Recent Advances in the Synthesis of Diarylheptanoids

In the quest for synthesizing biologically important natural products, medicinal chemists embark on an endless journey. This review focuses on the reports published towards the syntheses of diarylheptanoids, classifying them into linear, tetrahydropyran, diarylether, and biphenyl categories. The synthesis methods for each class from 2013 to 2023 are discussed, providing a comprehensive overview of the advancements in the field. Representative natural product examples are highlighted for each category. The review emphasizes the importance of diarylheptanoids in the realms of chemistry and medicine, showcasing their potential as valuable compounds for medicinal and synthetic chemists.

Regio- and Stereospecific Hydrative Cloke-Wilson Rearrangement

The Cloke-Wilson rearrangement of unsymmetrical β-diketone-derived cyclopropanes inevitably yields a mixture of two 4-acylated 2,3-dihydrofuran regiomers. By using alkynes as masked acyls, Tf2NH-promoted Cloke-Wilson rearrangement of polysubstituted 1-(1-alkynyl)cyclopropyl ketones followed by alkyne hydration is described, regioselectively affording 2,3-dihydrofurans bearing 4-acyls nonequivalent to that involved in the Cloke-Wilson rearrangement. The 2,3-dihydrofuran rings with cis 2,3-diaryls are unexpectedly more stable than their trans diastereomers under the reaction conditions, guaranteeing the regiospecificity of this hydrative Cloke-Wilson rearrangement with high fidelity.

Palladium-Catalyzed Intermolecular Dearomatization Annulation Cascade Reaction of Furans for Stereoselective Access to 2,5-Dihydrofurans

A novel palladium-catalyzed intermolecular dearomatization of furans with alkynes via a three-component formal [3 + 2] spiroannulation/allylic substitution cascade reaction has been successfully developed for the stereoselective assembly of spiro 2,5-dihydrofuran frameworks. High step economy and efficacy as well as excellent stereoselectivity were achieved for a broad substrate scope. Two new C-C bonds and one new C-O bond were generated sequentially in a one-pot manipulation. The yielded spiro 2,5-dihydrofuran skeleton bearing a tetrasubstituted carbon center constitutes the core structure for plenty of useful natural products or corresponding analogues. This work represents a significant advancement in the dearomatization strategy for furan heterocycles and provides a practical methodology for expedited access to complex spiro dihydrofuran scaffolds.

Metal-free synthesis of dihydrofuran derivatives as anti-vicinal amino alcohol isosteres

Dihydrofuran cores are commonly incorporated into synthetically and pharmacologically significant scaffolds in natural product and drug discovery chemistry. Herein, we report a concise and practical strategy to construct spiro-dihydrofuran and amino dihydrofuran scaffolds as anti-vicinal amino alcohol isosteres. Hypervalent iodine (PhI(OAc)(NTs₂))-mediated C-H activation of alkynes resulted in two-bond formations with one pi bond cleavage: (i) C(sp²)-N(sp³) and O(sp³)-C(sp²); (ii) C(sp²)-N(sp³) and C(sp³)-C(sp²). The metal-free 5-endo-dig oxidative cyclization provided versatile amino 2,3- and 2,5-dihydrofurans bearing the C₅ quaternary carbon. The non-toxicity of all synthesised dihydrofurans was verified via in vitro cell viability assay.

Influence of Substituents in a Six-Membered Chelate Ring of HG-Type Complexes Containing an N→Ru Bond on Their Stability and Catalytic Activity

An efficient approach to the synthesis of olefin metathesis HG-type catalysts containing an N→Ru bond in a six-membered chelate ring was proposed. For the most part, these ruthenium chelates can be prepared easily and in high yields based on the interaction between 2-vinylbenzylamines and Ind II (the common precursor for Ru-complex synthesis). It was demonstrated that the increase of the steric volume of substituents attached to the nitrogen atom and in the α-position of the benzylidene fragment leads to a dramatic decrease in the stability of the target ruthenium complexes. The bulkiest ⁱPr substituent bonded to the nitrogen atom or to the α-position does not allow the closing of the chelate cycle. N,N-Diethyl-1-(2-vinylphenyl)propan-1-amine is a limiting case; its interaction with Ind II makes it possible to isolate the corresponding ruthenium chelate in a low yield (5%). Catalytic activity of the synthesized complexes was tested in RCM reactions and compared with α-unsubstituted catalysts obtained previously. The structural peculiarities of the final ruthenium complexes were thoroughly investigated using XRD and NMR analysis, which allowed making a reliable correlation between the structure of the complexes and their catalytic properties.

