Asymmetric Syntheses of the Flavonoid Diels-Alder Natural Products Sanggenons C and O

Asymmetric catalytic [1,3]- or [3,3]-sigmatropic rearrangement of 3-allyloxy-4H-chromenones and their analogues

A highly efficient asymmetric [1,3]- and [3,3]-O-to-C sigmatropic rearrangement of 3-allyloxy-4H-chromenones and their analogues was developed. Chiral N,N'-dioxide complexes of 3d late transition metal complexes enabled two mechanistically different processes, giving a series of optically active 2,2-disubstituted chromane-3,4-diones and 2-allyl-3-hydroxy-4H-chromen-4-ones as well as their related compounds in excellent yield and enantioselectivity. Systemic mechanistic studies and DFT calculation revealed the nature of the vinyl ether unit of the substrate, which biased regioselectivity via a stepwise tight ion pair pathway and a concerted pericyclic pathway, respectively. The enantioselectivity of the two processes is also disclosed.

Hunting for the Intermolecular Diels-Alderase

ConspectusThe Diels-Alder reaction is well known as a concerted [4 + 2] cycloaddition governed by the Woodward-Hoffmann rules. Since Prof. Otto Diels and his student Kurt Alder initially reported the intermolecular [4 + 2] cycloaddition between cyclopentadiene and quinone in 1928, it has been recognized as one of the most powerful chemical transformations to build C-C bonds and construct cyclic structures. This named reaction has been widely used in synthesizing natural products and drug molecules. Driven by the synthetic importance of the Diels-Alder reaction, identifying the enzyme that stereoselectively catalyzes the Diels-Alder reaction has become an intriguing research area in natural product biosynthesis and biocatalysis. With significant progress in sequencing and bioinformatics, dozens of Diels-Alderases have been characterized in microbial natural product biosynthesis. However, few are evolutionally dedicated to catalyzing an intermolecular Diels-Alder reaction with a concerted mechanism.This Account summarizes our endeavors to hunt for the naturally occurring intermolecular Diels-Alderase from plants. Our research journey started from the biomimetic syntheses of D-A-type terpenoids and flavonoids, showing that plants use both nonenzymatic and enzymatic intermolecular [4 + 2] cycloadditions to create complex molecules. Inspired by the biomimetic syntheses, we identify an intermolecular Diels-Alderase hidden in the biosynthetic pathway of mulberry Diels-Alder-type cycloadducts using a biosynthetic intermediate probe-based target identification strategy. This enzyme, MaDA, is an endo-selective Diels-Alderase and is then functionally characterized as a standalone intermolecular Diels-Alderase with a concerted but asynchronous mechanism. We also discover the exo-selective intermolecular Diels-Alderases in Morus plants. Both the endo- and exo-selective Diels-Alderases feature a broad substrate scope, but their mechanisms for controlling the endo/exo pathway are different. These unique intermolecular Diels-Alderases phylogenetically form a subgroup of FAD-dependent enzymes that can be found only in moraceous plants, explaining why this type of [4 + 2] cycloadduct is unique to moraceous plants. Further studies of the evolutionary mechanism reveal that an FAD-dependent oxidocyclase could acquire the Diels-Alderase activity via four critical amino acid mutations and then gradually lose its original oxidative activity to become a standalone Diels-Alderase during the natural evolution. Based on these insights, we designed new Diels-Alderases and achieved the diversity-oriented chemoenzymatic synthesis of D-A products using either naturally occurring or engineered Diels-Alderases.Overall, this Account describes our decade-long efforts to discover the intermolecular Diels-Alderases in Morus plants, particularly highlighting the importance of biomimetic synthesis and chemical proteomics in discovering new intermolecular Diels-Alderases from plants. Meanwhile, this Account also covers the evolutionary and catalytic mechanism study of intermolecular Diels-Alderases that may provide new insights into how to discover and design new Diels-Alderases as powerful biocatalysts for organic synthesis.

