Lead(IV) acetate mediated cleavage of β-hydroxy ethers: enantioselective synthesis of α-acetoxy carbonyl compounds

α-Acetoxy aldehydes or α-acetoxy ketones can be efficiently synthesized by treating 2,3-epoxy primary alcohols with lead tetraacetate. The reaction, which proceeds with complete regio- and stereoselectivity facilitates the enantioselective synthesis of α-acetoxy carbonyl compounds from allyl alcohols, via Sharpless epoxidation. Cyclic β-hydroxy ethers, with an oxygenated five-, six- or seven-membered ring, are transformed into α-acetoxy ethers.     

A highly stereoselective conversion of α,β-epoxy esters to α-hydroxy esters. An efficient route to optically active α-hydroxyesters

α,β-Epoxy esters were cleanly converted to α-hydroxy esters with retention of the configurations at the α-carbon atoms via MgI₂-promoted regioselective oxirane ring-opening followed by tributyltin hydride-reduction.     

Enantioselective oxidation of vic-diols to optically active α-hydroxy ketones by a fructose-derived dioxirane

Optically active α-hydroxy ketones 3 have been prepared in moderate to good enantioselectivities through asymmetrization of meso-, and kinetic resolution of, racemic vic-diols 2 by enantioselective oxidation with the in situ generated dioxirane from the fructose-derived ketone 1; the opposite sense in the enantioselectivity of the two processes is explained in terms of the hydrogen-bonded transition-state structures for the concerted C–H oxygen insertion.     

Highly enantioselective zinc/BINOL-catalyzed alkynylation of α-ketoimine ester: a new entry to optically active quaternary α-CF3 α-amino acids

An effective method for highly enantioselective alkynylation of ketoimine (α-trifluoromethyl ketoimine ester) has been developed via a zinc/BINOL catalyzed process. This protocol provides a useful and facile access to optically active quaternary α-trifluoromethyl α-amino acids and related derivatives of interest in life sciences.     

A novel synthesis of epoxides and allylic alcohols from carbonyl compounds through α, β-epoxy sulfoxides

Treatment of α,β-epoxy sulfoxides, easily prepared from carbonyl compounds with 1 equivalent of n-butyllithium at −100 °C gave the desulfinated epoxides in good yields. The similar α,β-epoxy sulfoxides having arylmethyl group at α-position, on treatment with excess n-butyllithium at −70 °C afforded 3-aryl allylic alcohols.     

Phosphorous acid promoted isomerization of propargyl alcohols to α,β-unsaturated carbonyl compounds

A metal-free and two-phase protocol for the Meyer-Schuster isomerization of propargyl alcohols to the corresponding α,β-unsaturated carbonyl compounds has been achieved in the presence of stoichiometric phosphorous acid aqueous solution, which produces the desired products in high yields with excellent stereoselectivity. Compared with the traditional methods, the procedure features broad scope of the substrates, mild conditions, and easy separation, providing an appealing alternative to the Meyer-Schuster reaction.     

A gold(I)-catalyzed route to α-sulfenylated carbonyl compounds from propargylic alcohols and aryl thiols

A one-step atom efficient gold(I)-catalyzed route to α-sulfenylated ketones and aldehydes from propargylic alcohols and aryl thiols is described.     

Rhodium/diene-catalyzed asymmetric arylation of N-sulfonyl indolylimines: a new access to highly optically active α-aryl 3-indolyl-methanamines

A new and efficient method for the preparation of highly enantiomerically enriched α-aryl 2- or 3-indolyl-methanamines by rhodium-catalyzed asymmetric arylation of N-sulfonyl indolylimines with arylboronic acids using chiral bicyclo[3.3.0] diene was developed.     

Synthesis of optically active α-hydroxy ketones by enantioselective oxidation of silyl enol ethers with a fructose-derived dioxirane

Optically active α-hydroxy ketones 3 have been prepared in moderate to high enantioselectivities by the asymmetric oxidation of the silyl enol ethers 2 with in situ generated dioxirane from the fructose-derived ketone 1. Best results (ee values up to 82%) for this novel non-transition metal mediated asymmetric α-hydroxylation may be obtained, when an excess of the fructose-derived ketone 1 is employed at pH ca. 8 and short reaction times. Valuable mechanistic information on the spiro versus planar transition states for the oxygen-transfer process has been aquired through the absolute configuration of the resulting α-hydroxy ketone 3 products.     

Organoselenium-mediated reduction of α,β-epoxy ketones to β-hydroxy ketones: A new access to inter- and intramolecular aldols

The organoselenium-mediated reduction of α,β-epoxy ketones has been demonstrated to be a promising entry to a variety of cyclic and acyclic aldols.     

