1,3-Diols

Synthesis, Equilibration, and Coupling of 4-Lithio-1,3-dioxanes: Synthons for syn- and anti-1,3-Diols

Deoxy‐nitrosugars. 6th communication. Stereoelectronic control in the reductive denitration of tertiary nitro ethers. A synthesis of ‘C‐glycosides’

Carbon−Carbon Bond Formation from Small- and Medium-Ring Lactol Acetates via Radical and Oxonium Ion Intermediates. Synthesis of (±)-Laurenan

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