Electrochemical Dearomatizing Methoxylation of Phenols and Naphthols: Synthetic and Computational Studies

The electrochemical oxidative dearomatizing methoxylation of phenols and naphthols was developed. It provides an alternative route for the preparation of methoxycyclohexadienones, important and versatile synthetic intermediates, that eliminates the need for stoichiometric high-energy chemical oxidants and generates hydrogen as a sole by-product. The reaction proceeds in a simple constant current mode, in an undivided cell, and it employs standardized instrumentation. A collection of methoxycyclohexadienones derived from various 2,4,6-tri-substituted phenols and 1-substituted-2-naphthols was obtained in moderate to excellent yields. These include a complex derivative of estrone, as well as methoxylated dearomatized 1,1'-bi-2-naphthols (BINOLs). The mechanism of the reaction was subject to profound investigations using density functional theory calculations. In particular, the reactivity of two key intermediates, phenoxyl radical and phenoxenium ion, was carefully examined. The obtained results shed light on the pathway leading to the desired product and rationalize experimentally observed selectivities regarding a side benzylic methoxylation and the preference for the functionalization at the para over the ortho position. They also uncover the structure-selectivity relationship, inversely correlating the steric bulk of the substrate with its propensity to undergo the side-reaction. Moreover, the loss of stereochemical information from enantiopure BINOL substrates during the reaction is rationalized by the computations.

Asymmetric Brominative Dearomatization of 2-Naphthols Using a Cinchona Alkaloid-Based Organocatalyst

A cinchona alkaloid-based organocatalyst enables asymmetric brominative dearomatization of 2-naphthols, providing the corresponding bromonaphthalenones with high enantioselectivities. The first metal-free reaction can accommodate a variety of functional groups and give useful frameworks bearing a Br-containing tetrasubstituted stereogenic center.

Dinuclear Zn-Catalytic System as Brønsted Base and Lewis Acid for Enantioselectivity in Same Chiral Environment

Zinc (Zn) is a crucial element with remarkable significance in organic transformations. The profusion of harmless zinc salts in the Earth's outer layer qualifies zinc as a noteworthy contender for inexpensive and eco-friendly reagents and catalysts. Recently, widely recognized uses of organo-Zn compounds in the field of organic synthesis have undergone extensive expansion toward asymmetric transformations. The ProPhenol ligand, a member of the chiral nitrogenous-crown family, exhibits the spontaneous formation of a dual-metal complex when reacted with alkyl metal (R-M) reagents, e.g., ZnEt2. The afforded Zn complex possesses two active sites, one Lewis acid and the other Brønsted base, thereby facilitating the activation of nucleophiles and electrophiles simultaneously within the same chiral pocket. In this comprehensive analysis, we provide a thorough account of the advancement and synthetic potential of these diverse catalysts in organic synthesis, while emphasizing the reactivity and selectivities, i.e., dr and ee due to the design/structure of the ligands employed.

Concise and Free-Metal Access to Lactone-Annelated Pyrrolo[2,1-a]isoquinoline Derivatives via a 1,2-Rearrangement Step

Here, An efficient approach to obtaining previously unknown furo[2',3':2,3]pyrrolo[2,1-a]isoquinoline derivatives from readily available 1-R-1-ethynyl-2-vinylisoquinolines is described. The reaction features a simple procedure, occurs in hexaflouroisopropanol and does not require elevated temperatures. It has been found that the addition of glacial acetic acid significantly increases the yields of the target spirolactone products. Using trifluoroethanol instead of hexaflouroisopropanol results in the formation of pyrido[2,1-a]isoquinolines.

Switchable multipath cascade cyclization to synthesize bicyclic lactams and succinimides via chemodivergent reaction

Herein, a novel switchable multipath cascade cyclization via chemodivergent reaction between readily available ketoamides and deconjugated butenolides was developed to efficiently synthesize γ-lactone fused γ-lactams and succinimide fused hemiketals. The Aldol/aza-Michael reaction and Aldol/imidation/hemiketalization reaction were enabled by catalytic amounts of two bases, namely tetramethyl guanidine and NaOAc. A wide range of substrate scope with diverse functional group compatibility was demonstrated to deliver the corresponding products with good yield and excellent diastereoselectivity (>60 examples).

