First green protocols for the large-scale preparation of γ-diisoeugenol and related dihydro(1H)indenes via formal [3+2] cycloaddition reactions

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Valorisation of biobased olefins via Rh-catalyzed transfer hydroformylation and isomerization using formaldehyde as a CO/H2 surrogate

The use of biomass as a new platform of chemical substrates has become a subject of intensive research. In this article the selective functionalization and isomerization of allylbenzenes by transfer hydroformylation with formaldehyde is reported.

Acid-Mediated Decarboxylative C–H Coupling between Arenes and O-Allyl Carbamates

Treatment of O-allyl N-tosyl carbamates with aromatic compounds in the presence of Cu(OTf)2 or TMSOTf as promoters, affords N-substituted 1-arylpropan-2-amines, 1,2-diarylpropanes, 1,1-diarylpropanes, or indanes, depending on the nature of the...

Pharmaceutically relevant (hetero)cyclic compounds and natural products from lignin-derived monomers: Present and perspectives

Lignin, the richest source of renewable aromatics on the planet, is an intriguing raw material for the construction of value-added aromatics. In the past decade, much progress has been made regarding the development of efficient lignin depolymerization methods, able to produce specific monophenol derivatives in high-enough selectivity and yields. This now serves as an excellent basis for developing powerful downstream conversion strategies toward a wide range of products, including fine chemical building blocks. The inherent structural features of lignin-derived platform chemicals undoubtedly inspire the development of novel, creative, atom-economic synthetic routes toward biologically active molecules or natural products. In this perspective we attempt to bridge the structural features of lignin-derived platform chemicals with existing synthetic strategies toward the construction of heterocycles and provide a summary of efforts for the production of natural products from aromatics that can be, in principle, obtained from lignin. Last, we comment on the latest efforts that present entire value-chains from wood to valuable pharmaceutically relevant compounds.

Bioactivity of semisynthetic eugenol derivatives against Spodoptera frugiperda (Lepidoptera: Noctuidae) larvae infesting maize in Colombia

The anti-acetylcholinesterase, larvicidal, antifeedant activities and general toxicity of 15 semisynthetic eugenol derivatives based on clove oil (including the own oil), were evaluated against the maize armyworm, Spodoptera frugiperda (J.E. Smith). Therefore, promising eugenol molecules were classified with larvicidal, anti-acetylcholinesterase and antifeedant activities for controlling this pest. During structure–activity relationship studies and physicochemical profile analysis, it was found that among tested molecules 1–15, eugenol 1, prenyl eugenol 4, isoeugenol 8 and isoeugenol acetate 11 exhibited lethal effects LD₅₀ at concentrations <1 mg/g of insect. On the other hand, eugenol 1, metallyl eugenol 3, isoeugenol 8 and isoeugenol acetate 11 showed a good antifeedant activity (CE₅₀ = 158–209 µg/mL) with a high antifeedant index (70–78%) at concentration 1000 µg/mL, possessing a weak anti-acetylcholinesterase activity (IC₅₀ = 21–31 μg/mL). According to their ecotoxicological profiles (LC₅₀ = 2033.1–6303.8 µg/mL on Artemia salina larvae), isoeugenol 8 and its acetate derivative 11 could be potential used in control of the growth, feeding, or reproduction of S. frugiperda larvae, acting as moderate insecticidal acetylcholinesterase inhibitors and/or antifeedant molecules. Such structure–activity relationship studies could stimulate the identification of lead structures from natural sources for the development of larvicidal and deterrent products against S. frugiperda and related insect pests.

Bright Side of Lignin Depolymerization: Toward New Platform Chemicals

Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the future considered as potential lignin-derived platform chemicals.

A solvent-directed stereoselective and electrocatalytic synthesis of diisoeugenol

A stereoselective and electrocatalytic coupling reaction of isoeugenol has been demonstrated for the first time, wherein 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) plays a crucial role.

Effect of methoxyl groups on the NMR spectra: configuration and conformation of natural and synthetic indanic and tetralinic structures

Here, we studied the influence of the methoxyl groups attached at C-7 and C-2' of natural and synthetic 1-arylindanes on the chemical shift of the signal of bibenzylic hydrogen and carbon atoms and J_1,2 coupling constants. This influence was also analysed in natural 1-aryltetralins and related compounds that possess methoxyl and/or hydroxyl groups bound at C-8 and C-2'. The methoxyl groups attached at C-7 in indanes or at C-8 in tetralins produce a deshielding signal at H-1 and shield at C-1 and a strong decrease in the value of J_1,2 due to the pseudoequatorial location adopted by the aryl group bound at C-1, avoiding an 'A^1,3 strain'. Furthermore, compounds with hydroxyl or methoxyl groups in C-2', in the absence of substituents of C-7 or C-8, present a strong deshielding signal at H-1, strong shield of the C-1 signal and a decrease in the value of J_1,2 . This is attributed to the stereoelectronic effects of the methoxyl or hydroxyl groups, which we have called 'Asarone effect'. NOESY experiments were conducted to confirm the configuration and conformation of some of the compounds included in this work. This study shows that both effects, A^1,3 strain and Asarone effect, must be taken into account when the structure of natural indanes and tetralins is analysed by using ¹ H-NMR and ¹³ C-NMR spectra. Copyright © 2016 John Wiley & Sons, Ltd.

Ru-Catalyzed Estragole Isomerization under Homogeneous and Ionic Liquid Biphasic Conditions

The isomerization of estragole to trans-anethole is an important reaction and is industrially performed using an excess of NaOH or KOH in ethanol at high temperatures with very low selectivity. Simple Ru-based transition-metal complexes, under homogeneous, ionic liquid (IL)-supported (biphasic) and "solventless" conditions, can be used for this reaction. The selectivity of this reaction is more sensitive to the solvent/support used than the ligands associated with the metal catalyst. Thus, under the optimized reaction conditions, 100% conversion can be achieved in the estragole isomerization, using as little as 4 × 10^-3 mol % (40 ppm) of [RuHCl(CO)(PPh₃)₃] in toluene, reflecting a total turnover number (TON) of 25 000 and turnover frequencies (TOFs) of up to 500 min^-1 at 80 °C. Using a dimeric Ru precursor, [RuCl(μ-Cl)(η³:η³-C₁₀H₁₆)]₂, in ethanol associated with P(OEt)₃, a TON of 10 000 and a TOF of 125 min^-1 are obtained with 100% conversion and 99% selectivity. These two Ru catalytic systems can be transposed to biphasic IL systems by using ionic-tagged P-ligands such as 1-(3-(diphenylphosphanyl)propyl)-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide immobilized in 1-(3-hydroxypropyl)-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl) imide with up to 99% selectivity and almost complete estragole conversion. However, the reaction is much slower than that performed under solventless or homogeneous conditions. The use of ionic-tagged ligands significantly reduces the Ru leaching to the organic phase, compared to that in reactions performed under homogeneous conditions, where the catalytic system loses catalytic performance after the second recycling. Detailed kinetic investigations of the reaction catalyzed by [RuHCl(CO)(PPh₃)₃] indicate that a simplified kinetic model (a monomolecular reversible first-order reaction) is adequate for fitting the homogeneous reaction at 80 °C and under biphasic conditions. However, the kinetics of the reaction are better described if all of the elementary steps are taken into consideration, especially at 40 °C.