Palladium-Catalyzed [2+3] Cycloaddition/Cross-Coupling Reaction: Z/E and Diastereoselective Synthesis of Dendralene-Functionalized Dihydrofurans

Herein, we describe a Pd-catalyzed [2+3] cycloaddition/cross-coupling reaction of allenyl acetates for the Z/E selective and diastereoselective synthesis of dendralene-functionalized dihydrofurans. Remarkably, mechanistic studies show the formation of an epoxide from a carbonyl bond via cycloaddition, which is practically and mechanistically significant for the construction of other bioactive heterocyclic epoxides. This research also revealed the utility and potential of allenic esters as C2 synthons and 1,2-biselectrophiles in cycloaddition reactions.

Time-economical synthesis of selenofunctionalized heterocycles via I2O5-mediated selenylative heterocyclization

A time-economical and robust synthesis of various selenofunctionalized heterocycles was accomplished via I₂O₅-mediated selenocyclizations of olefins with diselenides. Using this method, 116 selenomethyl-substituted heterocycles were synthesized with up to 97% isolated yield in minutes. Additional features of this new protocol include the use of an inorganic oxidant, mild conditions, and easy operation. Preliminary investigations suggest that the transformation operates through selenenyl iodide-induced electrophilic cyclization.

Cobalt-catalyzed modular assembling toward multi-functionalized furan derivatives

A cobalt-catalyzed modular [3+2] assembling of unsaturated hydrocarbons and β-dicarbonyls is reported. This protocol features mild reaction conditions and a broad substrate scope, providing facile entries toward diverse multi-functionalized dihydrofuran...

Gold-catalyzed formal (3 + 2) and (4 + 2) cycloaddition reactions using propiolates: assembly of 2,3-dihydrofurans and 3,4-dihydropyrans via a multistep cascade process

A gold-catalyzed formal dipolar cycloaddition reaction was developed using polarized alkynes as dipolarophiles and butenediol or pentenediol derivatives as formal dipoles. Silyl groups were used to solve the selectivity issue of unsymmetrical diols.

Asymmetric Synthesis of 2,3-Dihydrofurans via Squaramide Catalyzed Michael-Alkylation Reaction

Abstract:

Asymmetric Catalysis, Dihydrofuran, Michael-Alkylation Reaction, Tertiary Amine-Squaramide, Organocatalysis, α-Bromonitroalkene

Stereoselective Synthesis of Biologically Relevant Tetrahydropyridines and Dihydro-2H-pyrans via Ring-Expansion of Monocyclopropanated Heterocycles

A stereoselective, scalable, and metal-free ring-expansion of monocyclopropanated pyrroles and furans has been developed, leading to value-added highly functionalized tetrahydropyridine and dihydro-2H-pyran derivatives. Featuring a cyclopropylcarbinyl cation rearrangement as the key step, the selective cleavage of the unactivated endocyclic cyclopropane C-C bond is achieved. Targeted transformations of the thus obtained six-membered heterocycles give access to versatile building blocks with relevance for drug synthesis.

Asymmetric Sequential Corey-Chaykovsky Cyclopropanation/Cloke-Wilson Rearrangement for the Synthesis of 2,3-Dihydrofurans

The first sequential Corey-Chaykovsky cyclopropanation/Cloke-Wilson rearrangement between propargyl sulfonium salts and acrylonitrile derivatives has been developed, affording the tetra-substituted 2,3-dihydrofurans in generally excellent yields (57-98%) with good diastereoselectivities (7:1-18:1). In addition, chiral propargyl sulfonium salt is also suitable for this strategy, giving the optically active 2,3-dihydrofurans with good enantioselectivities. This reaction sequence was designed upon in situ generated 10π-conjugated structures from the dearomatization of indole fragments and subsequent intramolecular 1,6-addition.