Aerobic oxidative α-iodination of carbonyl compounds using molecular iodine activated by a nitrate-based catalytic system

The novel reaction system comprising air/NH₄NO_3(cat.)/I₂/H₂SO_4(cat.) is introduced as a simple, safe, cheap, efficient, and regioselective mediator for direct aerobic oxidative α-iodination of aryl, heteroaryl, alkyl, and cycloalkyl methyl ketones. The reaction system enabled the moderate to quantitative regioselective iodination of a large range of different methyl ketone derivatives including those bearing oxidizable heteroatom (S, N) substituents. Several activated aromatic compounds were also efficiently and selectively iodinated. The practical applicability of the presented reaction system was shown on 20 mmol scale under ambient pressure and 100% conversion of substrate was achieved.     

Enantioselective Diels-Alder reaction of α-(acylthio)acroleins: a new entry to sulfur-containing chiral quaternary carbons

A catalytic and enantioselective Diels-Alder reaction of α-(carbamoylthio)acroleins induced by an organoammonium salt of chiral triamine is described. α-(Carbamoylthio)acroleins are designed and synthesized as new sulfur-containing dienophiles for the first time. The Diels-Alder reaction affords chiral tertiary thiol precursors with up to 91% ee.     

Pyridine assisted oxidations of alcohols to carbonyl compounds by means of 3-car☐ypyridinium dichromate (ndc) reagent

3-Car☐ypyridinium dichromate (NDC),readly prepared from nicotinic acid and chromium trioxide, is an efficient reagent for the oxidation of alcohols into carbonyl compounds in the presence of pyridine. The optimum molar ratio substrate:reagent:pyridine to ensure complete oxidation of starting material in a short reaction time was found 1:2.5:20 respectively. A brief comparison between this reagent and pyridinium dichromate (PDC) is made. In contrast to the PDC reagent, NDC allows selective oxidation between benzylic alcohols and aliphatic alcohols. The NDC-pyridine system has been successfully extended to the oxidation of N-(2-hidroxy-2-phenyl or 2-methylethyl)-β-lactams into their corresponding carbonyl compounds as N-H azetidin-2-one precursors. In contrast, primary N-(2-hydroxyethyl)-β-lactams upon treatment with this reagent system afforded N-formylazetidin-2-ones.The influence of pyridine in oxidations by means of NDC is further shown in the conversion of hydroquinones into quinones. Another interesting feature associated with the use of this reagent is the ease of purification of the final products.     

Zinc-copper(I) chloride or -silver(I) acetate promoted aldol reaction of chlorodifluoromethyl ketones with carbonyl compounds. A general and effective route to α,α-difuluoro-β-hydroxy ketones

Chlorodifluoromethyl ketones efficiently underwent the aldol reaction with a variety of aldehydes or ketones in the presence of zinc, a catalytic amount of copper(I) chloride or silver(I) acetate and molecular sieves to give the corresponding α,α-difluoro-β-hydroxy ketones in good to excellent yields.     

Silafunctional compounds in organic synthesis. 29.1 Oxidation of (alkenyl)alkoxysilanes to α-hydroxy ketones

(Alkenyl)alkoxysilanes are readily transformed into α-hydroxy ketones via epoxidation and the subsequent oxidative cleavage of the carbon—silicon bonds.     

Copper catalysed 1,4-addition of organozinc reagents to α,β-unsaturated carbonyl compounds: a mechanistic investigation

A mechanistic study of the 1,4-addition of diethylzinc to 2-cyclohexenone catalysed by copper complexes of the Schiff base ligand H₂L1 was performed. The kinetic law of this system was determined and the nature of the different copper complexes involved in the reaction was investigated. The experimental results indicate a first-order dependence of the reaction rate on 2-cyclohexenone concentration, a zero-order on diethylzinc concentration, and a first-order dependence with respect to the concentration of a 1:1.2 mixture of Cu(OTf)₂ and H₂L1. A sharp change in the kinetics of the reaction was observed at catalyst concentrations higher than 9 mM, indicating the possible formation of catalytically inactive species. An aggregate copper complex, with the molecular formula [(CuL1)₂ · Cu(OTf)₂](TfOH)_1/3 (1), is formed upon mixing Cu(OTf)₂ and ligand H₂L1 in toluene. Complex 1 is reduced in situ to a catalytically active copper(I) species by addition of 12 equivalents of Et₂Zn. This species is able to perform the conjugate addition to 2-cyclohexenone under stoichiometric conditions and resumes its catalytic activity in the presence of 2-cyclohexenone and Et₂Zn.