Asymmetric Synthesis of α-Spiro-γ-lactones and α-Substituted γ-Lactones via Chiral Bifunctional Sulfide-Catalyzed Bromolactonizations

An efficient enantioselective synthesis of γ-chiral α-spiro-γ-lactones, which are important building blocks for pharmaceuticals, was achieved via BINOL-derived chiral bifunctional sulfide-catalyzed bromolactonizations of α-allyl carboxylic acids containing either hetero- or carbocyclic structures. Transformations of the resultant α-spiro-type bromolactonization product were examined to obtain optically active γ-functionalized α-spiro-γ-lactones. The utility of this catalytic system was also demonstrated in the asymmetric synthesis of α,α-diaryl- and dialkyl-substituted γ-lactones.

Construction of Vicinal All-Carbon Stereogenic Centers via Copper-Catalyzed Asymmetric Decarboxylative Propargylation: Enantio- and Diastereoselective Synthesis of Substituted Spirolactones

The formation of vicinal all-carbon quaternary stereocenters remains a formidable challenge. We report herein the synthesis of such highly congested structural dyads by copper-catalyzed decarboxylative propargylation between propargyl carbonates and indanone-based nucleophiles. The implementation of diphenylethylenediamine (DPEN)-based ligands is the key to success. A wide range of functional groups was tolerated, delivering the indanone-based spirolactones in good yields with high diastereo- and enantioselectivity. The mechanistic observations suggest the capability of the new copper complex to enable stereocontrolled addition to copper-allenylidene species.

An Unexpected Photoinduced Cyclization to Synthesize Fully Substituted γ-Spirolactones via Intramolecular Hydrogen Abstraction with Allyl Acrylates

A photoinduced intramolecular cyclization of allyl acrylates to synthesize fully substituted spiro-γ-butyrolactone compounds is described. The reaction is believed to undergo activation of the carbonyl group upon ultraviolet irradiation, 1,4-hydrogen...

Synthesis of Spirolactones via a BF3·Et2O-Promoted Cascade Annulation of α-Keto Acids and 1,3-Enynes

A novel and effective method for the synthesis of spirolactones from readily available α-keto acids and 1,3-enynes is developed via a BF₃·Et₂O-promoted cascade annulation. This sequential process is conducted at room temperature, and it provides the functionalized spirolactones in good to excellent yield under metal-free conditions.

A short route to access oxaspiro[n,3,3]propellanes

Novel access to oxaspiro[n,3,3]propellanes has been developed from bicyclic lactones directly prepared by a photochemical hydroxymethylation or alternatively by a three-step sequence. Thanks to the presence of additional hydroxy- and propargylic groups, a second cyclization catalyzed by silver or bismuth salts, led to the propellane structure which was finally transformed into spiranic derivatives by a Simmons-Smith reaction or condensation with α-ketoesters.

Synthesis of novel 2-oxospiro[indoline-3,4'-[1,3]dithiine]-5'-carbonitrile derivatives by new spiro[indoline-3,4'-[1,3]dithiine]@Cu(NO3)2 supported on Fe3O4@gly@CE MNPs as efficient catalyst and evaluation of biological activity

New spiro[indoline-3,4'-[1,3]dithiine]@Cu(NO₃)₂ supported on Fe₃O₄@gly@CE magnetic nanoparticle were synthesized and used as efficient and recyclable catalyst in the synthesis of 2-oxospiro[indoline-3,4'-[1,3]dithiine]-5'-carbonitrile derivatives. The structure of magnetic nanoparticles were confirmed using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), thermogravimetric analysis (TGA), infrared spectroscopy (FT-IR) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Subsequently, antibacterial and antifungal activities in terms of inhibition zone diameter, minimum inhibitory concentration, minimum bactericidal concentration and antioxidant activity against the DPPH free radical of the derivatives were investigated. The results revealed acceptable biological effects of the synthetic derivatives and a significant relationship between their structure and biological activity were observed.

