- On the stereochemical course of the addition of allylsilanes to aldehydes
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Model compounds 3 and 5 have been studied to determine the orientation of the reacting double bonds in the transition state of the allylmetal-aldehyde addition. These models were designed to remove any intrinsic steric bias for the formation of the bicyclic products that would obfuscate a stereoelectronic contribution to the transition states. Model system 3 revealed a modest preference for the synclinal transition state, albeit in very low yields. Model system 5 underwent selective and largely Lewis acid independent cyclization primarily via a synclinal transition state. The high proximal selectivity observed in these cyclizations likely reflects the selectivity of an unhindered allylmetal-aldehyde addition for the synclinal transition state and results from a stereoelectronic preference, not an intrinsic steric bias, for the synclinal arrangement of double bonds.
- Denmark, Scott E.,Weber, Eric J.,Almstead, Neil G.,Wolf, Larry M.
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experimental part
p. 7701 - 7718
(2012/09/11)
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- Indirect electroreductive cyclization and electrohydrocyclization using catalytic reduced nickel(II) salen
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We describe efforts to achieve the electroreductive cyclization (ERC) and the electrohydrocyclization (EHC) reactions using catalytic nickel(II) salen as a mediator. While nickel(II) salen proved effective, the analogous cobalt complex as well as nickel(II) cyclam were not. The transformations were achieved in yields ranging from 60 to 94% using either a mercury pool or an environmentally preferable reticulated vitreous carbon (RVC) cathode. These examples represent the first instances wherein a nickel salen complex has been used in this manner. Clear differences between the voltammetric behavior of the ERC and EHC substrates were observed. The bisenoate 14, for example, displays a substantially larger catalytic current. When the structurally modified mediator 31 was used, the electron-transfer pathway shuts down. Instead, the reduced form of 31 behaves as an electrogenerated base, leading to the formation of the intramolecular Michael adduct 23. Presumably, the methyl groups of the modified ligand diminish the ability of the reduced form of the complex to serve as a nucleophile but not as a base. Aldehyde 23 was also characterized as a side product of the nickel(II) salen mediated electroreductive cyclization of 11. Given that it is absent from nonmediated processes, its formation is linked to the presence of the mediator. To account for the results, we favor the existence of a mechanistic continuum involving an equilibrium between nickel(II) salen (15) and two reduced forms, one being the metal-centered species 16, the other being a ligand-centered species 17. We postulate that one form may be more prominently involved with the chemistry than another, depending upon the electronic properties/requirements of the substrate, and suggest that the equilibrium will shift to accommodate the need. Thus, for a hard electrophile like an alkyl halide, the properties of 16 ought to dominate, whereas 17 ought to predominate as the reactive species accounting for the chemistry described herein since it properly matches a soft ligand-centered nucleophile with a soft electron deficient alkene.
- Miranda, James A.,Wade, Carolyn J.,Little, R. Daniel
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p. 8017 - 8026
(2007/10/03)
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- Reduction of alkyl (2-oxocyclohexyl)acetates by baker's yeast.
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Baker's yeast reduction of methyl and ethyl (2-oxocyclohexyl) acetates proceeded with enantio- and diastereo-selectivity, affording the corresponding (2S)-trans-alcohols (major), (2S)-cis-alcohols (minor), and the unaltered (1S)-ketones with high optical purity.
- Ganaha,Funabiki,Motoki,Yamauchi,Kinoshita
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p. 181 - 184
(2007/10/03)
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- Stereoselectivity in hydrosilylative reduction of substituted cyclohexanone derivatives with chiral rhodium-bis(oxazolinyl)pyridine catalyst
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Stereoselectivity in the reduction of substituted cyclohexanones, 4-tert-butylcyclohexanone, 2-methylcyclohexanone, 2-phenylcyclohexanone, and 2-methoxycarbonyl-methylcyclohexanone, was examined with chiral rhodium-bis(oxazolinyl)pyridine catalyst and diphenylsilane. 4-tert-Butylcyclohexanone gave the corresponding trans(equatorial)-alcohol predominantly; the ratio of the trans/cis alcohols, 67:33. Other 2-substituted cyclohexanones showed exclusive enantioselectivities for each diastereomer in terms of the kinetic resolution; e.g. from 2-phenylcyclohexanone, 99% ee of (1S,2R)-trans-2-phenylcyclohexanol and 96% ee of (1S,2S)-cis-2-phenylcyclohexanol in 92% yield (the trans/cis ratio = 51:49).
- Nishiyama,Park,Itoh
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p. 1029 - 1034
(2007/10/02)
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- TRIBUTYLTIN HYDRIDE-INDUCED O-STANNYL KETYLS IN THE CYCLIZATION OF ALDEHYDES AND KETONES WITH ALKENES
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An aldehyde or a ketone connected by a tether to an olefin efficiently cyclizes in a free radical reaction mediated by tributyltin hydride.The effects of activated olefins on this reaction, which provides functionalized cyclopentane rings and γ-lactones from O-stannyl ketyls in a mild and regiocontrolled manner, were also studied.
- Enholm, Eric J.,Prasad, Girija
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p. 4939 - 4942
(2007/10/02)
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- Electroreductive Cyclization. Ketones and Aldehydes Tethered to α,β-Unsaturated Esters (Nitriles). Fundamental Investigations
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The intramolecular electrochemically initiated cyclization of a variety of α,β-unsaturated esters and one nitrile, each of which is tethered to an aldehyde or a ketone, has been investigated.Good yields (70-79percent) of mono- and bicyclic products, resulting from closure between the β-carbon of the α,β-unsaturated unit and the aldehyde or ketone carbonyl carbon, were obtained.Cyclic voltammetry was used to determine that the α,β-unsaturated unit corresponded to the electrophore.In all but one instance, cyclization favored formation of the product wherein the hydroxy and (methoxycarbonyl)methyl units were trans to one another.The stereoselectivity was studied as a function of temperature, nature of the proton donor, proton availability, and percent conversion (i.e., as a function of time).Attempts to use the reaction to synthesize the marine natural product ambliol A were unsuccessful.A mechanistic scheme in which a reversible cyclization of the initially formed radical anion is followed by an irreversible proton transfer is suggested to account for the experimental observations.
- Little, R. Daniel,Fox, Dennis P.,Hijfte, Luc Van,Dannecker, Robert,Sowell, Gregory,et al.
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p. 2287 - 2294
(2007/10/02)
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- Ouverture de dichlorocyclopropanes en presence d'un nucleophile interne. Absence de participation intramoleculaire. Rearrangement concerte en chlorures allyliques
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It is shown that a conveniently placed internal nucleophile (carboxyl group) is not involved with the rearrangement of a diclorocyclopropane into an allylic chloride.This result appears to support a concerted mechanism of a ?s2 + ?a2 type for this rearrangement.In the products obtained, the allylic chloride may undergo displacement either by solvent (H2O), leading to alcohols, or by the internal carboxyl group, leading to a lactone.
- Chiche, Laurent,Christol, Henri,Coste, Jacques,Pietrasanta, Francine,Plenat, Francoise
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p. 164 - 174
(2007/10/02)
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