Chiral-at-Iron Catalyst for Highly Enantioselective and Diastereoselective Hetero-Diels-Alder Reaction

This study demonstrates that chiral-at-iron complexes, in which all coordinated ligands are achiral and the overall chirality the consequence of a stereogenic iron center, are capable of catalyzing asymmetric transformations with very high enantioselectivities. The catalyst is based on a previously reported design (J. Am. Chem. Soc. 2017, 139, 4322), in which iron(II) is surrounded by two configurationally inert achiral bidentate N-(2-pyridyl)-substituted N-heterocyclic carbenes in a C2 -symmetric fashion and complemented by two labile acetonitriles. By replacing mesityl with more bulky 2,6-diisopropylphenyl substituents at the NHC ligands, the steric hindrance at the catalytic site was increased, thereby providing a markedly improved asymmetric induction. The new chiral-at-iron catalyst was applied to the inverse electron demand hetero-Diels-Alder reaction between β,γ-unsaturated α-ketoester and enol ethers provide 3,4-dihydro-2H-pyrans in high yields with excellent diastereoselectivities (up to 99 : 1 dr) and excellent enantioselectivities (up to 98 % ee). Other electron rich dienophiles are also suitable as demonstrated for a reaction with a vinyl azide.

Iodine-Catalyzed Synthesis of Substituted Furans and Pyrans: Reaction Scope and Mechanistic Insights

Substituted pyrans and furans are core structures found in a wide variety of natural products and biologically active compounds. Herein, we report a practical and mild catalytic method for the synthesis of substituted pyrans and furans using molecular iodine, a simple and inexpensive catalyst. The method described is performed under solvent-free conditions at an ambient temperature and atmosphere, thus offering a facile and practical alternative to currently available reaction protocols. A combination of experimental studies and density functional theory calculations revealed interesting mechanistic insights into this seemingly simple reaction.

Electrophilic fluorosulfoxonium cations as hidden Brønsted acid catalysts in (n + 2) annulations of strained cycloalkanes

A fluorosulfoxonium cation and triflic acid were shown to promote (n + 2) annulations of donor–acceptor cyclopropanes or -butanes with 1,2-dipoles, with different activity and selectivity but a presumed similar role as (hidden) Brønsted acids.

Rh-Catalyzed oxidative homo-coupling cyclization of 2,3-allenols to conjugated furylenones

An unexpected rhodium-catalyzed highly regio- and stereo-selective bimolecular nucleophilic oxyrhodation/insertion/1,4-Rh migration/β-H elimination reaction of 2,3-allenols affording (2,5-dihydrofuran-3-yl)but-2(E)-en-1-one derivatives is reported.

Silver(I) Oxide-/DBU-Promoted Synthesis of Dihydrofuran Units through Allenyl Silver Formation

A formal [3+2] cyclization mediated by silver(I) oxide and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is described herein. Through a broad variety of carbonyl compounds, this system can promote cyclization reactions with high yield (up to 85 %) and diastereoselectivity (up to 95:5) for a straightforward access to complex and congested dihydrofuran derivatives in one step under mild conditions. Based on DFT studies, the proposed mechanism would involve an allenyl silver intermediate.

Dehydrative Re2O7-Catalyzed Approach to Dihydropyran Synthesis

Monoallylic 1,3- and 1,5-diols undergo Re₂O₇-mediated ionization to form allylic cations that engage in cyclization reactions to form dihydropyran products. The reactions give the 2,6-trans-stereoisomer as the major products as a result of minimizing steric interactions in a boat-like transition state. The results of these studies are consistent with cationic intermediates, with an intriguing observation of stereochemical retention in one example.

Gold(I)-Catalyzed One-Pot and Diastereoselective Synthesis of trans-2-Silyl-4,5-dihydrofurans from Propargylsilanes and Aldehydes

A diastereoselective and high-yielding gold-catalyzed synthesis of trans-2-silyl-4,5-dihydrofurans is described. In addition to a sequential manner, this reaction could be performed in a one-pot procedure from propargylsilanes and aldehydes. A mechanistic proposal for the cis-trans isomerization step is formulated. To provide experimental support for this proposal, which involves ring opening/ring closing steps of the dihydrofuran, several isotopically labeled experiments, intramolecular capture of a proposed intermediate, and construction of a Hammett plot have been performed.