Competing Domino Knoevenagel-Cyclization Sequences with N-Arylcinnamylamines

Domino Knoevenagel-cyclization reactions of N-arylcinnamylamines were carried out with active methylene reagents, which took place with five competing cyclization mechanisms: intramolecular hetero Diels-Alder reaction, stepwise polar [2 + 2] cycloaddition, styryl or aza-Diels-Alder reactions followed by rearomatization, and [1,5]-hydride shift-6-endo cyclization. In the stepwise aza-Diels-Alder reaction, the N-vinylpyridinium moiety acted as an azadiene, producing a condensed heterocycle with tetrahydroquinolizinium and tetrahydroquiniline subunits. Antiproliferative activity with low micromolar IC50 values was identified for some of the novel scaffolds.

Chiral-Boron-Complex-Catalyzed Asymmetric [3 + 2] Cycloaddition of β-Trifluoromethyl α,β-Unsaturated Ketones with N,N'-Cyclic Azomethine Imines

The (R)-3,3'-(3,5-(CF3)2-C6H3)2-BINOL-boron-complex-catalyzed asymmetric 1,3-dipolar cycloaddition of β-trifluoromethyl α,β-unsaturated ketone with N,N'-cyclic azomethine imines was developed to afford N,N'-bicyclic pyrazolidine derivatives bearing a stereogenic carbon center containing CF3 motifs in high yields with excellent diastereo- and enantioselectivities (up to >20:1 dr, and >99% ee). This catalytic system features mild reaction conditions, high efficiency, and a broad substrate scope.

Chiral-Boron-Complex Catalyzed Asymmetric Inverse-Electron-Demand Aza-Diels-Alder Reaction of β-Trifluoromethyl α,β-Unsaturated Ketones with Cyclic N-Sulfonyl Ketimines

A highly efficient asymmetric inverse-electron-demand aza-Diels-Alder reaction of β-trifluoromethyl α,β-unsaturated ketone with cyclic N-sulfonyl ketimines catalyzed by (R)-3,3'-I2-BINOL-boron-complex was developed. A broad range of fused piperidine derivatives bearing stereogenic carbon containing CF3 motifs were prepared in high yields with excellent diastereo- and enantioselectivities (up to >20:1 dr, and >99% ee). This protocol had the characteristics of mild reaction conditions, high efficiency, and high stereoselectivity.

Catalytic Asymmetric Diels-Alder Reaction of 2'-Hydroxychalcone as a Dienophile with a VANOL-Borate Ester Complex

Numerous flavonoid Diels-Alder-type natural products have been isolated and received great attention from the synthetic community. Herein, we reported a catalytic strategy for an asymmetric Diels-Alder reaction of 2'-hydroxychalcone with a range of diene substrates using a chiral ligand-boron Lewis acid complex. This method enables the convenient synthesis of a wide range of cyclohexene skeletons in excellent yields with moderate to good enantioselectivities, which is critical to prepare natural product congeners for further biological studies.

A bioinspired, one-step total synthesis of peshawaraquinone

A concise synthesis of a stereochemically complex meroterpenoid, peshawaraquinone, via the unsymmetrical dimerization of its achiral precursor, dehydro-α-lapachone, is reported. Enabled by reversible oxa-6π-electrocyclizations of 2H-pyran intermediates, the base-catalyzed dimerization sets up an intramolecular (3 + 2) cycloaddition, with the formation of six stereocenters during the cascade. Combining the generation and in situ dimerization of dehydro-α-lapachone allows a one-step total synthesis of peshawaraquinone from lawsone and prenal.

