Unveiling the Reactivity of Propargylic Hydroperoxides under Gold Catalysis

Late-stage modification of complex drug: Base-controlled Pd-catalyzed regioselective synthesis and bioactivity of arylated osimertinibs

Achieving regioselective synthesis in complex molecules with multiple reactive sites remains a tremendous challenge in synthetic chemistry. Regiodivergent palladium-catalyzed C─H arylation of complex antitumor drug osimertinib with various aryl bromides via the late-stage functionalization strategy was demonstrated here. This reaction displayed a switch in regioselectivity under complete base control. Potassium carbonate (K2CO3) promoted the arylation of acrylamide terminal C(sp2)-H, affording 34 derivatives. Conversely, sodium tert-butoxide (t-BuONa) mediated the aryl C(sp2)-H arylation of the indole C2 position, providing 27 derivatives. The derivative 3r containing a 3-fluorophenyl group at the indole C2 position demonstrated similar inhibition of EGFRT790M/L858R and superior antiproliferative activity in H1975 cells compared to osimertinib, as well as similar antiproliferative activity in A549 cells and antitumor efficacy in xenograft mouse model bearing H1975 cells. This approach provides a "one substrate-multi reactions-multiple products" strategy for the structural modification of complex drug molecules, creating more opportunities for the fast screening of pharmaceutical molecules.

Multigram Synthesis of a Combustion-Relevant δ-Ketohydroperoxide through Sulfonylhydrazine Substitution

A synthesis of a δ-ketohydroperoxide is described, addressing potential functional-group compatibilities in these elusive species relevant to combustion and atmospheric chemistries. The hydroperoxide is installed via sulfonylhydrazine substitution, which was found to be more effective than displacement of secondary halides. As part of this protocol, it was observed that 1,2-dimethoxyethane is an advantageous medium for the reaction, avoiding the formation of a tetrahydrofuran hydroperoxide side product. This discovery facilitated the multigram synthesis (6 steps, 41 % yield overall) and discrete characterization of the target δ-ketohydroperoxide.

Atroposelective Construction of Nine-Membered Carbonate-Bridged Biaryls

We herein demonstrated an efficient method for the atroposelective construction of nine-membered carbonate-bridged biaryls through vinylidene ortho-quinone methide (VQM) intermediates. Diverse products with desirable pharmacological features were synthesized in satisfying yields and good to excellent enantioselectivities. In subsequent bioassays, several agents showed considerable antiproliferative activity via the mitochondrial-related apoptosis mechanism. Further transformations produced more structural diversity and may inspire new ideas for developing functional molecules.

Gold-Catalyzed Hydroarylation Reactions: A Comprehensive Overview

The hydroarylation of alkynes, alkene, and, allene is a cost-effective and efficient way to incorporate unsaturated moieties into aromatic substrates. This review focuses on gold-catalyzed hydroarylation, which produces aromatic alkenes,...

An Approach for the Generation of γ-Propenylidene-γ-butenolides and Application to the Total Synthesis of Rubrolides

Design and synthesis of a new class of γ-butenolides, viz. β-aryl-γ-propenylidene-γ-butenolides, have been reported from β-aryl-Z-enoate propargylic alcohols in the presence of acid. Isolation of β-aryl-γ-propenylidene-γ-butenolides and their O¹⁸-isomer confirmed the intermediacy of the allenyl-lactonium ion as well as the cyclic-hemiacetal during the proposed mechanism. By utilizing the β-aryl-γ-methylenecyclohexenylidene-γ-butenolides as starting materials, a highly stereoselective and efficient approach has been developed for the syntheses of frameworks of rubrolide natural products. This strategy was further extended for the total synthesis of rubrolide E.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C-C and/or C-O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a-O bond formation: (route a). (2.2) O-C2 bond formation: (route b). (2.3) C2-C3 bond formation: (route c). (2.4) C3-C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3-C3a bond formation (route a + d). (3.2) O-C2 and C2-C3 bond formation: (route b + c). (3.3) O-C2 and C3-C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

Benzylic Hydroperoxidation via Visible-Light-Induced Csp3-H Activation

A highly efficient benzylic hydroperoxidation has been realized through a visible-light-induced Csp³-H activation. We believe that this reaction undergoes a direct HAT mechanism catalyzed by eosin Y. This approach features the use of a metal-free catalyst (eosin Y), an energy-economical light source (blue LED), and a sustainable oxidant (molecular oxygen). Primary, secondary, and tertiary hydroperoxides as well as silyl, benzyl, and acyl peroxides were successfully prepared with good yields and excellent functional group compatibility.

