Mechanistic studies on the photochemical deprotection of 3',5'-dimethoxybenzoin esters

Several mechanistic alternatives proposed for the photochemical deprotection of dimethoxybenzoin esters are presented. Both experimental and theoretical evidence suggest the mechanism is heterolysis of the singlet excited state to form a carbox‐ylate and the α‐ketocation. The α‐ketocation has been observed by transient spectroscopy. We propose the α‐ketocation undergoes electrocyclization to an intermediate with extended conjugation, whose deprotonation gives the observed benzofuran product. A Brønsted study of the rates of benzofuran formation with dimethoxybenzoin esters derived from acids of varying pK_a shows the rate is independent of the basicity of the leaving group. In this multistep reaction, benzofuran formation by a final deprotonation is slower than α‐ketocation generation.     

Preparation of Highly Functionalised Benzofurans from ortho‐Hydroxyphenones and Dichloroethylene: Applications and Mechanistic Investigations

Highly functionalised benzofurans have been prepared from ortho‐hydroxyphenones and 1,1‐dichloroethylene. The key intermediate, a chloromethylene furan, smoothly rearranged into the corresponding benzofuran carbaldehyde under acidic conditions. Some mechanistic investigations have been performed and several biologically active benzofurans have been synthesised.     

Facile Chemoselective synthesis of 2‐(2‐(Methoxycarbonyl)‐3‐oxo‐2,3‐dihydrobenzofuran‐2‐yl)benzoic acids and 3H,3’H‐Spiro[benzofuran‐2,1′‐isobenzofuran]‐3,3′‐dione derivatives

Oxidation of some derivatives of 4b,9b–dihydroxyindeno[1,2‐b]benzofuran‐10‐one have been investigated in detail using lead(IV) acetate in acetic acid under reflux conditions and periodic acid in aqueous ethanol at room temperature. We realized that during the first 5–15 minutes of the oxidation reactions in lead(IV) acetate/acetic acid system, 3H,3’H‐spiro[benzofuran‐2,1′‐isobenzofuran]‐3,3′‐dione derivatives have been synthesized chemo selectively, while, if the reaction mixtures stirred for additional 3 hours, the main products would be 2‐(2‐(Methoxycarbonyl)‐3‐oxo‐2,3‐dihydrobenzofuran‐2‐yl)benzoic acids. Moreover, room temperature oxidation of 4b,9b–dihydroxyindeno[1,2‐b]benzofuran‐10‐ones by periodic acid (H₅IO₆), leads to the formation of 3H,3’H‐spiro[benzofuran‐2,1′‐isobenzofuran]‐3,3′‐dione derivatives in good to excellent yields.     

Effective Conversion of Three Diacetyl‐C‐(β‐D‐Glycopyranosyl) Phloroglucinols to Spiroketal Derivatives by Refluxing in Water

Refluxing of diacetylphloroglucinol C‐β‐D‐gluco‐, ‐galacto‐, and ‐allopyranosides in water for 1 d gave two kinds of spiroketal derivatives in total yields of 77%, 74%, and 64%, respectively. The structure and stereochemistry of the six new spiro(benzofuran‐[2H]pyran and ‐[2H]furan) derived from galactoside and alloside were verified by NMR analysis. The production ratios of the spiro derivatives were measured by HPLC analysis at regular time intervals. Since the majority of spiro(benzofuran‐[2H]furan) were produced after 8 to 12 h of refluxing and most spiro(benzofuran‐[2H]pyran) produced after 2 d of refluxing, it is assumed that formation of spirofuran and spiropyran is a kinetic‐ and thermodynamic‐controlled reaction, respectively.     

Computational insight on the chalcone formation mechanism by the Claisen–Schmidt reaction

New insight of the formation mechanism of chalcones is presented in the current study. Ab initio calculations were applied in studying the mechanistic pathways for the base‐catalyzed Claisen–Schmidt condensation for obtaining chalcones (1,3‐diphenyl‐2‐propen‐1‐ons). The energies of the stationary points along the reaction coordinate were obtained at two levels of theory—MP2/6‐31 + G(d,p) and SCS‐MP2/6‐31 + G(d,p). The role of water in the reaction mechanisms is examined. The theoretical results show that the process is catalyzed by an ancillary water molecule. The reaction mechanism, proposed in this study, consists of two reactions—an activation of the acetophenone by a removal of proton is followed by the attack of the formed acetophenone anion to the aromatic aldehyde, which through few steps leads to the formation of the final product—chalcone. The first reaction proceeds very fast in one step while the second reaction goes through four steps and three intermediate complexes before the formation of the final product.     

