- Direct organocatalytic stereoselective transfer hydrogenation of conjugated olefins of steroids
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Kinetically controlled and organocatalytic syn-selective transfer hydrogenation has been successfully demonstrated for the reduction of the enone functional group of various steroids. Herein, diastereoselective synthesis of many 5β-steroids have been reported through organocatalysis, which have broad medicinal and pharmaceutical applications. The mechanistic studies and the selectivity of the products clearly indicated that the catalyst 1b·d-CSA is mild enough to activate the various chiral cyclic enones through iminium ion formation during the organocatalytic transfer hydrogenations with Hantzsch ester 2a as a hydrogen source. Further, clear evidence for the selective formation of intermediate iminium species [I]+ have been characterized through on-line monitoring of controlled experiments by NMR and ESI-HRMS analyses.
- Ramachary, Dhevalapally B.,Sakthidevi, Rajasekar,Reddy, P. Srinivasa
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p. 13497 - 13506
(2013/09/02)
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- One-Pot Bi(OTf)catalyzed oxidative deprotection of tert -butyldimethyl silyl ethers with TEMPO and co-oxidants
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A sequential one-pot synthesis for the oxidation of primary and secondary tert-butyldimethylsilyl (TBDMS) ethers, using catalytic amounts of metal triflates and TEMPO in combination with PhIO or PhI(OAc)in THF or acetonitrile, is described. Acid-sensitive protecting groups such as methylidene, isopropylidene, acetals, and Boc are unaffected under the reaction conditions. Another feature of this procedure is its high selectivity for TBDMS ethers over tert-butyldiphenylsilyl ethers and of aliphatic TBDMS groups over phenolic TBDMS groups. Georg Thieme Verlag Stuttgart - New York.
- Barnych, Bogdan,Vatèle, Jean-Michel
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experimental part
p. 2048 - 2052
(2011/10/19)
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- Application of the excited state meta effect in photolabile protecting group design
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A novel photolabile protecting group for carbonyl compounds has been developed, based on the excited state meta effect.
- Wang, Pengfei,Hu, Ayou,Wang, Yun
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p. 2831 - 2833
(2008/02/07)
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- Investigation on the regioselectivities of intramolecular oxidation of unactivated C-H bonds by dioxiranes generated in Situ
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We found that dioxiranes generated in situ from ketones 1-6 and Oxone underwent intramolecular oxidation of unactivated C-H bonds at δ sites of ketones to yield tetrahydropyrans. From the trans/cis ratio of oxidation products 1a and 2a as well as the retention of the configuration at the δ site of ketone 5, we proposed that the oxidation reaction proceeds through a concerted pathway under a spiro transition state. The intramolecular oxidation of ketone 6 showed the preference for a tertiary δ C-H bond over a secondary one. This intramolecular oxidation method can be extended to the oxidation of the tertiary γ′ C-H bond of ketones 9 and 10. For ketone 11 with two δ C-H bonds and one γ′ C-H bond linked respectively by a sp3 hydrocarbon tether and a sp2 ester tether, the oxidation took place exclusively at the δ C-H bonds. Finally, by introducing proper tethers, regioselective hydroxylation of steroid ketones 12-14 have been achieved at the C-17, C-16, C-3, and C-5 positions.
- Wong, Man-Kin,Chung, Nga-Wai,He, Lan,Wang, Xue-Chao,Yan, Zheng,Tang, Yeung-Chiu,Yang, Dan
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p. 6321 - 6328
(2007/10/03)
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- Controlling the reactive state through cation binding: Photochemistry of enones within zeolites
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The nature of the lowest triplet state of enones is altered by the cations present within Y zeolites. Alkali metal ions, such as Li+, are predicted to interact with the carbonyl unit of enones in a collinear fashion and significantly lower both the p-type n and π-2 orbitals. Excited state energies, estimated at the CIS(D)/6-31+G* level, show that the lowest triplet is n-π* in character for the enones, but switch to π-π* on coordination with Li+. Observed product distribution within zeolite is consistent with this theoretical prediction.
- Uppili, Sundararajan,Takagi, Shinsuke,Sunoj,Lakshminarasimhan,Chandrasekhar,Ramamurthy
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p. 2079 - 2083
(2007/10/03)
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- Palladium(II)-catalyzed oxidation of alcohols to aldehydes and ketones by molecular oxygen
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A novel combination of Pd(OAc)2/pyridine/MS3A catalyzes the aerobic oxidation in toluene of a variety of primary and secondary alcohols into the corresponding aldehydes and ketones in high yields. Various substituents and protecting groups are compatible with this oxidation. The ca. 2:3 ratio of O2 uptake to product yield is observed, whereas in the absence of MS3A, the ratio is ca. 1:1, suggesting the in situ formation of H2O2 and its decomposition by MS3A into water and oxygen. A catalytic cycle including the formation of a Pd(II)-alcoholate followed by β-elimination of a Pd(II)H species and a carbonyl compound and then the formation of a Pd(II)OOH species is proposed.
