Asymmetric oxidation of 1,2-diols using N-bromosuccinimide in the presence of chiral copper catalyst

Copper-Catalyzed Asymmetric Oxidative Desymmetrization of 2-Substituted 1,2,3-Triols

Asymmetric oxidative desymmetrization of 2-substituted glycerols has been achieved by using a new chiral bisoxazoline ligand/copper catalyst system with 1,3-dibromo-5,5-dimethylhydantoin and MeOH. The present transformation smoothly proceeds with readily accessible 2-(hetero)aryl- and alkyl-substituted glycerols and provides straightforward access toward various glycerate derivatives in good to high yields with high enantioselectivities. The synthetic utility of the present protocol was demonstrated by the transformation of the optically active glycerol into a glyceraldehyde derivative.

Visible-light enabled photochemical reduction of 1,2-dicarbonyl compounds by Hünig's base

A visible-light enabled, chemoselective photoreduction of 1,2-dicarbonyl compounds by using Hünig's base as reductant is reported.

Catalytic Reductive Cross-Coupling between Aromatic Aldehydes and Arylnitriles

A reductive cross-coupling reaction between aromatic aldehydes and arylnitriles using a copper catalyst and a silylboronate as a reductant is reported. This protocol represents an unprecedented approach to the chemoselective synthesis of α-hydroxy ketones by electrophile-electrophile cross-coupling.

Microbial Synthesis of (S)- and (R)-Benzoin in Enantioselective Desymmetrization and Deracemization Catalyzed by Aureobasidium pullulans Included in the Blossom Protect™ Agent

In this study, we examined the Aureobasidium pullulans strains DSM 14940 and DSM 14941 included in the Blossom Protect™ agent to be used in the bioreduction reaction of a symmetrical dicarbonyl compound. Both chiral 2-hydroxy-1,2-diphenylethanone antipodes were obtained with a high enantiomeric purity. Mild conditions (phosphate buffer [pH 7.0, 7.2], 30 °C) were successfully employed in the synthesis of (S)-benzoin using two different methodologies: benzyl desymmetrization and rac-benzoin deracemization. Bioreduction carried out with higher reagent concentrations, lower pH values and prolonged reaction time, and in the presence of additives, enabled enrichment of the reaction mixture with (R)-benzoin. The described procedure is a potentially useful tool in the synthesis of chiral building blocks with a defined configuration in a simple and economical process with a lower environmental impact, enabling one-pot biotransformation.

Chiral Benzoins via Asymmetric Transfer Hemihydrogenation of Benzils: The Detail that Matters

The synthesis of enantiomerically pure benzoins by hydrogenation of readily available benzils has been long thwarted by their base-sensitivity. We show here that an iron(II) hydride complex catalyzes the asymmetric transfer hydrogenation of benzils from 2-propanol. When strictly base-free conditions are granted, excellent enantioselectivity is achieved even with o-substituted substrates, which are particularly challenging to prepare with other methods. Hence, under optimized reaction conditions, chiral benzoins were prepared in good yields (up to 83%) and excellent enantioselectivity (up to 98% ee) in short reaction times (30-75 min). Also, this work confirms that both enantiomers of the benzoin products can be accessed when a metal catalyst is used, which is a clear advantage over enzymatic methods.

Visible-light-mediated iodine-catalyzed α-hydroxylation of α-methylene ketones under aerobic conditions

A visible-light-mediated α-hydroxylation of α-methylene ketones using atmospheric oxygen as a green oxidant has been developed with novel substrate selectivity.

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

Bromination is one of the most important transformations in organic synthesis and can be carried out using bromine and many other bromo compounds. Use of molecular bromine in organic synthesis is well-known. However, due to the hazardous nature of bromine, enormous growth has been witnessed in the past several decades for the development of solid bromine carriers. This review outlines the use of bromine and different bromo-organic compounds in organic synthesis. The applications of bromine, a total of 107 bromo-organic compounds, 11 other brominating agents, and a few natural bromine sources were incorporated. The scope of these reagents for various organic transformations such as bromination, cohalogenation, oxidation, cyclization, ring-opening reactions, substitution, rearrangement, hydrolysis, catalysis, etc. has been described briefly to highlight important aspects of the bromo-organic compounds in organic synthesis.

Nitrogen enriched mesoporous organic polymer anchored copper(ii) material: an efficient and reusable catalyst for the synthesis of esters and amides from aromatic systems

New mesoporous polymer anchored copper acetate (Cu-mPMF) has been synthesized and well characterized. The catalytic performance of this complex has been tested for the synthesis of esters and amides.

Biocatalyzed asymmetric reduction of benzils to either benzoins or hydrobenzoins: pH dependent switch

Different enzyme activities present in a whole-cell biocatalyst have been selectively harnessed to asymmetrically reduce bulky–bulky 1,2-diketones to either 2-hydroxyketones or 1,2-diols.