Remote Participation during Glycosylation Reactions of Galactose Building Blocks: Direct Evidence from Cryogenic Vibrational Spectroscopy

The stereoselective formation of 1,2‐cis‐glycosidic bonds is challenging. However, 1,2‐cis‐selectivity can be induced by remote participation of C4 or C6 ester groups. Reactions involving remote participation are believed to proceed via a key ionic intermediate, the glycosyl cation. Although mechanistic pathways were postulated many years ago, the structure of the reaction intermediates remained elusive owing to their short‐lived nature. Herein, we unravel the structure of glycosyl cations involved in remote participation reactions via cryogenic vibrational spectroscopy and first principles theory. Acetyl groups at C4 ensure α‐selective galactosylations by forming a covalent bond to the anomeric carbon in dioxolenium‐type ions. Unexpectedly, also benzyl ether protecting groups can engage in remote participation and promote the stereoselective formation of 1,2‐cis‐glycosidic bonds.     

Copper‐Catalyzed Aerobic Oxidations of 3‐N‐Hydoxyaminoprop‐1‐ynes to Form 3‐Substituted 3‐Amino‐2‐en‐1‐ones: Oxidative Mannich Reactions with a Skeletal Rearrangement

Cu‐catalyzed aerobic oxidations of readily available 3‐N‐hydroxyaminopro‐1‐ynes with water, alcohols, or thiols to form diverse 3‐substituted 3‐amino‐2‐en‐1‐ones are described. The utility of this catalysis is manifested by a wide scope of applicable N‐hydroxyl propargylamines and nucleophiles, thus enabling the design of one‐pot cascade or two‐step sequential reactions. Besides synthetic significances, such oxidative Mannich reactions are mechanistically interesting because structurally reorganized products were obtained. Our mechanistic studies reveal that the aerobic oxidations involve initial formation of nitrone intermediates, followed by the attack of nucleophiles. Herein, water and MeOH implement the conversion of nitrone intermediates to reaction products in two distinct pathways.     

A Convenient Palladium‐Catalyzed Carbonylative Synthesis of Benzofuran‐2(3 H)‐ones with Formic Acid as the CO Source

A general and convenient palladium‐catalyzed carbonylation procedure for the synthesis of benzofuran‐2(3 H)‐ones from phenols and aldehydes has been developed. With formic acid as the CO source, a variety of benzofuran‐2(3 H)‐ones were obtained in moderate to good yields.     

Total Synthesis of (+)‐Machaeriols B and C and of Their Enantiomers with a Cannabinoid Structure

An efficient and concise synthesis of the biologically interesting (+)‐machaeriol B ( 2 ) and its enantiomer 5 was accomplished from O‐phenylhydroxylamine ( 7 ) in four steps (Scheme 2). In addition, the first total synthesis of natural (+)‐machaeriol C ( 3 ) and its enantiomer 6 was achieved from the readily available ester 15 in eight steps (Scheme 4). The key strategies in the syntheses of 2 and 5 involved benzofuran formation through a [3,3]‐sigmatropic rearrangement and trans‐hexahydrodibenzopyran formation by a domino aldol‐type/hetero‐Diels–Alder reaction. In the case of 3 and 6 , the key steps were stilbene formation by a Horner–Wadsworth–Emmons reaction and trans‐hexahydrodibenzopyran formation by domino reactions.     

Synthesis of New Spirocyclopropane and Oxadiazole Products

In explorations of synthesis and chemistry of spiroheterocycles, we found that the reaction of 2‐diazopropane with arylidene‐benzofuran‐2(3H)‐one and arylidene‐benzofuran‐3(2H)‐one derivatives generated the spirocyclopropane products. In addition to the expected cycloadducts, an unexpected oxadiazole was formed in some cases. The structures of the obtained adducts have been assigned by means of spectroscopic methods.     

O−O Bond Formation and Liberation of Dioxygen Mediated by N5‐Coordinate Non‐Heme Iron(IV) Complexes

Formation of the O?O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O?O bond in solution, from non‐heme, N₅‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O?O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [Fe^IV(O)] and [Fe^V(O)] being the resting and active principles of the O?O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O?O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.     

