Switchable chemoselective aryne reactions between nucleophiles and pericyclic reaction partners using either 3-methoxybenzyne or 3-silylbenzyne

Arynes are known to serve as highly reactive benzene-based synthons, which have gained numerous successes in preparing functionalized arenes. Due to the superb electrophilic nature of these fleeting species, however, it is challenging to modulate the designated aryne transformation chemoselectively, when substrates possess multiple competing reaction sites. Here, we showcase our effort to manipulate chemoselective control between two major types of aryne transformations using either 3-methoxybenzyne or 3-silylbenzyne, where nucleophilic addition-triggered reactions and non-polar pericyclic reactions could be differentiated. This orthogonal chemoselective protocol is found to be applicable between various nucleophiles, i.e., imidazole, N-tosylated/N-alkyl aniline, phenol, and alcohol, and an array of pericyclic reaction partners, i.e., furan, cyclopentadiene, pyrrole, cycloheptatrienone, and cyclohexene. Beyond arylation reactions, C-N bond insertion, Truce-Smiles rearrangement, and nucleophilic annulation are appropriate reaction modes as well. Moreover, this chemoselective protocol can find potential synthetic application.

Mechanistic Details of Asymmetric Bromocyclization with BINAP Monoxide: Identification of Chiral Proton-Bridged Bisphosphine Oxide Complex and Its Application to Parallel Kinetic Resolution

The mechanism of our previously reported catalytic asymmetric bromocyclization reactions using 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) monoxide was examined in detail by the means of control experiments, NMR studies, X-ray structure analysis, and CryoSpray electrospray ionization mass spectrometry (ESI-MS) analysis. The chiral BINAP monoxide was transformed to a key catalyst precursor, proton-bridged bisphosphine oxide complex (POHOP·Br), in the presence of N-bromosuccinimide (NBS) and contaminating water. The thus-formed POHOP further reacts with NBS to afford BINAP dioxide and molecular bromine (Br₂) simultaneously in equimolar amounts. While the resulting Br₂ is activated by NBS to form a more reactive brominating reagent (Br₂─NBS), BINAP dioxide serves as a bifunctional catalyst, acting as both a Lewis base that reacts with Br₂─NBS to form a chiral brominating agent (P═O⁺─Br) and also as a Brønsted base for the activation of the substrate. By taking advantage of this novel concerted Lewis/Brønsted base catalysis by BINAP dioxide, we achieved the first regio- and chemodivergent parallel kinetic resolutions (PKRs) of racemic unsymmetrical bisallylic amides via bromocyclization.

Recent Advances on Diels-Alder-Driven Preparation of Bio-Based Aromatics

The preparation of high value-added chemicals from renewable resources is a crucial approach towards a sustainable economy. One prominent alternative to the production of petroleum-based chemicals from fossil resources is through the sequential Diels-Alder/aromatization reactions of biomass-derived furan platforms. This Concept is focused on the recent boom in bio-based furan DA strategies for aromatization of bio-based platform chemicals, particularly that of furfurals, ranging from indirect use and activation strategies to recent examples of direct DA reaction of these electron-withdrawing biomass-derived furans.

Catalytic asymmetric transformations of oxa- and azabicyclic alkenes

This review provides an overview of the fundamental concepts and recent developments in a wide range of enantioselective transformations involving oxa- and azabicyclic alkenes.

Activation of allylic esters in an intramolecular vinylogous kinetic resolution reaction with synergistic magnesium catalysts

Kinetic resolution (KR) of racemic starting materials is a powerful and practical alternative to prepare valuable enantiomerically enriched compounds. A magnesium-catalyzed kinetic resolution based on a designed intramolecular vinylogous Michael reaction is disclosed. Here we show a synergistic catalytic strategy based on the development of chiral ligands. Substrates containing linear allylic ester structures are designed and synthesized to construct key [6.6.5]-tricyclic chiral skeletons via this kinetic resolution process. Detailed mechanistic studies reveal a rational mechanism for the current intramolecular vinylogous KR reaction. The desired direct intramolecular asymmetric vinylogous Michael reaction of linear allylic esters is realized in high efficiency and enantioselectivity with the synergistic catalytic system.

Kinetic resolution allows to obtain enantioenriched compounds from racemic mixtures. Here, the authors report a synergistic magnesium catalyst promoting kinetic resolution of an intramolecular vinylogous Michael reaction to access [6.6.5]-tricyclic chiral skeletons.

Copper-catalyzed diastereoselective hydrothioetherification of oxa(aza)benzonorbornadienes

A novel copper-catalyzed hydrothioetherification of oxa(aza)bicyclic alkenes with potassium thioacetate and aryl or alkyl iodides to synthesize unsymmetrical thioethers has been developed. Notably, the reaction with complete diastereoselectivity went through a syn-selective addition process to give exo-adducts. In addition, this protocol exhibited high efficiency and good functional group tolerance to afford the target thioethers in moderate to good yields. Based on the results of mechanistic investigations, a plausible mechanism was proposed.

