1,2-Benzoquinones in Diels-Alder reactions, dipolar cycloadditions, nucleophilic additions, multicomponent reactions and more

Total Synthesis of Brevitaxin

Brevitaxin was prepared in nine steps from commercially available carnosic acid. The construction of the 1,4-benzodioxin moiety involved an unique stepwise ortho-quinone-engaged [4+2] cycloaddition. Two strategic stages were employed to prepare the highly unsaturated cycloaddition precursor 3: (1) synthesizing the diene moiety (C1-C2 and C10-C20 double bonds) by regioselective ortho-quinone tautomerization, and (2) installing four sp2-hybridized carbon atoms (C3, C5, C6 and C7) in one step using a SeO2-promoted chemo- and regioselective oxidation reaction.

Asymmetric Total Synthesis of (+)-Chuanxiongnolide L1 via a Stereoselective Oxidative Dearomatization/Diels-Alder Strategy

The first asymmetric total synthesis of chuanxiongnolide L1 was achieved in 16 steps and 1.9% overall yield by employing a bioinspired chiral auxiliary strategy. The key steps involving asymmetric oxidative dearomatization of chiral amino ether and subsequent asymmetric Diels-Alder reaction of the resulting masked chiral ortho-benzoquinone were adopted.

Diversity-Oriented Catalytic Asymmetric Dearomatization of Indoles with o-Quinone Diimides

Herein, the first diversity-oriented catalytic asymmetric dearomatization of indoles with o-quinone diimides (o-QDIs) is reported. The catalytic asymmetric dearomatization (CADA) of indoles is one of the research focuses in terms of the structural and biological importance of dearomatized indole derivatives. Although great achievements have been made in target-oriented CADA reactions, diversity-oriented CADA reactions are regarded as more challenging and remain elusive due to the lack of synthons featuring multiple reaction sites and the difficulty in precise control of chemo-, regio-, and enantio-selectivity. In this work, o-QDIs are employed as a versatile building block, enabling the chemo-divergent dearomative arylation and [4 + 2] cycloaddition reactions of indoles. Under the catalysis of chiral phosphoric acid and mild conditions, various indolenines, furoindolines/pyrroloindolines, and six-membered-ring fused indolines are collectively prepared in good yields with excellent enantioselectivities. This diversity-oriented synthesis protocol enriches the o-quinone chemistry and offers new opportunities for CADA reactions.

Isolation, Structural Analysis and Biological Activity Assays of Biselisabethoxanes A and B: Two Dissymmetric Bis-Diterpenes from the Southwestern Caribbean Sea Gorgonian Coral Pseudopterogorgia elisabethae

Two novel dissymmetric diterpenoids, biselisabethoxanes A and B (1 and 2), were isolated from the hexane extracts of the gorgonian coral Pseudopterogorgia elisabethae. Biselisabethoxane A (1) represents the first example of a marine-derived C₄₀ dimer made of two distinct diterpene fragments, whereas biselisabethoxane B (2) is a fused heterodimer stemming from coupling of two amphilectane-based fragments. The structures of 1 and 2 were elucidated based on 1D and 2D NMR spectral data analysis. The molecular structure of 1 was subsequently confirmed by X-ray crystallographic analysis. When evaluated for their inhibitory effects in a series of well-established biological activity assays the isolated compounds were shown to moderately inhibit the growth of Mycobacterium tuberculosis.

Catalytic Atroposelective Electrophilic Amination of Indoles

Reported here is the first catalytic atroposelective electrophilic amination of indoles, which delivers functionalized atropochiral N-sulfonyl-3-arylaminoindoles with excellent optical purity. This reaction was furnished by 1,6-nucleophilic addition to p-quinone diimines. Control experiments suggest an ionic mechanism that differs from the radical addition pathway commonly proposed for 1,6-addition to quinones. The origin of 1,6-addition selectivity was investigated through computational studies. Preliminary studies show that the obtained 3-aminoindoles atropisomers exhibit anticancer activities. This method is valuable with respect to enlarging the toolbox for atropochiral amine derivatives.