Microbial Transformation of Some Natural and Synthetic Aromatic Compounds by Fungi: Aspergillus and Neurospora Strains

Microbial transformation of chalcone (1), 4-hydroxychalcone (2) and 4′-hydroxychalcone (3), 1,1-diphenylmethane (4), 1,3-diphenylacetone (5), 1,3-diphenylpropane (6), bibenzyl (7), ( E)-stilbene (8a)- and ( Z)-stilbenes (8b), and phenylcyclohexane (9), (1 R,2 S)-1-phenyl-2-hydroxycyclohexane (9a) and (1 S,2 R)-1-phenyl-2-hydroxycyclohexane (9b), and a naturally occurring bis-bibenzyl, marchantin A (10) were performed by using Aspergillus niger TBUYN-2 and the other Aspergillus strains, and Neurospora crassa which were capable to hydrogenation and epoxidation of a conjugated double bond, and direct hydroxylation and hydroperoxidation on benzene ring, and hydroxylation and carbonization on cyclohexane ring. Aspergillus species converted chalcone (1) to dihydrochalcone (1a) almost quantitatively.

A coherent study on the Z-enoate assisted Meyer–Schuster rearrangement

The impact of temperature, solvent, concentration of the counter ion and the nature of the arene nucleophile on the Z-enoate assisted Meyer–Schuster rearrangement of propargylic alcohols was studied.

Cyclohepta[b]indoles: A Privileged Structure Motif in Natural Products and Drug Design

Seven-membered rings fused with an indole are termed cyclohepta[b]indoles. Compounds exhibiting this structure motif display a broad spectrum of biological activities, ranging from inhibition of adipocyte fatty-acid-binding protein (A-FABP), deacetylation of histones, inhibition of leukotriene production p53, antituberculosis activities, and anti-HIV activities. These biological profiles are found in natural products containing the cyclohepta[b]indole motif, as well as in pharmaceuticals that contain this structure motif. Therefore, the biology of molecules derived from the skeleton of cyclohepta[b]indoles, as well as cyclopenta- and cyclohexa[b]indoles, has attracted considerable interest from the pharmaceutical industry as potential therapeutics in recent years. This is reflected by more than two dozen patents that have been issued in the past decade, solely based on the cyclohepta[b]indole structure motif. The efficient preparation of highly functionalized and unsymmetrically substituted cyclohepta[b]indoles has therefore become of central interest for synthetic organic chemists. Historically, this structure motif most often has been prepared by means of a Fischer indole synthesis. Although very robust and useful, this reaction poses certain limitations. Especially unsymmetrically functionalized cyclohepta[b]indoles are not suitable for a Fischer indole type synthesis, since product mixtures are inevitable. Therefore, novel methodologies to overcome these synthetic obstacles have been developed in recent years. This Account introduces all natural products and pharmaceutical compounds exhibiting the cyclohepta[b]indole motif. The structural variability within cyclohepta[b]indole alkaloids in combination with the broad range of organisms where these alkaloids have been isolated from, strongly suggests that the cyclohepta[b]indole is somehow a "privileged" structure motif. The organisms producing these compounds range from evergreen trees (actinophyllic acid) to cyanobacteria (ambiguinines). The synthetic methodologies to construct these molecular scaffolds (natural and unnatural in origin) are in turn highlighted and discussed with regard to their potential to access highly functionalized and unsymmetrical cyclohepta[b]indoles, for which they specifically have been designed. The methods are classified with respect to reaction type and whether or not they are enantioselective. Finally, the syntheses of cyclohepta[b]indole natural products are presented, thereby in each case, focusing on the construction of this structure motif in the course of the respective total synthesis. As a conclusion, we end by contrasting the methodological progress in the field with the actual successful application of the newly developed methods to the synthesis of complex structures to pinpoint the urgent requirement for further synthetic development for efficient synthetic design of this "privileged" structure motif.

Regioselective, cascade [3+2] annulation of β-naphthols (resorcinols) with Z-enoate propargylic alcohols: a novel entry for the synthesis of complex naphtho(benzo)furans

An unprecedented regioselective, first cascade [3+2] annulation process has been reported between propargylic alcohols and β-naphthols (resorcinols) for the synthesis of naphtho(benzo)furans.

The Z-enoate assisted, Meyer–Schuster rearrangement cascade: unconventional synthesis of α-arylenone esters

Brønsted acid promoted nucleophilation of propargylic alcohols during the Meyer–Schuster rearrangement (M–S) has been introduced. The reverse polarization of the M–S intermediate allenyl cation has been realized by employing a novel concept of cis-enoate assistance strategy.

Total synthesis of protosappanin A and its derivatives via palladium catalyzed ortho C–H activation/C–C cyclization under microwave irradiation

Protosappanin A, a complex natural product with high bioactivity, and 25 of its derivatives were synthesized through an intramolecular ortho C–H activation/C–C cyclization, ring-enlargement and deprotection reaction. C–H activation as the key step was investigated and optimized.

Computational and Experimental Studies of Phthaloyl Peroxide-Mediated Hydroxylation of Arenes Yield a More Reactive Derivative, 4,5-Dichlorophthaloyl Peroxide

The oxidation of arenes by the reagent phthaloyl peroxide provides a new method for the synthesis of phenols. A new, more reactive arene oxidizing reagent, 4,5-dichlorophthaloyl peroxide, computationally predicted and experimentally determined to possess enhanced reactivity, has expanded the scope of the reaction while maintaining a high level of tolerance for diverse functional groups. The reaction proceeds through a novel "reverse-rebound" mechanism with diradical intermediates. Mechanistic insight was achieved through isolation and characterization of minor byproducts, determination of linear free energy correlations, and computational analysis of substituent effects of arenes, each of which provided additional support for the reaction proceeding through the diradical pathway.

