Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Silver-catalyzed chemodivergent assembly of aminomethylated isochromenes and naphthols

A silver-catalyzed chemodivergent cyclization of alkyne-tethered aldehydes with aminals to aminomethylated 1H-isochromenes and naphthols is described by tuning the reaction conditions. The reaction exhibits broad substrate generality and functional group compatibility. Mechanistic studies have disclosed that the aminomethylated naphthols are generated from the resulting N,O-aminal containing isochromenes via a silver-catalyzed unusual rearrangement process.

Unveiling a key catalytic pocket for the ruthenium NHC-catalysed asymmetric heteroarene hydrogenation

The chiral ruthenium(ii)bis-SINpEt complex is a versatile and powerful catalyst for the hydrogenation of a broad range of heteroarenes. This study aims to provide understanding of the active form of this privileged catalyst as well as the reaction mechanism, and to identify the factors which control enantioselectivity. To this end we used computational methods and in situ NMR spectroscopy to study the hydrogenation of 2-methylbenzofuran promoted by this system. The high flexibility and conformational freedom of the carbene ligands in this complex lead to the formation of a chiral pocket interacting with the substrate in a "lock-and-key" fashion. The non-covalent stabilization of the substrate in this particular pocket is an exclusive feature of the major enantiomeric pathway and is preserved throughout the mechanism. Substrate coordination leading to the minor enantiomer inside this pocket is inhibited by steric repulsion. Rather, the catalyst exhibits a "flat" interaction surface with the substrate in the minor enantiomer pathway. We probe this concept by computing transition states of the rate determining step of this reaction for a series of different substrates. Our findings open up a new approach for the rational design of chiral catalysts.

Cu(i) catalysis for selective condensation/bicycloaromatization of two different arylalkynes: direct and general construction of functionalized C-N axial biaryl compounds

Selective condensation/bicycloaromatization of two different arylalkynes is firstly developed under ligand-free copper(i)-catalysis, which allows the direct synthesis of C-N axial biaryl compounds in high yields with excellent selectivity and functional group tolerance. Due to the critical effects of Cu(i) catalyst and HFIP, many easily occurring undesired reactions are suppressed, and the coupled five-six aromatic rings are constructed via the selective formation of two C(sp2)-N(sp2) bonds and four C(sp2)-C(sp2) bonds. The achievement of moderate enantioselectivity verifies its potential for the simplest asymmetric synthesis of atropoisomeric biaryls. Western blotting demonstrated that the newly developed compounds are promising targets in biology and pharmaceuticals. This unique reaction can construct structurally diverse C-N axial biaryl compounds that have never been reported by other methods, and might be extended to various applications in materials, chemistry, biology, and pharmaceuticals.

Addition of benzyl ethers to alkynes: a metal-free synthesis of 1H-isochromenes

Bromotrimethylsilane (TMSBr)-promoted intramolecular cyclization of (o-arylethynyl)benzyl ethers to form 1H-isochromenes at room temperature is reported. Further studies indicated that vinyl carbocations are the reaction intermediates which are stabilized by the conjugated aryl groups. Thus, O-addition of benzyl ethers/tetrahydropyrans to alkynes was achieved under metal-free, acidic conditions. These reaction conditions were compatible with an alkynyl Prins reaction; therefore, 1H-isochromenes were produced directly from alkynyl benzaldehydes and alkynyl alcohols using a one-pot procedure.

Rhodium-Catalyzed Asymmetric Hydrogenation of 3-Benzoylaminocoumarins for the Synthesis of Chiral 3-Amino Dihydrocoumarins

An asymmetric hydrogenation of 3-benzoylaminocoumarins was achieved for the first time using our BridgePhos-Rh catalytic system, providing chiral 3-amino dihydrocoumarins in high yields (up to 98 %) and with excellent enantioselectivities (up to 99.7 % ee). The relationship between the enantioselectivities of the hydrogenations and the dihedral angles and the resulting π-π stacking effects of the BridgePhos-Rh complexes, which were determined by X-ray diffraction analysis, are discussed. The corresponding hydrogenated products allow for many transformations, providing several chiral skeletons with important physiological and pharmacological activities.