Stereoselective Synthesis of Flavonoids: A Brief Overview

Stereoselective synthesis has been emerging as a resourceful tool because it enables the obtaining of compounds with biological interest and high enantiomeric purity. Flavonoids are natural products with several biological activities. Owing to their biological potential and aiming to achieve enantiomerically pure forms, several methodologies of stereoselective synthesis have been implemented. Those approaches encompass stereoselective chalcone epoxidation, Sharpless asymmetric dihydroxylation, Mitsunobu reaction, and the cycloaddition of 1,4-benzoquinone. Chiral auxiliaries, organo-, organometallic, and biocatalysis, as well as the chiral pool approach were also employed with the goal of obtaining chiral bioactive flavonoids with a high enantiomeric ratio. Additionally, the employment of the Diels-Alder reaction based on the stereodivergent reaction on a racemic mixture strategy or using catalyst complexes to synthesise pure enantiomers of flavonoids was reported. Furthermore, biomimetic pathways displayed another approach as illustrated by the asymmetric coupling of 2-hydroxychalcones driven by visible light. Recently, an asymmetric transfer hydrogen-dynamic kinetic resolution was also applied to synthesise (R,R)-cis-alcohols which, in turn, would be used as building blocks for the stereoselective synthesis of flavonoids.

Synthesis, Biosynthesis, and Biological Activity of Diels-Alder Adducts from Morus Genus: An Update

The plants of the Moraceae family are producers of a great variety of polyphenolic natural products. Among these, the Diels-Alder type adducts (DAAs) are endowed with a unique cyclohexene scaffold, since they are biosynthesized from [4+2] cycloaddition of different polyphenolic precursors such as chalcones and dehydroprenyl polyphenols. To date, more than 150 DAAs have been isolated and characterized from Moraceous and related plants. The main source of DAAs is the mulberry root bark, also known as "Sang-Bai-Pi" in Traditional Chinese Medicine, but they have also been isolated from root bark, stem barks, roots, stems or twigs, leaves, and callus cultures of Moraceous and other related plants. Since 1980, many biological activities of DAAs have been identified, including anti-HIV, antimicrobial, anti-inflammatory, and anticancer ones. For these reasons, natural DAAs have been intensively investigated, and a lot of efforts have been made to study their biosynthesis and to establish practical synthetic access. In this review, we summarized all the updated knowledge on biosynthesis, chemoenzymatic synthesis, racemic and enantioselective total synthesis, and biological activity of natural DAAs from Moraceous and related plants.

Mulberry Diels-Alder-type adducts: isolation, structure, bioactivity, and synthesis

Mulberry Diels-Alder-type adducts (MDAAs) are unique phenolic natural products biosynthetically derived from the intermolecular [4 + 2]-cycloaddition of dienophiles (mainly chalcones) and dehydroprenylphenol dienes, which are exclusively distributed in moraceous plants. A total of 166 MDAAs with diverse skeletons have been isolated and identified since 1980. Structurally, the classic MDAAs characterized by the chalcone-skeleton dienophiles can be divided into eight groups (Types A - H), while others with non-chalcone dienophiles or some variations of classic MDAAs are non-classic MDAAs (Type I). These compounds have attracted significant attention of natural products and synthetic chemists due to their complex architectures, remarkable biological activities, and synthetic challenges. The present review provides a comprehensive summary of the structural properties, bioactivities, and syntheses of MDAAs. Cited references were collected between 1980 and 2021 from the SciFinder, Web of Science, and China National Knowledge Internet (CNKI).

Chiral-Boron-Complex-Catalyzed Asymmetric [3 + 2] Cycloaddition of 2'-Hydroxychalcones with N,N'-Cyclic Azomethine Imines

We report the (R)-3,3'-I₂-BINOL-boron-complex-catalyzed asymmetric 1,3-dipolar cycloaddition of 2'-hydroxychalcones with N,N'-cyclic azomethine imines, providing the corresponding N,N'-bicyclic pyrazolidine derivatives with three contiguous tertiary stereocenters in high yields with excellent diastereo- and enantioselectivities (up to >99:1 diastereomeric ratio and >99% enantiomeric excess). This catalytic system exhibits advantages of mild reaction conditions, high efficiency, and broad substrate scopes.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Total Synthesis of Palodesangrens A and C

Palodesangrens A and C along with the common tetracyclic core are prepared from simple benzaldehyde and acetophenone derivatives in a 10-step longest linear sequence which featured the Diels-Alder reaction forming the cyclohexene moiety, LiAlH₄ isomerization, stereoselective acid-catalyzed cyclization forming the chroman moiety, regioselective iodination/vinyl Suzuki cross-coupling reaction, and ring-closing metathesis (RCM) forming the 2H-pyran-2-one. Overall, the desired palodesangrens A and C are obtained in 6.1% and 6.4% yields, respectively.