Biocatalytic route to chiral acyloins: P450-catalyzed regio- and enantioselective α-hydroxylation of ketones

P450-BM3 and mutants of this monooxygenase generated by directed evolution are excellent catalysts for the oxidative α-hydroxylation of ketones with formation of chiral acyloins with high regioselectivity (up to 99%) and enantioselectivity (up to 99% ee). This constitutes a new route to a class of chiral compounds that are useful intermediates in the synthesis of many kinds of biologically active compounds.

Microwave-assisted kinetic resolution of homochiral (Z)-cyclooct-5-ene-1,2-diol and (Z)-2-acetoxycyclooct-4-enyl acetate using lipases

Over the last decade, the use of biocatalysts has become an attractive alternative to conventional chemical methods, especially for organic synthesis, due to their unusual properties. Among these enzymes, lipases are the most widely used, because they are cheap, easily available, cofactor-free, and have broad substrate specificity. Combined to microwave heating in non-aqueous medium, recent results suggest that irradiation may influence the enzyme activity. This Communication reports the benefits of lipases and the microwave irradiation on the kinetic resolution of racemic homochiral (Z)-cyclooct-5-ene-1,2-diol and (Z)-2-acetoxycyclooct-4-enyl acetate. In order to best achieve the kinetic resolution, different parameters were studied including the type of lipase, the temperature, the impact of microwave power compared to conventional heating. Optimization of the reaction parameters lead to the obtainment of highly enriched or enantiopure diols and diesters in a clean, efficient and safe way.

An efficient oxidation of alcohols using a new trinuclear copper complex as a reusable catalyst under solvent free conditions

A new copper complex has shown efficacy as a catalyst for the selective oxidation of different alcohols using H₂O₂ as an oxidant under solvent free conditions.

Regio- and enantio-selective oxidation of diols by Candida parapsilosis ATCC 7330

Optically pure (S)-diols and their corresponding hydroxy ketones were prepared via regio- and enantio-selective oxidation of the corresponding diols using whole cells of Candida parapsilosis ATCC 7330.

Chemoselective Pd-catalyzed oxidation of polyols: synthetic scope and mechanistic studies

The regio- and chemoselective oxidation of unprotected vicinal polyols with [(neocuproine)Pd(OAc)]2(OTf)2 (1) (neocuproine = 2,9-dimethyl-1,10-phenanthroline) occurs readily under mild reaction conditions to generate α-hydroxy ketones. The oxidation of vicinal diols is both faster and more selective than the oxidation of primary and secondary alcohols; vicinal 1,2-diols are oxidized selectively to hydroxy ketones, whereas primary alcohols are oxidized in preference to secondary alcohols. Oxidative lactonization of 1,5-diols yields cyclic lactones. Catalyst loadings as low as 0.12 mol % in oxidation reactions on a 10 g scale can be used. The exquisite selectivity of this catalyst system is evident in the chemoselective and stereospecific oxidation of the polyol (S,S)-1,2,3,4-tetrahydroxybutane [(S,S)-threitol] to (S)-erythrulose. Mechanistic, kinetic, and theoretical studies revealed that the rate laws for the oxidation of primary and secondary alcohols differ from those of diols. Density functional theory calculations support the conclusion that β-hydride elimination to give hydroxy ketones is product-determining for the oxidation of vicinal diols, whereas for primary and secondary alcohols, pre-equilibria favoring primary alkoxides are product-determining. In situ desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed catalytic cycle.

Highly efficient Cu(I)-catalyzed oxidation of alcohols to ketones and aldehydes with diaziridinone

A novel and efficient Cu(I)-catalyzed oxidation of alcohols has been achieved with di-tert-butyldiaziridinone as the oxidant under mild conditions. A wide variety of primary and secondary alcohols with various functional groups can be oxidized to aldehydes and ketones in high yields. The reaction proceeds under neutral conditions making it compatible with acid- or base-sensitive substrates, and it is amenable to gram scale.

Enantioselective Synthesis of 2-Hydroxy-5-Methyl-3-Hexanone

2-Hydroxy-5-methyl-3-hexanone was synthesised enantioselectively by Sharpless asymmetric dihydroxylation (Sharpless AD) or Shi's asymmetric epoxidation (Shi's AE) of 5-methyl-3-trimethylsiloxy-2-hexene. This was obtained in an E/Z ratio of 7.5/1 by deprotonation of 5-methyl-3-hexanone with NaHMDS in n-hexane followed by reaction with chlorotrimethylsilane. Optically active 2-hydroxy-5-methyl-3-hexanone was obtained in a 76.9% yield with a 75.6% ee when AD-mix-β was used as oxidant. Its absolute configuration was tentatively assigned to be R according to the AD face selection rule. AD-mix-α gave the product in a 69.2% yield only with a 15.7% ee. The major product had the same configuration as that obtained by AD-mix-β. Optically active 2-hydroxy-5-methyl-3-hexanone was obtained in a 54% yield with a 74.6% ee by Shi's AE when a catalyst derived from D-fructose was used, while in a 50% yield with a 72.8% ee when a catalyst derived from L-fructose was used. The absolute configuration of the product obtained by a catalyst from D-fructose was deduced to be R whilst a catalyst from L-fructose gave the ( S)-enantiomer by comparison of the retention time on a chiral column G-TA with the product obtained by AD-mix-β.