First Total Syntheses of Tetracenomycins C and X

Divergent and chemoselective deuteration of N-unsubstituted imidazoles enabled by precise acid/base control

Herein, we report acid/base-controlled and divergent deuteration of N-unsubstituted imidazoles in an imidazole-selective manner. This protocol enabled the deuteration of not only the 4-arylimidazoles but also the 2-arylimidazoles without labelling the aromatic rings. We demonstrated the advantages of this protocol by the synthesis of deuterated pharmaceuticals, which is difficult to achieve by means of transition metals.

Defluoroalkylation of gem-Difluoroalkenes with Alcohols via C-F/C-H Coupling

A feasible and effective method to synthesize α-fluoroalkenyl alcohols was reported. With the cooperation of photoredox and hydrogen atom transfer (HAT) processes, defluoroalkylations of gem-difluoroalkenes occurred smoothly with alcohols under visible-light irradiation. Notably, the protocols feature broad scopes, mild conditions, and validity for the late-stage functionalization of bioactive molecule derivatives. Mechanistic studies suggested that the reaction occurred through the radical coupling of the alkyl radical and the fluoroalkenyl radical.

Dearomatization of (Hetero)arenes through Photodriven Interplay between Polysulfide Anions and Formate

The facile construction of C(sp3 )-rich carbo- and heterocyclic compounds is a pivotal synthetic strategy to foster contemporary drug discovery programs. The downstream dearomatization of readily accessible two-dimensional (2D) planar arenes represents a direct pathway towards accessing three-dimensional (3D) aliphatic scaffolds. Here, we demonstrate that polysulfide anions are capable of catalyzing a dearomatization process of substituted naphthalenes, indoles, and other related heteroaromatic compounds in the presence of potassium formate and methanol under visible light irradiation. The developed protocol exhibits broad functional group tolerance, operational simplicity, scalability, and cost-effectiveness, representing a practical and sustainable synthetic tool for the arene dearomatization.

Visible-light-induced selective defluoroalkylations of polyfluoroarenes with alcohols

To provide α-polyfluoroarylalcohols, a novel protocol for the selective defluoroalkylation of polyfluoroarenes with easily accessible alcohols was reported via the cooperation of photoredox and hydrogen atom transfer (HAT) strategies with the assistance of Lewis acids under visible light irradiation. The protocol featured broad scope, excellent regioselectivity for both C-H and C-F bond cleavages, and mild conditions. Mechanistic studies suggested that the reaction occurred through Lewis acid-promoted HAT to provide an alkyl radical and sequential addition to polyfluoroarenes. Impressively, the regioselectivity for C-F cleavage was verified with the Fukui function. The feasibility and application of this protocol on fluoroarene synthesis were well illustrated by gram-scale synthesis under both batch and flow conditions, late-stage decoration of bioactive compounds, and further transformations of the fluoroarylalcohols.

Palladium-Catalyzed Access to Benzocyclobutenone-Derived Ketonitrones via C(sp2)-H Functionalization

The palladium-catalyzed C(sp2)-H functionalization of bromoaryl aldonitrones leading to benzocyclobutenone-derived ketonitrones is described. This method allows for the preparation of a wide range of strained, four-membered ketonitrones with broad functional group tolerance. Downstream transformations of the formed products were readily demonstrated, illustrating the synthetic utility of the obtained benzocyclobutenone-derived nitrones for the construction of polycyclic nitrogen-containing scaffolds.

Biological evaluation and spectral characterization of a novel tetracenomycin X congener

The aromatic polyketide tetracenomycin X (TcmX) was recently found to be a potent inhibitor of protein synthesis; its binding site is located in a unique locus within the tunnel of the large ribosomal subunit. The distinct mode of action makes this relatively narrow class of aromatic polyketides promising for drug development in the quest to prevent the spread of drug-resistant pathogens. Here we report the isolation and structure elucidation of a novel natural tetracenomycin X congener - 6-hydroxytetraceonomycin X (6-OH-TcmX). In contrast to TcmX, 6-OH-TcmX exhibited lower antimicrobial and cytotoxic activity, but comparable in vitro protein synthesis inhibition ability. A survey on spectral properties of tetracenomycins revealed profound differences in both UV-absorption and fluorescence spectra between TcmX and 6-OH-TcmX, suggesting a significant influence of 6-hydroxylation on the tetracenomycin X chromophore. Nonetheless, characteristic spectral properties of tetracenomycins make them suitable candidates for semi-synthetic drug development (e.g., for targeted delivery, chemical biology, or cell imaging).