Kinetic Resolution of Propargylic Ethers via [2,3]-Wittig Rearrangement to Synthesize Chiral α-Hydroxyallenes

An efficient kinetic resolution of propargyloxy dicarbonyl compounds via asymmetric [2,3]-Wittig rearrangement was achieved by using a chiral N,N'-dioxide/Ni^II complex catalyst. Various chiral α-allenyl alcohols were obtained in high enantioselectivities under mild conditions. The utility of this method was readily demonstrated in the asymmetric synthesis of the chiral 2,5-dihydrofuran derivative.

Efficient Stereoselective Carbocyclization to cis-1,4-Disubstituted Heterocycles Enabled by Dual Pd/Electron Transfer Mediator (ETM) Catalysis

An efficient Pd/ETM (ETM = electron transfer mediator)-cocatalyzed stereoselective oxidative carbocyclization of dienallenes under aerobic oxidation conditions has been developed to afford six-membered heterocycles. The use of a bifunctional cobalt complex [Co(salophen)-HQ] as hybrid ETM gave a faster aerobic oxidation than the use of separated ETMs, indicating that intramolecular electron transfer between the hydroquinone unit and the oxidized metal macrocycle occurs. In this way, a class of important cis-1,4-disubstituted six-membered heterocycles, including dihydropyran and tetrahydropyridine derivatives were obtained in high diastereoselectivity with good functional group compatibility. The experimental and computational (DFT) studies reveal that the pendent olefin does not only act as an indispensable element for the initial allene attack involving allenic C(sp³)–H bond cleavage, but it also induces a face-selective reaction of the olefin of the allylic group, leading to a highly diastereoselective formation of the product. Finally, the deuterium kinetic isotope effects measured suggest that the initial allenic C(sp³)–H bond cleavage is the rate-limiting step, which was supported by DFT calculations.

Formal [5+1] annulation reactions of dielectrophilic peroxides: facile access to functionalized dihydropyrans

Peroxides, functioning as unique five-atom bielectrophilic synthons, enable the new [5+1] annulation reactions to access dihydropyrans in high yields.

Stereoselective synthesis of hydrazinodihydrofurans via cascade Michael addition-substitution involving the reaction of curcumin and other β-dicarbonyls with α-hydrazinonitroalkenes

Highly diastereoselective synthesis of 2-hydrazinated 2,3-dihydrofurans in good to excellent yields involving an interrupted Feist-Bénary type reaction by treating a wide variety of 1,3-dicarbonyl compounds, including curcumins, with α-hydrazinated nitroalkenes is reported here. The first ever enantioselective reaction of α-hydrazinonitroalkenes has also been carried out with two selected 1,3-dicarbonyls, dimedone and cyclohexanone by employing an l-t-leucine derived squaramide as the chiral organocatalyst to afford the enantio-enriched 2-hydrazinodihydrofurans as single diastereomers in good yields and with good enantioselectivities.

Catalytic asymmetric synthesis of 2,5-dihydrofurans using synergistic bifunctional Ag catalysis

We report a bifunctional Ag catalyst promoted intramolecular capture of oxonium ylides with alkynes for the enantioselective synthesis of 2,5-dihydrofurans. This represents unprecedented synergistic catalysis of a bifunctional Ag catalyst. Mechanistic studies revealed that [(R)-3,5-DM-BINAP](AgSbF₆)₂ (9) is likely to be the active catalytic species and that the reaction involves second order kinetics with respect to 9, suggesting that two molecules of 9 are involved in the intramolecular trapping of a Ag-associated oxonium ylide with a Ag-activated alkyne. Based on our mechanistic hypothesis, we further optimized the reaction, rendering a facile approach to 2,5-dihydrofurans in good to excellent yields in a highly chemo- and enantioselective fashion.