Chiral Hydroxytetraphenylene-Boron Complex Catalyzed Asymmetric Diels-Alder Cycloaddition of 2'-Hydroxychalcones

(S)-2,15-Cl₂-DHTP-boron complex catalyst for the asymmetric Diels-Alder cycloaddition of 2'-hydroxychalcones and dienes was developed and tested. The resulting cyclohexenes with three chiral centers were obtained in high yields (up to 98%) with excellent stereoselectivities (up to >20:1 endo/exo, >99% ee). This catalytic system features high efficiency, broad substrate scopes, and mild reaction conditions. In addition, a DFT study was performed to explain the stereochemical course of the asymmetric induction.

Chemoenzymatic Total Syntheses of Artonin I with an Intermolecular Diels-Alderase

Diels-Alder reaction is one of the most important transformations used in organic synthesis, with the ability to construct two new CC bonds and up to four chiral centers simultaneously. However, the biggest synthetic challenge in Diels-Alder reaction lies in controlling its regio-, diastereo-, and enantioselectivity. Using Stille cross-coupling and enzymatic Diels-Alder reaction as the key steps, the first chemoenzymatic total synthesis of artonin I is achieved in 30% overall yield over only seven steps. This enzymatic Diels-Alder reaction catalyzed by MaDA is featured with excellent endo- and enantioselectivity and high catalytic efficiency (k_cat /K_M = 362 ± 54 mm^-1  s^-1 ). These successful chemoenzymatic total syntheses of artonin I and dideoxyartonin I demonstrated the remarkable potential of the intermolecular Diels-Alderase MaDA in biocatalysis.

Synthesis, Characterization, Cytotoxic Activity, and Metabolic Studies of Ruthenium(II) Polypyridyl Complexes Containing Flavonoid Ligands

Four novel monocationic Ru(II) polypyridyl complexes were synthesized with the general formula [Ru(DIP)₂flv]X, where DIP is 4,7-diphenyl-1,10-phenanthroline, flv stands for the flavonoid ligand (5-hydroxyflavone in [Ru(DIP)₂(5-OHF)](PF₆), genistein in [Ru(DIP)₂(gen)](PF₆), chrysin in [Ru(DIP)₂(chr)](OTf), and morin in [Ru(DIP)₂(mor)](OTf)), and X is the counterion, PF₆^-, and OTf ̅ (triflate, CF₃SO₃̅), respectively. Following the chemical characterization of the complexes by ¹H and ¹³C NMR, mass spectrometry, and elemental analysis, their cytotoxicity was tested against several cancer cell lines. The most promising complex, [Ru(DIP)₂(gen)](PF₆), was further investigated for its biological activity. Metabolic studies revealed that this complex severely impaired mitochondrial respiration and glycolysis processes, contrary to its precursor, Ru(DIP)₂Cl₂, which showed a prominent effect only on the mitochondrial respiration. In addition, its preferential accumulation in MDA-MB-435S cells (a human melanoma cell line previously described as mammary gland/breast; derived from metastatic site: pleural effusion), which are used for the study of metastasis, explained the better activity in this cell line compared to MCF-7 (human, ductal carcinoma).