A unique two‐dimensional coordination network of 1‐benzofuran‐2,3‐dicarboxylate with lanthanum(III) obtained by solvothermal synthesis

The title compound, poly[bis[diaqualanthanum(III)]‐tris(μ‐1‐benzofuran‐2,3‐dicarboxylato)], [La₂(C₁₀H₄O₅)₃(H₂O)₄]_n, was obtained under solvothermal conditions by reacting lanthanum trinitrate hexahydrate with 1‐benzofuran‐2,3‐dicarboxylic acid in a strongly basic environment. It forms an extended two‐dimensional coordination network, wherein every lanthanum ion links to four deprotonated diacid ligands, while two of the latter bridge between adjacent metal cations. The component species are additionally linked to one another by hydrogen bonds. The polymeric arrays are tightly stacked one on top of the other, without incorporating any solvent in the interface zones between them, which are lined with the lipophilic benzofuran residues. This study provides the first example of coordination networking with the aid of the 1‐benzofuran‐2,3‐dicarboxylate ligand.     

N‐Alkylsulfonylimines as Dipolarophiles in Cycloaddition Reactions

First described in the late 1960s, N‐alkylsulfonylimines are heterocumulenes that participate in reactions with a range of 1,3‐dipoles to afford interesting 3‐, 4‐, 5‐, and 6‐membered heterocycles. The distribution of adducts obtained suggests that multistage, stepwise mechanistic pathways rather than a concerted process are in operation.     

Divergent Pathways and Competitive Mechanisms of Metathesis Reactions between 3‐Arylprop‐2‐ynyl Esters and Aldehydes: An Experimental and Theoretical Study

Mechanistic studies of the reaction between 3‐arylprop‐2‐ynyl esters and aldehydes catalyzed by BF₃ ? Et₂O were performed by isotopic labeling experiments and quantum chemical calculations. The reactions are shown to proceed by either a classical alkyne–carbonyl metathesis route or an unprecedented addition–rearrangement cascade. Depending on the structure of the starting materials and the reaction conditions, the products of these reactions can be Morita–Baylis–Hillman (MBH) adducts that are unavailable by traditional MBH reactions or E‐ and Z‐α,β‐unsaturated ketones. ¹⁸O‐Labeling studies suggested the existence of two different reaction pathways to the products. These pathways were further examined by quantum chemical calculations that employed the DFT(wB97XD)/6‐311+G(2d,p) method, together with the conductor‐like screening model for realistic solvation (COSMO‐RS). By using the wB97XD functional, the accuracy of the computed data is estimated to be 1–2 kcal mol^?1, shown by the careful benchmarking of various DFT functionals against coupled cluster calculations at the CCSD(T)/aug‐cc‐pVTZ level of theory. Indeed, most of the experimental data were reproduced and explained by theory and it was convincingly shown that the branching point between the two distinct mechanisms is the formation of the first intermediate on the reaction pathway: either the four‐membered oxete or the six‐membered zwitterion. The deep mechanistic understanding of these reactions opens new synthetic avenues to chemically and biologically important α,β‐unsaturated ketones.     

Density functional computational studies on the glucose and glycine Maillard reaction: Formation of the Amadori rearrangement products

Theoretical energy changes of various intermediates leading to the formation of the Amadori rearrangement products (ARPs) under different mechanistic assumptions have been calculated, by using open chain glucose (O‐Glu)/closed chain glucose (A‐Glu and B‐Glu) and glycine (Gly) as a model for the Maillard reaction. Density functional theory (DFT) computations have been applied on the proposed mechanisms under different pH conditions. Thus, the possibility of the formation of different compounds and electronic energy changes for different steps in the proposed mechanisms has been evaluated. B‐Glu has been found to be more efficient than A‐Glu, and A‐Glu has been found more efficient than O‐Glu in the reaction. The reaction under basic condition is the most favorable for the formation of ARPs. Other reaction pathways have been computed and discussed in this work. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Synthesis of Optically Enriched Spirocyclic Benzofuran‐2‐ones by Bifunctional Thiourea‐Base Catalyzed Double‐Michael Addition of Benzofuran‐2‐ones to Dienones

A highly enantioselective catalytic double‐Michael addition reaction of substituted benzofuran‐2‐ones with divinyl ketones promoted by readily accessible tertiary amine–thiourea Cinchona alkaloids has been developed. A number of optically enriched spirocyclic benzofuran‐2‐ones were prepared in very good yields (up to 99 %), diastereoselectivities (up to 19:1 d.r.), and very good enantioselectivities (up to 92 % ee). Density functional theory (DFT) calculations were performed to investigate the origin of stereoselectivity.     