Design, Synthesis, and Phenotypic Profiling of Pyrano-Furo-Pyridone Pseudo Natural Products

Natural products (NPs) inspire the design and synthesis of novel biologically relevant chemical matter, for instance through biology-oriented synthesis (BIOS). However, BIOS is limited by the partial coverage of NP-like chemical space by the guiding NPs. The design and synthesis of "pseudo NPs" overcomes these limitations by combining NP-inspired strategies with fragment-based compound design through de novo combination of NP-derived fragments to unprecedented compound classes not accessible through biosynthesis. We describe the development and biological evaluation of pyrano-furo-pyridone (PFP) pseudo NPs, which combine pyridone- and dihydropyran NP fragments in three isomeric arrangements. Cheminformatic analysis indicates that the PFPs reside in an area of NP-like chemical space not covered by existing NPs but rather by drugs and related compounds. Phenotypic profiling in a target-agnostic "cell painting" assay revealed that PFPs induce formation of reactive oxygen species and are structurally novel inhibitors of mitochondrial complex I.

Very short highly enantioselective Grignard synthesis of 2,2-disubstituted tetrahydrofurans and tetrahydropyrans

Phenones with elongated chains are shown to be excellent substrates for ligand-promoted asymmetric Grignard synthesis of tertiary alcohols. In turn this enables the simple, short and highly enantioselective (up to 96% ee) preparation of chiral 2,2-disubstituted THFs and THPs. Thus, asymmetric addition of Grignard reagents to γ-chlorobutyrophenones and δ-chlorovalerophenones takes place in the presence of a chiral diaminocyclohexyl-derived tridentate ligand and subsequent base-promoted intramolecular cyclisation occurs with complete retention of asymmetry. As examples of the methodology, we report the shortest syntheses of gossonorol, γ-ethyl-γ-phenylbutyrolactone and δ-methyl-δ-tolylvalerolactone, the joint-shortest and flexible synthesis of boivinianin A and the shortest formal syntheses of boivinianin B and yingzhaosu C.

Ruthenium-Catalyzed Intramolecular Double Hydroalkoxylation of Internal Alkynes

Intramolecular double hydroalkoxylation of internal alkynes could be achieved using a Grubbs-type ruthenium carbene complex or its modified species to deliver a series of bridged- and spiroacetal derivatives in moderate to good yields. This study represents a new example of nonmetathetic reactions catalyzed by Grubbs-type ruthenium carbene complexes.

Preparation of polysubstituted dihydrofurans through a PhI(OAc)2-promoted haloenolcyclization of olefinic dicarbonyl compounds

A metal-free cyclization of olefinic dicarbonyl compounds for the synthesis of various 5-halomethyl-4,5-dihydrofurans is presented. Using (diacetoxyiodo)benzene as the reaction promoter and halotrimethylsilane as the halogen source, the intramolecular haloenolcyclization of the 2-allyl-1,3-dicarbonyl compounds smoothly proceeded, leading to the corresponding 5-halomethyl-4,5-dihydrofurans in good to excellent isolated yields. Moreover, the resulting 5-iodomethyl products could be converted to functionalized furans in almost quantitative yields by treatment with DBU followed by acid-catalyzed rearrangement. The reactions could be carried out on a gram scale and did not require harsh reaction conditions. The good isolated yields, mild conditions, and operational simplicity make this reaction a viable method for the construction of different dihydrofuran and furan structures.

Tandem SN2' nucleophilic substitution/oxidative radical cyclization of aryl substituted allylic alcohols with 1,3-dicarbonyl compounds

A novel and efficient tandem S_N2' nucleophilic substitution/oxidative radical cyclization reaction of aryl substituted allylic alcohols with 1,3-dicarbonyl compounds has been developed by using Mn(OAc)₃ as an oxidant, which enables the expeditious synthesis of polysubstituted dihydrofuran (DHF) derivatives in moderate to high yields. The use of weakly acidic hexafluoroisopropanol (HFIP) as the solvent rather than AcOH has successfully improved the yields and expanded the substrate scope of this type of radical cyclization reactions. Mechanistic studies confirmed the cascade reaction process involving a final radical cyclization.