- Nishimura, Takahiro,Onoue, Tomoaki,Ohe, Kouichi,Uemura, Sakae
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p. 6750 - 6755
(2007/10/03)
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- Study of the reductive Cleavage of Selenides with Nickel Boride. A Convenient Deselenization Procedure
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The reductive deselenization of a variety of organoselenium compounds can be performed rapidly, conveniently, and in high yield by using nickel boride.The latter reagent was generated in situ by adding sodium borohydride to nickel chloride hexahydrate and the substrate selenide in THF-methanol at 0 deg C.Under these conditions, ca. 3-3.5 mol of borohydride was required to reduce all of the nickel salt to nickel boride and an excess of nickel boride was generally required to effect complete deselenization.Preformed nickel boride lost virtually all of its ability to cleave n-dodecyl phenyl selenide (2) after only 5 min, but continued to catalyze the decomposition of sodium borohydride.Deselenizing ability was not restored by the addition of hydrogen or further sodium borohydride to preformed nickel boride.Deuterium-labeling studies indicated that the hydrogen required for reductive deselenization originates from both the borohydride and the methanol solvent in a ratio of ca. 3:1.The use of sodium borodeuteride in methanol-d afforded 1-deuteriododecane of high isotopic purity, indicating that the procedure comprises a convenient method for preparing deuteriated products.The mechanism of nickel boride deselenization may involve transient nickel hydride or Ni(0) intermediates.The chemoselectivity of the reaction permits the cleavage of C-Se bonds in the presence of chlorides, nitriles, esters, sulfides, sulfones, ketones, and some olefins, but iodides and sulfinate esters undergo concomitant or preferential reduction.Electrochemical deselenization of 2 was studied and resulted in anodic oxidation of the selenide to 1-dodecene instead of cathodic reduction to n-dodecane.
- Back, Thomas G.,Birss, Viola I.,Edwards, Mark,Krishna, M. Vijaya
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p. 3815 - 3822
(2007/10/02)
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- Isomerization of Homoallyl Alcohols over Raney Nickel
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Homoallyl alcohols undergo ready isomerization to the corresponding aldehydes or ketones when refluxed in a solvent with Raney nickel.Reaction pathways, based on identification of intermediates of the reactions, involve dehydrogenation-hydrogenation for the cyclic alcohols and double bond shift for the open chain alcohols.
- Prasad, J. V. N. Vara,Pillai, C. N.
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p. 998 - 999
(2007/10/02)
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- Highly Chemoselective Palladium-Catalyzed Conjugate Reduction of α,β-Unsaturated Carbonyl Compounds with Silicon Hydrides and Zinc Chloride Cocatalyst
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A three-component system comparised of a soluble palladium catalyst, hydridosilane, and zinc chloride is capable of efficient conjugate reduction of α,β-unsaturated ketones and aldehydes.The optimal set of condition includes diphenylsilane as the most effective hydride donor, any soluble palladium complex in either the 0 or II oxidation state, when it is stabilized by phosphine ligands, and ZnCl2 as the best Lewis acid cocatalyst.The reaction is very general with respect to a broad range of unsaturated ketones and aldehydes, and it is highly selective for these Michael acceptors, as reduction of α,β-unsaturated carboxylic acid derivatives is very sluggish under these conditions.When dideuteriodiphenylsilane is used to reduce unsaturated ketones, deuterium is stereoselectively introduced at the less-hindered face of the substrate and regioselectively at the β-position.Conversely, when reductions are carried out in the presence of traces of D2O, deuterium incorporation occurs at the α-position.On the basis of deuterium-incorporation experiments and 1H NMR studies, a catalytic cycle is postulated in which the first step involves reversible coordination of the palladium complex to the electron-deficient olefin and oxidative addition of silicon hydride to form a hydropalladium olefin complex.Migratory insertion of hydride into the coordinated olefin produces an intermediate palladium enolate which, via reductive elimination, collapses back to the Pd(0) complex and a silyl enol ether, which is then hydrolyzed to the saturated ketone.In addition to catalyzing that hydrolysis, ZnCl2 facilitates the hydrosilation process.
- Keinan, Ehud,Greenspoon, Noam
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p. 7314 - 7325
(2007/10/02)
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- ZINC CHLORIDE-MEDIATED CONJUGATE REDUCTION WITH SILICON HYDRIDES AND PALLADIUM(0) CATALYST
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The chemistry of diphenylsilane/Pd(0), a system previously known to reduce only allylic heterosubstituents, has been radically altered by addition of catalytic amounts of zinc chloride, enabling efficient conjugate reduction of α,β-unsaturated ketones and aldehydes.
- Keinan, Ehud,Greenspoon, Noam
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p. 1353 - 1356
(2007/10/02)
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- A Convenient Procedure for the Reductive Deselenization of Selenides with Nickel Boride
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Nickel boride, readily produced in situ from nickel chloride hexahydrate and sodium borohydride, efficiently reduces selenides to hydrocarbons under mild conditions.
- Back, Thomas G.