Ruthenium-catalyzed (spiro)annulation of N-aryl-2,3-dihydrophthalazine-1,4-diones with quinones to access pentacyclic spiro-indazolones and fused-cinnolines

Ru(II)-catalyzed strategies were developed for the [4 + 1] and [4 + 2] oxidative coupling between N-aryl-2,3-dihydrophthalazine-1,4-diones and 1,4-benzoquinones, achieving spiro-indazolones and fused-cinnolines, respectively. Mild, aerobic and external oxidant-free conditions, as well as the use of a ruthenium catalyst for such (spiro)annulative strategies with quinones over reported Rh/Ir-catalyts, underline the rewards of the disclosed protocols.

Recent advances in catalytic oxidative reactions of phenols and naphthalenols

This critical review aims to provide an overview of oxidative phenol and naphthalenol transformations in nature and synthetic chemistry.

Formal dual C(sp2)–H cross-dehydrogenative C–O bond formation to construct highly functionalized diaryl ethers with O2

A formal dual C(sp2)–H cross-dehydrogenative C–O bond formation reaction between phenols and naphthylamine derivatives to construct diaryl ethers has been developed under mild conditions.

Switchable Oxidative Reactions of N-allyl-2-Aminophenols: Palladium-Catalyzed Alkoxyacyloxylation vs an Intramolecular Diels-Alder Reaction

The Pd(II)-catalyzed reaction of N-allyl-2-aminophenols in the presence of PhI(OCOR)₂ as the oxidant resulted in an alkoxyacyloxylation process, with the formation of functionalized dihydro-1,4-benzoxazines. The reaction performed in the absence of palladium catalyst switched to an intramolecular Diels–Alder reaction (IMDA) pathway, which was the result of an oxidative dearomatization of the 2-aminophenol, nucleophilic addition, and Diels–Alder reaction cascade, highlighting the role of the oxidant as both a nucleophilic donor and an oxidizing agent.

Asymmetric Total Syntheses of 8,9-Seco-ent-kaurane Diterpenoids Enabled by an Electrochemical ODI-[5+2] Cascade

An electrochemical ODI-[5+2] cascade reaction was developed which enables the rapid assembly of diversely functionalized bicyclo[3.2.1]octadienones from sensitive ethynylphenols. By combining a directed retro-aldol/aldol process, a [2,3]-sigmatropic rearrangement, and an Al(O-iPr)₃ -promoted reductive 1,3-transposition, the asymmetric total syntheses of five 8,9-seco-ent-kauranoids-(-)-shikoccin, (-)-O-methylshikoccin, (-)-epoxyshikoccin, (+)-O-methylepoxyshikoccin, and (+)-rabdo-hakusin-have been achieved in a concise and efficient manner.

Recent advancements in enzyme-mediated crosslinkable hydrogels: In vivo-mimicking strategies

Enzymes play a central role in fundamental biological processes and have been traditionally used to trigger various processes. In recent years, enzymes have been used to tune biomaterial responses and modify the chemical structures at desired sites. These chemical modifications have allowed the fabrication of various hydrogels for tissue engineering and therapeutic applications. This review provides a comprehensive overview of recent advancements in the use of enzymes for hydrogel fabrication. Strategies to enhance the enzyme function and improve biocompatibility are described. In addition, we describe future opportunities and challenges for the production of enzyme-mediated crosslinkable hydrogels.

Triphenylantimony(V) Catecholates of the Type (3-RS-4,6-DBCat)SbPh3-Catechol Thioether Derivatives: Structure, Electrochemical Properties, and Antiradical Activity

A new series of triphenylantimony(V) 3-alkylthio/arylthio-substituted 4,6-di-tert-butylcatecholates of the type (3-RS-4,6-DBCat)SbPh3, where R = n-butyl (1), n-hexyl (2), n-octyl (3), cyclopentyl (4), cyclohexyl (5), benzyl (6), phenyl (7), and naphthyl-2 (8), were synthesized from the corresponding catechol thioethers and Ph3SbBr2 in the presence of a base. The crystal structures of 1, 2, 3, and 5 were determined by single-crystal X-ray analysis. The coordination polyhedron of 1-3 is better described as a tetragonal pyramid with a different degree of distortion, while that for 5- was a distorted trigonal bipyramid (τ = 0.014, 0.177, 0.26, 0.56, respectively). Complexes demonstrated different crystal packing of molecules. The electrochemical oxidation of the complexes involved the catecholate group as well as the thioether linker. The introduction of a thioether fragment into the aromatic ring of catechol ligand led to a shift in the potential of the "catechol/o-semiquinone" redox transition to the anodic region, which indicated the electron-withdrawing nature of the RS group. The radical scavenging activity of the complexes was determined in the reaction with DPPH radical.