Total Synthesis of Palodesangren B Trimethyl Ether and D Dimethyl Ether via a Late-Stage Formation of 2H-Pyran-2-one of the Tetrahydrobenzo[c]pyranochromenone Core

In four steps from the tricyclic core, palodesangren B trimethyl ether and palodesangren D dimethyl ether could be synthesized in 29 and 18% overall yields, respectively. A reaction sequence comprising the regioselective MgCl₂-mediated Casnati-Skattebøl ortho-formylation of phenol, Wittig methylenation, acryloylation, and Ru(II)-catalyzed ring-closing metathesis (RCM) led to the formation of the final 2H-pyran-2-one ring of the desired tetracyclic core.

Temperature-Controlled Stereodivergent Synthesis of 2,2'-Biflavanones Promoted by Samarium Diiodide

In this work, the first example of a radical stereodivergent reaction directed towards the stereoselective synthesis of both (R*,R*)- and (R*,S*)-2,2'-biflavanones promoted by samarium diiodide is reported. Control experiments showed that the selectivity of this reaction was exclusively controlled by the temperature. It was possible to generate a variety of 2,2'-biflavanones bearing different substitution patterns at the aromatic ring in good-to-quantitative yields, being both stereoisomers of the desired compounds obtained with total or high control of selectivity. A mechanism that explains both the generation of the corresponding 2,2'-biflavanones and the selectivity is also discussed. The structure and stereochemistry determination of each isomer was unequivocally elucidated by single-crystal X-ray diffraction experiments.

Stereodivergent Access to Enantioenriched Epoxy Alcohols with Three Stereogenic Centers via Ruthenium-Catalyzed Transfer Hydrogenation

The resolution technique of stereodivergent reaction on racemic mixtures (stereodivergent RRM) was employed for the first time in ruthenium complex catalyzed transfer hydrogenation of racemic epoxy ketones, providing a new and very simple method that allows access to enantioenriched epoxy alcohols with three stereogenic centers in a one-step fashion. The protocol features simple reaction conditions, practical operation, ability to scale up, and broad group tolerance.

Kinetic resolution of 2,2-disubstituted-1,3-diketones via carbene catalysis

Organocatalytic kinetic resolution of 1,3-diketones with central quaternary stereocenters was achieved for the first time. The resolution proceeds via two basic modes, and the inherent principles between the different combinations of ketone groups and the resolution patterns were also disclosed.

Zinc(II)-Catalyzed Asymmetric Diels-Alder Reaction of ( E)-1-Phenyl Dienes with β,γ-Unsaturated α-Ketoesters

A highly regio-, diastereo-, and enantioselective Diels-Alder reaction of β,γ-unsaturated α-ketoesters with ( E)-1-phenyl dienes has been accomplished by using a stable and easily available chiral N,N'-dioxide/zinc(II) complex as catalyst. Only one isomer of the corresponding cyclohexenes with three chiral centers was obtained in good to excellent yields with excellent ee values under mild reaction conditions. The configurations of the product and chiral N,N'-dioxide/zinc(II) catalyst were identified by X-ray diffraction analysis. In addition, a possible catalytic model was proposed to explain the origin of the asymmetric induction.

Novel binaphthyl and biphenyl α- and β-amino acids and esters: organocatalysis of asymmetric Diels-Alder reactions. A combined synthetic and computational study

Asymmetric catalysis of the Diels-Alder reaction between cyclopentadiene and cinnamaldehydes has been studied using as catalysts a range of novel α- and β-aminoacids and aminoesters with binaphthyl and biphenyl backbones, providing enantioselectivities of up to 62% ee. B3LYP/6-31G* calculations, including free energy corrections, have been carried out on a binaphthyl catalyst example to identify transition state structures and to aid in the identification of major enantiomers. The calculated product ratios agree well with the experimental data; the transition states identified involve preferential approach of cyclopentene along a trajectory adjacent to the acid/ester group. The four lowest energy transition states display a stabilizing dipolar interaction between the carbonyl group oxygen atom and a terminal proton of the diene unit.