Mild Intermolecular Synthesis of a Cyclopropane-Containing Tricyclic Skeleton: Unusual Reactivity of Isobenzopyryliums

Cyclopropanes embedded in a polycyclic bridged architecture are a versatile structural motif, but such complex frameworks often impose substantial synthetic challenges. Herein we introduce a new approach for the expedient access to such spring-loaded strained systems via an exceptionally mild intermolecular convergent process between the readily available isobenzopyryliums and vinyl boronic acids. Different from the typical conventional approaches, our protocol does not involve the highly active carbenoid intermediates or strong conditions in order to overcome the disfavored kinetic and thermodynamic problems. Instead, the key cyclopropane ring was formed between the well-positioned nucleophile and electrophile in the adduct from the regioselective [4+2] cycloaddition. Thus, this unusual process also represents a new reactivity of the versatile isobenzopyryliums. The choice of a Brønsted acid catalyst with proper acidity is crucial to the high efficiency and selectivity for this multiple bond-forming process. The strained products are precursors to other useful synthetic building blocks.

Iodine-Catalyzed Synthesis of Substituted Furans and Pyrans: Reaction Scope and Mechanistic Insights

Substituted pyrans and furans are core structures found in a wide variety of natural products and biologically active compounds. Herein, we report a practical and mild catalytic method for the synthesis of substituted pyrans and furans using molecular iodine, a simple and inexpensive catalyst. The method described is performed under solvent-free conditions at an ambient temperature and atmosphere, thus offering a facile and practical alternative to currently available reaction protocols. A combination of experimental studies and density functional theory calculations revealed interesting mechanistic insights into this seemingly simple reaction.

Organocatalytic Asymmetric Multicomponent Cascade Reaction for the Synthesis of Contiguously Substituted Tetrahydronaphthols

Isobenzopyrylium ions are unique, highly reactive, aromatic intermediates which are largely unexplored in asymmetric catalysis despite their high potential synthetic utility. In this study, an organocatalytic asymmetric multicomponent cascade via dienamine catalysis, involving a cycloaddition, a nucleophilic addition, and a ring-opening reaction, is disclosed. The reaction furnishes chiral tetrahydronaphthols containing four contiguous stereocenters in good to high yield, high diastereoselectivity (up to >20:1), and excellent enantioselectivity (93-98% ee). The obtained products are important synthetic intermediates, and it is demonstrated that they can be used for the generation of frameworks such as octahydrobenzo[h]isoquinoline and [2.2.2]octane scaffolds. Furthermore, mechanistic experiments involving oxygen-18-labeling studies and density functional theory calculations provide a vivid picture of the reaction mechanism. Finally, the bioactivity of 16 representative tetrahydronaphthol compounds has been evaluated in U-2OS cancer cells with some compounds showing a unique profile and a clear morphological change.

Interrupted Pyridine Hydrogenation: Asymmetric Synthesis of δ-Lactams

Metal-catalyzed hydrogenation is an effective method to transform readily available arenes into saturated motifs, however, current hydrogenation strategies are limited to the formation of C-H and N-H bonds. The stepwise addition of hydrogen yields reactive unsaturated intermediates that are rapidly reduced. In contrast, the interruption of complete hydrogenation by further functionalization of unsaturated intermediates offers great potential for increasing chemical complexity in a single reaction step. Overcoming the tenet of full reduction in arene hydrogenation has been seldom demonstrated. In this work we report the synthesis of sought-after, enantioenriched δ-lactams from oxazolidinone-substituted pyridines and water by an interrupted hydrogenation mechanism.

Gold-catalyzed synthesis of 1H-isochromene-4-carbaldehydes via oxidative cascade cyclization

An efficient gold-catalyzed formation of indenylidene-derived 1H-isochromene-4-carbaldehydes from substituted 1,5,10-triyne-O-silanes was developed under mild reaction conditions. In this reaction, gold-catalyzed selective oxidation, 1,2-migration, nucleophilic addition and then 5-endo-dig cyclization took place regioselectively. The indenylidene-derived isochromene-4-carbaldehydes were synthesized in moderate to very good yields via the formation of new C-C and C-O bonds in one pot.