Recent Advances in the Selective Oxidative Dearomatization of Phenols to o-Quinones and o-Quinols with Hypervalent Iodine Reagents

ortho-Quinones are valuable molecular frameworks with diverse applications across biology, materials, organic synthesis, catalysis, and coordination chemistry. Despite their broad utility, their synthesis remains challenging, in particular via the direct oxidation of readily accessible phenols, due to the need to affect regioselective ortho oxidation coupled with the sensitivity of the resulting o-quinone products. The perspective looks at the emergence of I(V) hypervalent iodine reagents as an effective class of oxidants for regioselective o-quinone synthesis. The application of these reagents in regioselective phenol oxidation to both o-quinones and o-quinols will be discussed, including a recent report from our laboratory on the first method for the oxidation of electron-deficient phenols using a novel nitrogen-ligated I(V) reagent. Also included are select examples of total syntheses utilizing this methodology as well as recent advancements in chiral I(V) reagent design for asymmetric phenol dearomatization.

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane as a useful precursor to 2,3-dimethylene-1,4-dioxane for [4+2] cycloaddition reaction

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane readily prepared from biacetyl serves as a stable precursor to 2,3-dimethylene-1,4-dioxane which undergoes a [4+2] cycloaddition reaction with dienophiles to give functionalized cyclohexene derivatives. The cycloaddition adducts obtained by the present procedure are transformed into potentially useful intermediates for biologically important materials.

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane serves as a stable precursor to 2,3-dimethylene-1,4-dioxane which undergoes a cycloaddition with dienophiles. The adducts are transformed into useful intermediates for biologically important materials.

Dual C–H activation: Rh(iii)-catalyzed cascade π-extended annulation of 2-arylindole with benzoquinone

A rhodium-catalyzed, N–H free indole directed cyclization reaction of benzoquinone via a dual C–H activation strategy is disclosed. This protocol has a good functional group tolerance and affords useful indole-fused heterocylces. Besides, it is insensitive to moisture, commercially available solvent can be directly used and work quite well for this transformation.

A Rh-catalyzed cascade annulation of N–H free 2-arylindole with benzoquinone via dual C–H activation strategy was reported.

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

Harnessing structurally unbiased ortho-benzoquinone monoimine for biomimetic oxidative [4+2] cycloaddition with enamines

Reported herein is the first catalytic oxidative [4+2] cycloaddition of 2-aminophenols with cyclic enamines.

Transition-Metal-Free Multicomponent Approach to Stereoenriched Cyclopentyl-isoxazoles through C-C Bond Cleavage

An efficient multicomponent reaction for the synthesis of stereoenriched cyclopentyl-isoxazoles from camphor-derived α-oximes, alkynes, and MeOH is reported. Our method involved a series of cascade transformations, including the in situ generation of an IIII catalyst, which catalyzed the addition of MeOH to a sterically hindered ketone. Oxidation of the oxime, and rearrangement of the α-hydroxyiminium ion generated a nitrile oxide in situ, which, upon [3+2] cycloaddition reaction with an alkyne, delivered the regioselective product. This reaction was very selective for the syn-oxime. This multicomponent approach was also extended to the synthesis of a new glycoconjugate, camphoric ester-isoxazole C-galactoside.

Photodegradation of Eumelanin and Pheomelanin and Its Pathophysiological Implications