Bioinspired Diastereoconvergent Synthesis of the Tricyclic Core of Palodesangrens via Diels-Alder Reaction, LiAlH4-Mediated Isomerization, and Acid-Mediated Cyclization

The cyclohexene moiety of the tricyclic 6,7-diaryl-tetrahydro-6 H-benzo[ c]chromene core of palodesangrens could be assembled in a biomimetic and step-economical fashion by the Diels-Alder reaction between the electron-rich ( E)-1,3-butadienylarenes as the diene and the electron-deficient chalcones as the dienophile. During the reduction of ketone to the corresponding alcohol by LiAlH₄, the mixture of endo and exo isomers underwent a novel diastereoconvergent LiAlH₄-mediated isomerization to install the desired stereochemistry at C10a. Subsequent pyran ring closure under acidic conditions installed the stereochemistry at the remaining C6. Overall, the tricyclic core of palodesangrens could be prepared in three steps and up to 38% yield.

Convenient Synthesis of Heterocyclic Azo Dyes in the Class of Pyranopyrazoles and Chromenes

A series of novel azo group fused 2 H-chromenes and 4 H-chromenes were synthesised as well as pyranopyrazole derivatives via the Knoevenagel condensation reaction of C–H acid compounds with 5-(arylazo)salicylaldehydes by a nucleophilic addition to the carbonyl group followed cyclisation.

Biomimetic Total Synthesis of (±)-Griffipavixanthone via a Cationic Cycloaddition-Cyclization Cascade

We report the concise, biomimetic total synthesis of the dimeric, Diels-Alder natural product griffipavixanthone from a readily accessible prenylated xanthone monomer. The key step utilizes a novel intermolecular [4+2] cycloaddition-cyclization cascade between a vinyl p-quinone methide and an in situ generated isomeric diene promoted by either Lewis or Brønsted acids. Experimental and computational studies of the reaction pathway suggest that a stepwise, cationic Diels-Alder cycloaddition is operative.

A cascade Claisen rearrangement/o-quinone methide formation/electrocyclization approach to 2H-chromenes

A new approach to 8-substituted 2H-chromenes is developed, featuring a novel cascade aromatic Claisen rearrangement/o-quinone methide formation/6π-electrocyclization.

Synthesis of a Fe3O4@P4VP@metal–organic framework core–shell structure and studies of its aerobic oxidation reactivity

A novel core–shell magnetic Fe₃O₄@P4VP@MIL-100(Fe) composite material has been synthesized and its aerobic oxidation activity has been extensively studied.

Picolinoyl functionalized MOF ligands for an air-promoted secondary alcohol oxidation with CuBr

A novel Zr-derived pyridine MOF ligand was designed and synthesized for an efficient Cu(i)-catalyzed secondary alcohol oxidation.

A novel modified MIL-101-NH2 ligand for CuI-catalyzed and air promoted oxidation of secondary alcohols

An efficient Cu(i)/MOF ligand catalyzed aerobic alcohol oxidation system was developed utilizing the ligand at room temperature.

The synthesis of a bifunctional copper metal organic framework and its application in the aerobic oxidation/Knoevenagel condensation sequential reaction

A novel one-pot aerobic oxidation/Knoevenagel condensation reaction system was developed employing a Cu(ii)/amine bifunctional, basic metal-organic framework (MOF) as the catalyst. The sequential aerobic alcohol oxidation/Knoevenagel condensation reaction was efficiently promoted by the Cu3TATAT MOF catalyst in the absence of basic additives. The benzylidenemalononitrile product was produced in high yield and selectivity from an inexpensive benzyl alcohol starting material under an oxygen atmosphere. The role of the basic functionality was studied to demonstrate its role in the aerobic oxidation and Knoevenagel condensation reactions. The reaction progress was monitored in order to identify the reaction intermediate and follow the accumulation of the desired product. Lastly, results showed that the yield was not significantly compromised by the reuse of a batch of catalyst, even after more than five cycles.

Development of a novel Brønsted acid UiO-66 metal–organic framework catalyst by postsynthetic modification and its application in catalysis

A novel Brønsted acid UiO-66 metal–organic framework catalyst has been developed and utilized for efficient acetalization and MBH reaction.