Leveraging Fleeting Strained Intermediates to Access Complex Scaffolds

Arynes, strained cyclic alkynes, and strained cyclic allenes were validated as plausible intermediates in the 1950s and 1960s. Despite initially being considered mere scientific curiosities, these transient and highly reactive species have now become valuable synthetic building blocks. This Perspective highlights recent advances in the field that have allowed access to structural and stereochemical complexity, including recent breakthroughs in asymmetric catalysis.

Concise Synthesis of Chromene/Chromane-Type Aryne Precursors and Their Applications

The gram-scale synthesis of 5,6-, 6,7-, and 7,8-chromene/chromane-type aryne precursors and their applications in regioselective transformation to other functional derivatives is reported. Chromene/chromane-type arynes are generated under mild conditions, which can further undergo [2 + 2], [3 + 2], and [4 + 2] cycloaddition reactions, nucleophilic addition reactions, and σ-insertion reactions to produce structurally novel substituted chromenes and chromanes. The excellent regioselectivity of the reaction is facilitated by the oxygen-containing guiding groups at the ortho-position of the triple bond, which can be removed or switched to other functional groups including alkenyl, aryl, heteroaryl, and arylamino groups.

Photochemical Conversion of Isoxazoles to 5-Hydroxyimidazolines

Photochemical conversion of isoxazole I to 5-hydroxyimidazoline VII proceeded via the trapping of the photogenerated acylazirine III with amines under UV light irradiation. This is the first report of the efficient synthesis of 5-hydroxyimidazolines that are not readily accessible by other means. 5-Hydroxyimidazolines were also converted into multisubstituted imidazoles in one step by treatment with trifluoroacetic anhydride (TFAA) and 2,6-lutidine in dichloromethane.

Dearomatization of Electron-Deficient Phenols to ortho-Quinones: Bidentate Nitrogen-Ligated Iodine(V) Reagents

Despite their broad utility, the synthesis of ortho-quinones remains a significant challenge, in particular, access to electron-deficient derivatives remains an unsolved problem. Reported here is the first general method for the synthesis of electron-deficient ortho-quinones by direct oxidation of phenols. The reaction is enabled by a novel bidentate nitrogen-ligated iodine(V) reagent, a previously unexplored class of compounds which we have termed Bi(N)-HVIs. The reaction is extremely general and proceeds with excellent regioselectivity for the ortho over para isomer. Functionalization of the ortho-quinone products was examined, resulting in a facile one-pot synthesis of catechols, as well as the incorporation of a variety of heteroatom nucleophiles. This method represents the first synthetic application of Bi(N)-HVIs and demonstrates their potential as a platform for the further development of highly reactive, but also highly tunable, I(V) reagents.

Iterative Polyketide Synthesis via a Consecutive Carbonyl-Protecting Strategy

To address the difficulty in protecting a β-polycarbonyl compound, a method for the sequential protection of elongating carbonyl groups was demonstrated. The iterative chain elongation of a carboxylic acid with malonic acid half thioester followed by the protection of the resulting β-ketothioester was performed via the stepwise formation of an isoxazole ring using an O-protected oxime functionality. Yangonin and isosakuranetin were synthesized according to this procedure.

Synthetic Approach to Benzocyclobutenones Using Visible Light and a Phosphonate Auxiliary

Reported herein is a two-step procedure to synthesize benzocyclobutenones from (o-alkylbenzoyl)phosphonates. It consists of a visible-light-driven cyclization reaction forming phosphonate-substituted benzocyclobutenols and subsequent elimination reaction of the H-phosphonate, which assumes a key role as the recyclable auxiliary. A wide variety of functionalized benzocyclobutenones, which include those difficult to synthesize by conventional methods, are efficiently synthesized.

Aryne-based strategy in the total synthesis of naturally occurring polycyclic compounds

This review has outlined the strategies and tactics of using arynes in the total syntheses of polycyclic natural products.