Organocatalytic oxidation. Asymmetric epoxidation of olefins catalyzed by chiral ketones and iminium salts

Desymmetrization of Diarylmethylamido Bis(phenols) through Peptide-Catalyzed Bromination: Enantiodivergence as a Consequence of a 2 amu Alteration at an Achiral Residue within the Catalyst

Diarylmethylamido bis(phenols) have been subjected to peptide-catalyzed, enantioselective bromination reactions. Desymmetrization of compounds in this class has been achieved such that enantioenriched products may be isolated with up to 97:3 er. Mechanistically, the observed enantioselectivity was shown to be primarily a function of differential functionalization of enantiotopic arenes, although additional studies unveiled a contribution from secondary kinetic resolution of the product (to afford the symmetrical dibromide) under the reaction conditions. Variants of the tetrapeptide catalyst were also evaluated and revealed a striking observation-enantiodivergent catalysis is observed upon changing the achiral amino acid residue in the catalyst (at the i+2 position) from an aminocyclopropane carboxamide residue (97:3 er) to an aminoisobutyramide residue (33:67 er) under a common set of conditions. An expanded set of catalysts was also evaluated, enabling structure/selectivity correlations to be considered in a mechanistic light.

Chemoselective P(NMe2)3-Mediated Reductive Epoxidation between Two Different Carbonyl Electrophiles: Synthesis of Highly Functionalized Unsymmetrical Epoxides

Herein, we report a chemoselective P(NMe₂)₃-mediated reductive epoxidation of α-dicarbonyl compounds such as isatins, α-keto esters, and α-diketones with aldehydes and ketones, leading to an efficient synthesis of a wide range of highly functionalized unsymmetrical epoxides in moderate to excellent yields and diastereoselectivities. The Kukhtin-Ramirez adduct, which is exclusively generated in situ from an α-dicarbonyl compound and P(NMe₂)₃, plays a key role in governing the chemoselectivity. It represents the first practical synthesis of unsymmetrical epoxides via direct reductive epoxidation of two different carbonyl electrophiles and also complements the existing methods of generating epoxides.

Asymmetric Epoxidation of 1,4-Naphthoquinones Catalyzed by Guanidine-Urea Bifunctional Organocatalyst

An enantioselective nucleophilic epoxidation of 2-substituted 1,4-naphthoquinones in the presence of a newly developed guanidine-bisurea bifunctional organocatalyst with tert-butyl hydroperoxide (TBHP) as an oxidant is presented. 1,4-Naphthoquinones bearing substituents at C6, C7, and C2 were available for the reaction, and the corresponding epoxides were obtained with 88:12-95:5 er in 71-98% yields. DFT calculations indicated that substituents at C2 and C6 in the terminal Ar group of the catalyst 9k play a key role in controlling the stereochemical outcome.

Substrate-Directed Catalytic Selective Chemical Reactions

The development of highly efficient reactions at only the desired position is one of the most important subjects in organic chemistry. Most of the reactions in current organic chemistry are reagent- or catalyst-controlled reactions, and the regio- and stereoselectivity of the reactions are determined by the inherent nature of the reagent or catalyst. In sharp contrast, substrate-directed reaction determines the selectivity of the reactions by the functional group on the substrate and can strictly distinguish sterically and electronically similar multiple reaction sites in the substrate. In this Perspective, three topics of substrate-directed reaction are mainly reviewed: (1) directing group-assisted epoxidation of alkenes, (2) ring-opening reactions of epoxides by various nucleophiles, and (3) catalytic peptide synthesis. Our newly developed synthetic methods with new ligands including hydroxamic acid derived ligands realized not only highly efficient reactions but also pinpointed reactions at the expected position, demonstrating the substrate-directed reaction as a powerful method to achieve the desired regio- and stereoselective functionalization of molecules from different viewpoints of reagent- or catalyst-controlled reactions.

A highly enantioselective asymmetric Darzens reaction catalysed by proline based efficient organocatalysts for the synthesis of di- and tri-substituted epoxides

A new class of easily available and readily tunable proline based chiral organocatalysts was found to efficiently catalyse an unprecedented highly enantioselective asymmetric Darzens reaction of α-chloroketones and substituted α-chloroketones with various aldehydes, which directly produces optically active di- and tri-substituted chiral epoxides with higher product yields (up to 97%) and excellent ee's (up to 99%) under mild reaction conditions.

Enantioselective total synthesis of sagittacin E and related natural products

The first enantioselective total synthesis of eremophilane-type sesquiterpenoids, sagittacin E and related natural products, was achieved.

Development of recyclable chiral macrocyclic metal complexes for asymmetric aminolysis of epoxides: application for the synthesis of an enantiopure oxazolidine ring

Macrocyclic Cr(iii)-salen complexes were synthesized for the ring opening reaction of various epoxides with anilines to furnish the corresponding β-amino-α-hydroxyl esters and β-amino alcohols with excellent ee/yield upto 99/95%.