Three New Phthalides from Gnaphalium adnatum

Three new phthalides, gnaphalides A–C ( 1 – 3 , resp.), together with three known phthalides, were isolated from the aerial part of Gnaphalium adnatum. The structures of the new compounds were elucidated as 6‐(1,1‐dimethylprop‐2‐en‐1‐yl)‐5,7‐dihydroxy‐2‐benzofuran‐1(3H)‐one ( 1 ), 5‐hydroxy‐7‐[(2‐hydroxy‐3‐methylbut‐3‐en‐1‐yl)oxy]‐2‐benzofuran‐1(3H)‐one ( 2 ), and 1,3‐dihydro‐7‐[(3‐methylbut‐2‐en‐1‐yl)oxy]‐1‐oxo‐2‐benzofuran‐5‐yl β‐D ‐glucopyranoside ( 3 ) on the basis of spectral analyses. The structure of 1 was also confirmed by X‐ray crystallographic analysis. The three known phthalides, identified as 5,7‐dihydroxyisobenzofuran‐1(3H)‐one ( 4 ), anaphatol ( 5 ), and 7‐O‐(β‐glucopyranosyl)‐5‐hydroxyisobenzofuran‐1(3H)‐one ( 6 ), were isolated from the genus Gnaphalium for the first time.     

Intermolecular Reactions of γ‐Halocarbanions – Stepwise Analogs of 1,3‐Dipolar Cycloaddition

γ‐Halocarbanions, short‐lived intermediates, add to electron‐deficient double bonds of aldehydes, Michael acceptors, and imines to form anionic adducts that enter intramolecular 1,5‐substitution to form five‐membered rings of tetrahydrofurans, cyclopentanes, and pyrrolidines, respectively. Although the γ‐halocarbanions can be generated by simple deprotonation of appropriate precursors, a wealth of other methods based on Lewis acid‐catalyzed opening of cyclopropanes with formation of dipolar species utilizes a similar mechanistic scheme. In our review, we analyze kinetic relations of elementary processes in the multistep transformations, and demonstrate how structural factors influence the mechanisms and selectivity of the reaction.     

Mechanistic Chemistry of Extraordinary Capacity of Salvianolic Acid B on Oxidatively‐damaged Mesenchymal Stem Cells

As the main bioactive component of Chinese herbal medicine Danshen, salvianolic acid B (Sal B, or lithospermic acid B) was observed to maintain the viability of mesenchymal stem cells (MSCs) treated by Fenton's reagent in the study. Interestingly, at a higher concentration, Sal B could even increase the viability rate to 175.1%. In mechanistic analysis experiments, Sal B was found to resist DNA destruction by ?OH radical, scavenge various radicals in vitro, reduce Cu²⁺ → Cu⁺, chelate Fe²⁺ to yield an absorption maximum at 710 nm. Based on these results, we concluded that, (1) Sal B can not only protect MSCs against ?OH‐induced damages, but also promote their proliferation. This extraordinary capacity makes Sal B an ideal candidate in MSCs transplantation especially when MSCs are polluted by iron‐overload or other oxidative stress factors and, can partly be responsible for the versatile properties of Sal B in pharmacology. The possible mechanisms of its protective effect are hypothesized to include Fe²⁺ chelating, and direct radical scavenging which is involved in electron transfer (ET) or hydrogen atom transfer (HAT) from the catechol moieties. Its proliferation‐promoting effect is presumed to be from its ester group, carboxylic group, or benzofuran ring.     

[RhIII(Cp*)]‐Catalyzed ortho‐Selective Direct C(sp2)H Bond Amidation/Amination of Benzoic Acids by N‐Chlorocarbamates and N‐Chloromorpholines. A Versatile Synthesis of Functionalized Anthranilic Acids

A Rh^III‐catalyzed direct ortho‐C?H amidation/amination of benzoic acids with N‐chlorocarbamates/N‐chloromorpholines was achieved, giving anthranilic acids in up to 85 % yields with excellent ortho‐selectivity and functional‐group tolerance. Successful benzoic acid aminations were achieved with carbamates bearing various amide groups including NHCO₂Me, NHCbz, and NHTroc (Cbz=carbobenzyloxy; Troc=trichloroethylchloroformate), as well as secondary amines, such as morpholines, piperizines, and piperidines, furnishing highly functionalized anthranilic acids. A stoichiometric reaction of a cyclometallated rhodium(III) complex of benzo[h]quinoline with a silver salt of N‐chlorocarbamate afforded an amido–rhodium(III) complex, which was isolated and structurally characterized by X‐ray crystallography. This finding confirmed that the C?N bond formation results from the cross‐coupling of N‐chlorocarbamate with the aryl–rhodium(III) complex. Yet, the mechanistic details regarding the C?N bond formation remain unclear; pathways involving 1,2‐aryl migration and rhodium(V)– nitrene are plausible.