Efficient construction of diverse 3-cyanoindoles under novel tandem catalysis

A novel and rapid construction of 3-cyanoindoles by palladium-catalyzed tandem reactions has been developed. "N-H" free unprotected, N-alkyl and N-aryl 3-cyanoindoles are obtained with good to excellent yields. The usefulness of this synthetic approach is further demonstrated by the successful synthesis of practical compounds such as the therapeutic estrogen receptor ligand A precursor. Mechanism study shows that the tandem catalysis exploits a Suzuki cross-coupling with subsequent base-induced isoxazole fragmentation, followed by the aldimine condensation.

Selective Arene Hydrogenation for Direct Access to Saturated Carbo- and Heterocycles

Arene hydrogenation provides direct access to saturated carbo- and heterocycles and thus its strategic application may be used to shorten synthetic routes. This powerful transformation is widely applied in industry and is expected to facilitate major breakthroughs in the applied sciences. The ability to overcome aromaticity while controlling diastereo-, enantio-, and chemoselectivity is central to the use of hydrogenation in the preparation of complex molecules. In general, the hydrogenation of multisubstituted arenes yields predominantly the cis isomer. Enantiocontrol is imparted by chiral auxiliaries, Brønsted acids, or transition-metal catalysts. Recent studies have demonstrated that highly chemoselective transformations are possible. Such methods and the underlying strategies are reviewed herein, with an emphasis on synthetically useful examples that employ readily available catalysts.

Redox deracemization of 1,3,4,9-tetrahydropyrano[3,4-b]indoles

The existing asymmetric synthesis of enantiopure α-substituted cyclic ethers predominantly relies on the enantioselective C-C bond formation involving a prochiral oxocarbenium ion intermediate. In such a strategy, enantioselectivity and efficiency are typically susceptible to the electronic and substituent effects of either nucleophile or electrophile partners. Here, we describe a strategically different redox deracemization of α-substituted 1,3,4,9-tetrahydropyrano[3,4-b]indoles. This method exhibits good compatibility with the regional variation of the electronic or substituent effect of substrates, thus providing a practical and efficient supplement to the traditional strategy.

Enantioselective Hydrogenation of Imidazo[1,2-a]pyridines

The enantioselective synthesis of tetrahydroimidazo[1,2-a]pyridines by direct hydrogenation was achieved using a ruthenium/N-heterocyclic carbene (NHC) catalyst. The reaction forgoes the need for protecting or activating groups, proceeds with complete regioselectivity, good to excellent yields, enantiomeric ratios of up to 98:2, and tolerates a broad range of functional groups. 5,6,7,8-Tetrahydroimidazo[1,2-a]pyridines, which are found in numerous bioactive molecules, were directly obtained by this method, and its applicability was demonstrated by the (formal) synthesis of several functional molecules.

Efficient access to chiral benzo[c]chromenes via asymmetric transfer hydrogenation of ketals

A highly enantioselective method to chiral α-substituted 6H-benzo[c]chromenes through chiral imidodiphosphoric acid-catalyzed asymmetric transfer hydrogenation of ketals has been established.

Donor- and acceptor-enynals/enynones

Three kinds of transformations based on benzo-fused donor- and acceptor-enynals/enynones, including reactions with alkenes, alkynes, and H₂O, were discussed.

Synthesis of Phenalenyl-Fused Pyrylium Cations: Divergent C-H Activation/Annulation Reaction Sequence of Naphthalene Aldehydes with Alkynes

Described herein is the synthesis of stable oxonium-doped polycyclic aromatic hydrocarbons (PAHs) by the rhodium-catalyzed C-H activation/annulations of naphthalene-type aldehydes with internal alkynes. This protocol provides four divergent reaction types, including two unexpected annulations with an oxygen transposition process, which lead to diverse types of phenalenyl-fused pyrylium cations comprising a four-, five-, or six-ring-fused π-conjugated core. The annulations exhibit an exquisite regioselectivity and a high tolerance of sensitive functional groups. These PAHs feature intriguing photophysical properties such as full-color tunable fluorescence emission, high quantum yield, and positively charged core, and can be reduced easily to the phenalenyl radicals.