Highly Stereoselective Synthesis of Tetrasubstituted Acyclic All-Carbon Olefins via Enol Tosylation and Suzuki-Miyaura Coupling

A highly stereocontrolled synthesis of tetrasubstituted acyclic all-carbon olefins has been developed via a stereoselective enolization and tosylate formation, followed by a palladium-catalyzed Suzuki-Miyaura cross-coupling of the tosylates and pinacol boronic esters in the presence of a Pd(OAc)2/RuPhos catalytic system. Both the enol tosylation and Suzuki-Miyaura coupling reactions tolerate an array of electronically and sterically diverse substituents and generate high yield and stereoselectivity of the olefin products. Judicious choice of substrate and coupling partner provides access to either the E- or Z-olefin with excellent yield and stereochemical fidelity. Olefin isomerization was observed during the Suzuki-Miyaura coupling. However, under the optimized cross-coupling reaction conditions, the isomerization was suppressed to <5% in most cases. Mechanistic probes indicate that the olefin isomerization occurs via an intermediate, possibly a zwitterionic palladium carbenoid species.

Molecular Recognition and Chirality Sensing of Epoxides in Water Using Endo-Functionalized Molecular Tubes

Chiral epoxides are important intermediates in chemistry and biology. The high-throughput screening of asymmetric epoxidation conditions requires fast determination of the absolute configurations and ee values of chiral epoxides. Herein, we report molecular recognition and chiroptical sensing of epoxides in water using endo-functionalized molecular tubes. The absolute configurations and ee values were simultaneously determined by circular dichroism spectroscopy. In addition, real-time monitoring as well as the application to real asymmetric epoxidation was demonstrated. The method is simple, environmentally friendly, and amenable to high-throughput screening.

Synthetic Route Development for the Laboratory Preparation of Eupalinilide E

Following the discovery that the guaianolide natural product eupalinilide E promotes the expansion of hematopoietic stem and progenitor cells; the development of a synthetic route to provide laboratory access to the natural product became a priority. Exploration of multiple synthetic routes yielded an approach that has permitted a scalable synthesis of the natural product. Two routes that failed to access eupalinilide E were triaged either as a result of providing an incorrect diastereomer or due to lack of synthetic efficiency. The successful strategy relied on late-stage allylic oxidations at two separate positions of the molecule, which significantly increased the breadth of reactions that could be used to this point. Subsequent to C-H bond oxidation, adaptations of existing chemical transformations were required to permit chemoselective reduction and oxidation reactions. These transformations included a modified Luche reduction and a selective homoallylic alcohol epoxidation.

Synthetic strategies towards mycolactone A/B, an exotoxin secreted by Mycobacterium ulcerans

Pitfalls and dead-ends pave the way to mycolactone A/B. This full account reports synthetic efforts towards this natural product that eventually culminated in a de novo total synthesis.

Selective C–H bond hydroxylation of cyclohexanes in water by supramolecular control

A new approach for selective hydroxylation of non-activated cyclohexanes using dioxirane generated in situ in water through supramolecular control has been developed.

One-pot synthesis of α,β-epoxy ketones through domino reaction between alkenes and aldehydes catalyzed by proline based chiral organocatalysts

A proline based metal free organocatalysts developed by new approach for the synthesis of epoxide derivatives through domino reaction. This domino reaction allows the direct access to epoxide from various alkene and aldehyde through C–H functionalization and C–C/C–O bond formation. The catalytic efficiencies of organocatalysts were also determined by domino reaction with very good yield (up to 95 %) and ee's (up to 99 %).

Organocatalysis using aldehydes: the development and improvement of catalytic hydroaminations, hydrations and hydrolyses

Aldehydes as organocatalysts? Simple aldehydes achieve difficult intermolecular reactions by exploiting temporary intramolecularity and inducing electrophilic activation.

Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles

An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N-protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP-catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X-ray crystal analysis, and by quantum-mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate-determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle.

Efficient photocatalytic oxygenation of alkenes by water soluble sensitizer in organic–water biphasic media

A new green, environmentally and economically photooxygenation of olefins by molecular oxygen in the presence of 4-carboxyl tetra phenyl porphyrin (H2TCPP), Rose Bengal (RB), Methylene Blue (MB) and some metallosensitizers (MS) such as cobalt phthalocyaninesulfonate (Coph(SO3H)[Formula: see text], MnTCPPCl, FeTCPPCl and SbTPP(OH)2 as water–soluble sensitizers in organic–water biphasic media at room temperature under visible light was illustrated. The products were obtained with 100% conversion by H2TCPP and 100% selectivity by Coph(SO3H)4. Also in this study, singlet oxygen production as the major route for oxidation of olefins was proved.

Metal-Templated Design: Enantioselective Hydrogen-Bond-Driven Catalysis Requiring Only Parts-per-Million Catalyst Loading

Based on a metal-templated approach using a rigid and globular structural scaffold in the form of a bis-cyclometalated octahedral iridium complex, an exceptionally active hydrogen-bond-mediated asymmetric catalyst was developed and its mode of action investigated by crystallography, NMR, computation, kinetic experiments, comparison with a rhodium congener, and reactions in the presence of competing H-bond donors and acceptors. Relying exclusively on weak forces, the enantioselective conjugate reduction of nitroalkenes can be executed at catalyst loadings as low as 0.004 mol% (40 ppm), representing turnover numbers of up to 20 250. A rate acceleration by the catalyst of 2.5 × 10(5) was determined. The origin of the catalysis is traced to an effective stabilization of developing charges in the transition state by carefully orchestrated hydrogen-bonding and van der Waals interactions between catalyst and substrates. This study demonstrates that the proficiency of asymmetric catalysis merely driven by hydrogen-bonding and van der Waals interactions can rival traditional activation through direct transition metal coordination of the substrate.