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p. 1417 - 1418
(2007/10/02)
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- Stereochemistry of the Palladium-catalyzed Hydrogenation of 3-Oxo-4-ene Steroids. V. A Kinetic Study in Basic and Acidic Media
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Effects of the β-methyl group at C-10 and some oxygen functions (=O, OH, OAc) at C-11, C-17, and C-20 on the rates of hydrogenation of 3-oxo-4-ene steroids have been studied with palladium catalyst in pyridine or THF/HBr as solvent.In contrast to the hydrogenation in pyridine, the rate in THF/HBr was greatly depressed by the presence of 10β-methyl group.The reactivity of the steroids was enhanced by the oxygen functions, in particular, by 11, 17-dioxo group.The effects of the substituents are discussed from a mechanistic consideration based on the obtained results.
- Nishimura, Shigeo,Momma, Yasuhiro,Kawamura, Hideo,Shiota, Michio
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p. 780 - 783
(2007/10/02)
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- Activation of Reducing Agents. Sodium Hydride Containing Complex Reducing Agents. 13. Selective Heterogeneous Hydrogenation of Polyfunctional Substrates over Nic
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Preparative-scale heterogeneous hydrogenation over Nic at room temperature and 1 atm are described.It is shown that these catalysts allow the selective hydrogenation of carbon-carbon double bonds in the presence of oxo groups without side reactions.Alkynes and functional alkynes are selectively hydrogenated to the corresponding cis alkenes in high yields.Carbonyl group hydrogenations were also performed in high yields.Selective hydrogenation properties of Nic were exemplified in the steroid series.Finally, it was demonstrated that this new catalyst-preparation concept is not limited to nickel and also applies to the preparation of cobalt and palladium catalysts.
- Gallois, Philippe,Brunet, Jean-Jaques,Caubere, Paul
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p. 1946 - 1950
(2007/10/02)
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- CONVERSION OF THE ALLYLIC AND HOMOALLYLIC STEROIDAL ALCOHOLS TO THE CORRESPONDING SATURATED KETONES BY MEANS OF THE ACTIVATED Ra/Ni
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Conversion of the allylic and homoallilyc alcohols (1) and (2) to the corresponding saturated ketones (3) and (4), by means of the activated Ra/Ni, in the presence of cyclohexanone, was achieved stereospecifically and high yields.
- Forsek, Joze
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p. 1071 - 1074
(2007/10/02)
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- NAD(P)H Models XV. Reduction of Δ4-3-ketosteroids via their iminium salts, by 1,4-dihydropyridine derivatives
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Pyrrolidinium salts derived from a series of 17-substituted Δ4-3-ketosteroids are reduced by 1-benzyl-1,4-dihydronicotinamide (BNAH) and 2,6-dimethyl-3,5-diethoxycarbonyl-1,4-dihydropyridine (Hantzsch ester, 1 eq.) to the corresponding isomeric 5α- and 5β-enamines which hydrolyse to the corresponding 3-ketosteroids.In the case of the Hantzsch ester reduction, proton transfer from the oxidized Hantzsch ester to the enamine results in the formation of iminium salts, which are further reduced to the corresponding ammonium salts.Mechanistic, stereochemical and biochemical implications of the reactions are discussed.
- Gase, Ronald A.,Pandit, Upendra K.
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p. 334 - 340
(2007/10/02)
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- Removal of Thioacetal Protecting Groups by Benzeneseleninic Anhydride
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A number of thioacetals were deprotected with benzeneseleninic anhydride in good yield.The reaction worked particularly well for hindered spiro-1,3-dithiolans of 2,2,6-trimethylcyclohexanone and fenchone, and in examples where other literature methods had failed.
- Cussans, Nigel J.,Ley, Steven V.,Barton, Derek H. R.
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p. 1654 - 1657
(2007/10/02)
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- Silanes in Organic Synthesis. 9. Enesilylation as a Method for 1,2-Carbonyl Migration within Ketones and for Conversion to 1,2-Transposed Allylic Alcohols
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Vinylsilanes are shown to be valuable synthetic intermediates in useful transformations of ketones.The epoxidation of vinylsilanes followed by lithium aluminium hydride reduction and oxidation with chromic acid and sulfuric acid in a two-phase (ether/water) system often gives high yields of 1,2-transposed ketones.With singlet oxygen and sequential sodium borohydride reduction, 2-trimethylsilyl alcohols are produced in which the α position of the parent ketone has been regiospecifically oxygenated.Fluoride ion promoted desilylation completes the conversion to the migrated allylic alcohol.
- Fristad, William E.,Bailey, Thomas R.,Paquette, Leo A.
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p. 3028 - 3037
(2007/10/02)
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- OXIDATION OF ALCOHOLS WITH OXOPEROXOBIS-(N-PHENYLBENZOHYDROXAMATO)MOLYBDENUM(VI)
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The title complex oxidizes primary and secondary alcohols to the corresponding carbonyl compounds.Stereoselective epoxidation of allylic alcohols is also described.
- Tomioka, Hiroki,Takai, Kazuhiko,Oshima, Koichiro,Nozaki, Hitosi,Toriumi, Koshiro
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p. 4843 - 4846
(2007/10/02)
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