Electrochemical asymmetric synthesis of biologically active substances

This review discusses the literature published in the last ten years on electrochemically driven oxidation and reduction reactions utilized in the asymmetric synthesis of biologically active substances.

A thermal decarboxylative Cloke–Wilson rearrangement of dispirocyclopropanes derived from para-quinone methides and bromo-Meldrum's acids: an approach to synthesize spirobutyrolactone para-dienones

An unprecedented approach to synthesize spirobutyrolactone para-dienones from para-quinone methides and bromo-Meldrum's acids has been developed.

First report of spiro-compounds from marine macroalga Gracilaria salicornia: prospective natural anti-inflammatory agents attenuate 5-lipoxygenase and cyclooxygenase-2

The inflammation pathology is an orchestrated biological process and the dual inhibition of pro-inflammatory enzymes 5-lipoxygenase and cyclooxygenase-2 has been found to be an effective approach against inflammation. This study involves the characterisation of two previously undescribed spiro[5.5]undecanes, 3-(hydroxymethyl)-7-(methoxymethyl)-3,11-dimethyl-9-oxospiro[5.5]undec-4-en-10-methylbutanoate (1) and 4-ethoxy-11,11-dimethyl-7-methylene-8-(propionyloxy)spiro[5.5]undec-2-en-10⁴,10⁶-dihydroxytetrahydro-2H-pyran-10-carboxylate (2) with potential anti-inflammatory properties, from seaweed Gracilaria salicornia by extensive-spectroscopic-experiments. These metabolites recorded prospective bioactivities against 5-lipoxygenase (IC₅₀ < 2.80 mM), whereas their selectivity indices were significantly greater (∼1) than ibuprofen (0.89) (p < 0.05), which attributed selective anti-inflammatory potencies of the studied spiro[5.5]undecane derivatives against inducible cyclooxygenase-2 than constitutive cyclooxygenase-1. Radical scavenging potential of spiro[5.5]undec-2-en-10⁴,10⁶-dihydroxytetrahydro-2H-pyran-10-carboxylate analogue (2) against oxidants, 2,2-diphenyl-1-picrylhydrazyl and 2,2'-azino-bis-3 ethylbenzothiozoline-6-sulfonic acid were found to be greater (IC₅₀ < 1.25 mM) than commercial standard, α-tocopherol (IC₅₀ 1.42-1.79 mM). The greater hydrogen-bonding interactions and binding affinity of 2 (-10.13 kcal/mol) with 5-LOX appropriately corroborated its prospective anti-inflammatory activity.

Facile Synthesis of 2,2-Diacyl Spirocyclohexanones via an N-Heterocyclic Carbene-Catalyzed Formal [3C + 3C] Annulation

A novel strategy for the construction of 2,2-diacyl spirocyclohexanones 3 has been demonstrated on the basis of an NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates with 2-ethylidene 1,3-indandiones. Furthermore, enantioenriched 3 was obtained in good to excellent yields with good enantioselectivities when chiral N-heterocyclic carbene (NHC) was employed. Notably, ring opening of the resulting 2,2-diacyl spirocyclohexanones 3 with hydrazine led to the formation of phthalazinones in good to excellent yields.

Ruthenium(0)-Catalyzed Cycloaddition of 1,2-Diols, Ketols, or Diones via Alcohol-Mediated Hydrogen Transfer

Merging the characteristics of transfer hydrogenation and carbonyl addition, a broad new class of ruthenium(0)-catalyzed cycloadditions has been developed. As discussed in this Minireview, fused or bridged bicyclic ring systems are accessible in a redox-independent manner in C-C bond-forming hydrogen transfer reactions of diols, α-ketols, or 1,2-diones with diverse unsaturated reactants.

Selective construction of quaternary stereocentres in radical cyclisation cascades triggered by electron-transfer reduction of amide-type carbonyls

Radical cyclisation cascades triggered by electron-transfer to amide-type carbonyls using SmI₂–H₂O–LiBr, result in the selective construction of quaternary carbon stereocentres.