Intramolecular Palladium-Catalyzed Ring Opening of Oxabenzonorbornadienes with C1-Tethered Aryl Halides

The novel intramolecular ring opening of oxabenzonorbornadienes with C₁-tethered aryl halides was investigated using palladium catalysts to form fused tetracyclic frameworks. The reaction was generally found to synthesize 1,2-dihydronaphthalen-1-ol products with mild selectivity but was capable of synthesizing dehydrated naphthalene products in excellent yield and selectivity. Substituent effects on oxabenzonorbornadiene and on the iodoarene were explored along with the effects of varying tether length, where an efficient reaction was observed in nearly all cases. A total of 16 examples are reported with yields ranging from 0 to 96%.

12,15-Dimethyl-8-oxatetracyclo[8.8.0.02,7.011,16]octadeca-1(18),2,4,6,11(16),12,14-heptaen-10-ol

In the title compound, he pyran ring is in a half-chair conformation and the fused ring system comprising the benzene and cyclo­hexene rings is essentially planar and forms a dihedral angle of 27.95 (6)° with the other benzene ring. In the crystal, O—H⋯O hydrogen bonds connect the mol­ecules into chains propagating along [001].

In the title compound, C₁₉H₁₈O₂, the pyran ring is in a half-chair conformation. The fused ring system comprising the benzene and cyclo­hexene rings is essentially planar (r.m.s. deviation = 0.053 Å) and forms a dihedral angle of 27.95 (6)° with the other benzene ring. In the crystal, O—H⋯O hydrogen bonds connect the mol­ecules into chains propagating along [001].

12-Ethyl-6a,10a-dihydro-5H-6-oxachrysene

In the title compound, the pyran ring is in a half-chair conformation. The essentially planar naphthalene ring system forms a dihedral angle of 14.37 (5)° with the fused benzene ring. In the crystal, pairs of mol­ecules are connected into inversion dimers by weak C—H⋯O hydrogen bonds to generate (6) loops.

In the title compound, C₁₉H₁₆O, the pyran ring is in a half-chair conformation. The essentially planar naphthalene ring system (r.m.s. deviation = 0.020 Å) forms a dihedral angle of 14.37 (5)° with the fused benzene ring. In the crystal, pairs of mol­ecules are connected into inversion dimers by weak C—H⋯O hydrogen bonds to generate R₂²(6) loops.

Rhodium-Catalyzed Enantioselective Synthesis of Oxazinones via an Asymmetric Ring Opening-Lactonization Cascade of Oxabicyclic Alkenes

The rhodium-catalyzed asymmetric ring opening reaction of oxabicyclic alkenes is shown to be an efficient method for synthesizing chiral heterocycles. We demonstrate that the pairwise combination of chiral catalyst with chiral amino-acid-derived pronucleophiles results in a stereodivergent synthesis of diastereomeric hydroxyesters. A favorable conformational preference induces the subsequent lactonization of one diastereomer leading to the highly enantioselective synthesis of oxazinones.

Parallel Kinetic Resolution of Unsymmetrical Acyclic Aliphatic syn-1,3-Diols

Disclosed is a mild, reliable, and enantioselective catalytic parallel kinetic resolution of unsymmetrical acyclic aliphatic syn-1,3-diol derived acetals mediated by chiral phosphoric acid. This method provides stereoselective access to a variety of syn-1,3-diols as valuable building blocks with high enantioselectivity. Moreover, this mild system allows for site-selective protection of optically pure syn-1,3-diols in excellent regioselectivity.

Desymmetrization of Achiral Heterobicyclic Alkenes through Catalytic Asymmetric Hydrophosphination

Asymmetric addition of diarylphosphines to oxa- and azabicyclic alkenes proceeded in the presence of a chiral phosphapalladacycle catalyst and a mild acid at room temperature to give exclusively the enantioenriched addition products in excellent yields and good selectivities. Three new chiral carbon centers were generated stereoselectively by the catalytic hydrophosphination reaction.

Divergent synthesis of functionalized pyrrolidines and γ-amino ketones via rhodium-catalyzed switchable reactions of vinyl aziridines and silyl enol ethers

Rhodium-catalyzed enantiospecific intermolecular [3+2] cycloadditions and ring-opening reactions of vinylaziridines with silyl enol ethers have been developed to synthesize functionalized pyrrolidines and γ-amino ketones, respectively.

Divergent Access to Functionalized Pyrrolidines and Pyrrolines via Iridium-Catalyzed Domino-Ring-Opening Cyclization of Vinyl Aziridines with β-Ketocarbonyls

A useful synthesis of five-membered N-heterocycles has been developed through an iridium-catalyzed domino-ring-opening cyclization of vinylaziridines with β-ketocarbonyls. α-Substituted 1,3-dicarbonyls reacted with vinylaziridines to give 2-methylenepyrrolidines bearing two adjacent sp³-carbon centers with moderate to excellent diastereoselectivity, while the reaction of α-unsubstituted 1,3-dicarbonyls afforded 2-pyrrolines in good yield.