Iodine-induced synthesis of sulfonate esters from sodium sulfinates and phenols under mild conditions

A facile and highly efficient method for the synthesis of sulfonate esters mediated by iodine at room temperature has been developed, without transition metal catalysts and oxidants.

Gold-catalyzed cyclization reactions of allenol and alkynol derivatives

Although gold is chemically inert as a bulk metal, the landmark discovery that gold nanoparticles can be effective catalysts has opened up new and exciting research opportunities in the field. In recent years, there has been growth in the number of reactions catalyzed by gold complexes [gold(I) and gold(III)], usually as homogeneous catalysts, because they are soft Lewis acids. In addition, alkynes and allenes have interesting reactivities and selectivities, notably their ability to produce complex structures in very few steps. In this Account, we describe our work in gold catalysis with a focus on the formation of C-C and C-O bonds using allenes and alkynes as starting materials. Of these, oxa- and carbo-cyclizations are perhaps the best known and most frequently studied. We have divided those contributions into sections arranged according to the nature of the starting material (allene versus alkyne). Gold-catalyzed carbocyclizations in allenyl C2-linked indoles, allenyl-β-lactams, and allenyl sugars follow different mechanistic pathways. The cyclization of indole-tethered allenols results in the efficient synthesis of carbazole derivatives, for example. However, the compound produced from gold-catalyzed 9-endo carbocyclization of (aryloxy)allenyl-tethered 2-azetidinones is in noticeable contrast to the 5-exo hydroalkylation product that results from allenyl sugars. We have illustrated the unusual preference for the 4-exo-dig cyclization in allene chemistry, as well as the rare β-hydride elimination reaction, in gold catalysis from readily available α-allenols. We have also observed in γ-allenols that a (methoxymethyl)oxy protecting group not only masks a hydroxyl functionality but also exerts directing effects as a controlling unit in a gold-catalyzed regioselectivity reversal. Our recent work has also led to a combined experimental and computational study on regioselective gold-catalyzed synthetic routes to 1,3-oxazinan-2-ones (kinetically controlled products) and 1,3-oxazin-2-one derivatives (thermodynamically favored) from easily accessible allenic carbamates. In addition, we discuss the direct gold-catalyzed cycloketalization of alkynyldioxolanes, as well as aminoketalization of alkynyloxazolidines. We performed labeling studies and density functional calculations to gain insight into the mechanisms of the bis-heterocyclization reactions. We also describe the controlled gold-catalyzed reactions of primary and secondary propargylic hydroperoxides with a variety of nucleophiles including alcohols and phenols, allowing the direct synthesis of β-functionalized ketones. Through computations and (18)O-labeling experiments, we discovered various aspects of the controlled reactivity of propargylic hydroperoxides with external nucleophiles under gold catalysis. The mechanism resembles a Meyer-Schuster rearrangement, but notably, the presence and geometry characteristics of the OOH functional group allow a new pathway to happen, which cannot apply to propargylic alcohols.

Gold(i)-catalysed [1,3] O→C rearrangement of allenyl ethers

Gold standard: a simple and rapid access to the α-substituted acryl aldehydes has been provided by developing a gold-catalysed [1,3] rearrangement of the allenyl ethers importantly with a record turnover frequency = 4600 h⁻¹ (at 0.05 mol% of the catalyst concentration) in homogeneous gold(i) catalysis.

Carbocyclization versus oxycyclization on the metal-catalyzed reactions of oxyallenyl C3-linked indoles

The preparation of previously unknown (indol-3-yl)-α-allenols and -allenones was accomplished from indole-3-carbaldehydes, through indium-mediated Barbier allenylation reaction taking advantage of the N-(2-pyridyl)sulfonyl group. Metal-catalyzed cyclizations of oxyallenyl C3-linked indoles proceeded in two ways depending on the presence or absence of the N-(2-pyridyl)sulfonyl group. For allenols, gold-catalyzed oxycyclization occurred in the presence of the protecting group; in the absence of the protecting group, palladium- and gold-catalyzed benzannulations operated. On the contrary, under gold catalysis furyl-indoles were obtained as exclusive products from NH-allenones, while 5-endo carbocyclization adducts were the major components starting from N-SO2py-protected allenones. These cyclization reactions have been developed experimentally, and their mechanisms have additionally been investigated by a computational study.

Gold-catalyzed oxycyclization of allenic carbamates: expeditious synthesis of 1,3-oxazin-2-ones

A combined experimental and computational study on regioselective gold-catalyzed synthetic routes to 1,3-oxazinan-2-ones (kinetically controlled products) and 1,3-oxazin-2-one derivatives (thermodynamically favored) from easily accessible allenic carbamates has been carried out.