Eumelanin is photoprotective for pigmented tissues while pheomelanin is phototoxic. In this review, we summarize current understanding of how eumelanin and pheomelanin structures are modified by ultraviolet A (UVA) and also by visible light and how reactive oxygen species participate in those processes. Alkaline hydrogen peroxide oxidation was employed to characterize eumelanin and benzothiazole-type pheomelanin, giving pyrrole-2,3,5-tricarboxylic acid (PTCA) and thiazole-2,4,5-tricarboxylic acid (TTCA), respectively. Reductive hydrolysis with hydroiodic acid gives 4-amino-3-hydroxyphenylalanine (4-AHP) from the benzothiazine moiety of pheomelanin. The results show that the photoaging of eumelanin gives rise to free PTCA (produced by peroxidation in situ) and pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA, produced by cross-linking). The TTCA/4-AHP ratio increases with photoaging, indicating the conversion of benzothiazine to the benzothiazole moiety. Analysis of those markers and their ratios show that both eumelanin and pheomelanin in human retinal pigment epithelium melanosomes undergo extensive structural modifications due to their lifelong exposure to blue light. Using synthetic melanins, we also found that singlet oxygen, in addition to superoxide anions, is photogenerated and quenched upon UVA irradiation. The (patho)physiological significance of those findings is discussed in relation to the tanning process, to melanomagenesis in the skin and to age-related macular degeneration in the eyes.

Roles of reactive oxygen species in UVA-induced oxidation of 5,6-dihydroxyindole-2-carboxylic acid-melanin as studied by differential spectrophotometric method

Eumelanin photoprotects pigmented tissues from ultraviolet (UV) damage. However, UVA-induced tanning seems to result from the photooxidation of preexisting melanin and does not contribute to photoprotection. We investigated the mechanism of UVA-induced degradation of 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-melanin taking advantage of its solubility in a neutral buffer and using a differential spectrophotometric method to detect subtle changes in its structure. Our methodology is suitable for examining the effects of various agents that interact with reactive oxygen species (ROS) to determine how ROS is involved in the UVA-induced oxidative modifications. The results show that UVA radiation induces the oxidation of DHICA to indole-5,6-quinone-2-carboxylic acid in eumelanin, which is then cleaved to form a photodegraded, pyrrolic moiety and finally to form free pyrrole-2,3,5-tricarboxylic acid. The possible involvement of superoxide radical and singlet oxygen in the oxidation was suggested. The generation and quenching of singlet oxygen by DHICA-melanin was confirmed by direct measurements of singlet oxygen phosphorescence.

A new four-component reaction involving the Michael addition and the Gewald reaction, leading to diverse biologically active 2-aminothiophenes

A Gewald-four component reaction has been successfully developed for the synthesis of a series of compounds containing an indole and a 2-aminothiophene moiety separated by a methylene spacer having anti-proliferative activity.

Multicomponent synthesis of new curcumin-based pyrano[2,3-d]pyrimidine derivatives using a nano-magnetic solid acid catalyst

An novel nano-magnetic solid acid catalyst was used for the efficient synthesis of new curcumin-based pyrano[2,3-d]pyrimidine derivatives via a multicomponent reaction under mild conditions.

A new paradigm for designing ring construction strategies for green organic synthesis: implications for the discovery of multicomponent reactions to build molecules containing a single ring

A new way of developing novel synthesis strategies for the construction of monocyclic rings found in organic molecules is presented. The method is based on the visual application of integer partitioning to chemical structures. Two problems are addressed: (1) the determination of the total number of possible ways to construct a given ring by 2-, 3-, and 4-component couplings; and (2) the systematic enumeration of those possibilities. The results of the method are illustrated using cyclohexanone, pyrazole, and the Biginelli adduct as target ring systems with a view to discover new and greener strategies for their construction using multicomponent reactions. The application of the method is also extended to various heterocycles found in many natural products and pharmaceuticals.

Employing Arynes in Diels-Alder Reactions and Transition-Metal-Free Multicomponent Coupling and Arylation Reactions

Arynes are highly reactive intermediates having several applications in organic synthesis for the construction of various ortho-disubstituted arenes. Traditionally, arynes are generated in solution from haloarenes under strongly basic conditions. However, the scopes of many of the aryne reactions are limited because of the harsh conditions used for their generation. The renaissance of interest in aryne chemistry is mainly due to the mild conditions for their generation by the fluoride-induced 1,2-elimination of 2-(trimethylsilyl)aryl triflates. This Account is focused on the Diels-Alder reaction of arynes and their transition-metal-free application in multicomponent couplings as well as arylation reactions. The Diels-Alder reaction of arynes is a powerful tool for constructing benzo-fused carbocycles and heterocycles. In 2012, we developed an efficient, broad-scope, and scalable Diels-Alder reaction of pentafulvenes with arynes affording benzonorbornadiene derivatives. Subsequently, we accomplished the Diels-Alder reaction of arynes with dienes such as 1,2-benzoquinones and tropones. Moreover, we uncovered a transition-metal-free protocol for the synthesis of 9,10-dihydrophenanthrenes by the reaction of arynes with styrenes that proceeds via a Diels-Alder/ene-reaction cascade. In addition, we demonstrated the reaction of arynes with indene/benzofurans, which proceeds via a tandem [4 + 2]/[2 + 2] sequence. Multicomponent coupling (MCC) involving arynes mainly comprises the initial addition of a nucleophile to the aryne followed by interception of the aryl anion intermediate with an electrophile (provided the nucleophilic and electrophilic moieties do not belong to the same molecule). We have disclosed aryne MCCs initiated by N-heterocycles such as (iso)quinoline, pyridine, and aziridines. When (iso)quinoline is used as the nucleophilic trigger and N-substituted isatin as the third component, the reaction affords spirooxazino(iso)quinolines via 1,4-dipolar intermediates. Unexpectedly, using pyridine affords indolin-2-ones, where the reaction proceeds via the pyridylidene intermediate. Additionally, we developed the phosphine-triggered aryne MCCs for the synthesis of functionalized benzooxaphospholes. In another phase of our work, we studied the synthetic utility of CO2 as a one-carbon synthon in aryne MCCs for the synthesis of phthalimides. Engaging arynes as an aryl source is one of the transition-metal-free methods for arylation reactions. We have demonstrated the N-arylation of aromatic tertiary amines and O-arylation of aliphatic alcohols using arynes. It is anticipated that the chemistry of arynes will continue to prosper and will lead to surprising developments for the synthesis of various 1,2-disubstituted arenes of molecular complexity and structural diversity. Future challenges in this area include the utility of arynes in enantioselective transformations and the synthesis and reactions of exotic heterocyclic arynes.

Quinone-induced protein modifications: Kinetic preference for reaction of 1,2-benzoquinones with thiol groups in proteins

Oxidation of polyphenols to quinones serves as an antioxidative mechanism, but the resulting quinones may induce damage to proteins as they react through a Michael addition with nucleophilic groups, such as thiols and amines to give protein adducts. In this study, rate constants for the reaction of 4-methylbenzoquinone (4MBQ) with proteins, thiol and amine compounds were determined under pseudo first-order conditions by UV-vis stopped-flow spectrophotometry. The chemical structures of the adducts were identified by LC-ESI-MS/MS. Proteins with free thiols were rapidly modified by 4MBQ with apparent second order rate constants, k2 of (3.1±0.2)×10(4)M(-1)s(-1) for bovine serum albumin (BSA) and (4.8±0.2)×10(3)M(-1)s(-1) for human serum albumin at pH 7.0. These values are at least 12-fold greater than that for α-lactalbumin (4.0±0.2)×10(2)M(-1)s(-1), which does not contain any free thiols. Reaction of Cys-34 of BSA with N-ethylmaleimide reduced the thiol concentration by ~59%, which resulted in a decrease in k2 by a similar percentage, consistent with rapid adduction at Cys-34. Reaction of 4MBQ with amines (Gly, Nα-acetyl-l-Lys, Nε-acetyl-l-Lys and l-Lys) and the guanidine group of Nα-acetyl-l-Arg was at least 5×10(5) slower than with low-molecular-mass thiols (l-Cys, Nα-acetyl-l-Cys, glutathione). The thiol-quinone interactions formed colorless thiol-phenol products via an intermediate adduct, while the amine-quinone interactions generated colored amine-quinone products that require oxygen involvement. These data provide strong evidence for rapid modification of protein thiols by quinone species which may be of considerable significance for biological and food systems.

Organocatalyzed benzannulation for the construction of diverse anthraquinones and tetracenediones

An efficient one-pot synthesis of anthraquinones and tetracenediones was achieved via L-proline catalyzed [4+2] cycloaddition of in situ generated azadiene from α,β-unsaturated aldehydes and 1,4-naphthoquinones or 1,4-anthracenedione in good to excellent yield. This protocol constitutes an unprecedented tandem benzannulation that allows one-pot construction of diverse anthraquinones and tetracenediones in the presence of organocatalysts. This methodology was applied successfully to the synthesis of naturally occurring molecules and photochemically interesting phenanthrenequinone derivatives.