Chiral Boron Complex-Promoted Asymmetric Diels–Alder Cycloaddition and Its Application in Natural Product Synthesis

Chiral-Boron Complex-Catalyzed Asymmetric [3 + 2] Cycloaddition of 2'-Hydroxychalcones with N-2,2,2-Trifluoroethylisatin Ketimines

A highly efficient asymmetric [3 + 2] cycloaddition reaction of 2'-hydroxychalcones with N-2,2,2-trifluoroethylisatin ketimines catalyzed by a (R)-3,3'-I2-BINOL-boron complex was developed. A broad range of 3,2'-pyrrolidinyl spirooxindole derivatives bearing a CF3-substituted pyrrolidine moiety with four contiguous stereocenters was 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 excellent stereocontrol.

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.

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.

Synthesis of Chalcones: An Improved High-Yield and Substituent-Independent Protocol for an Old Structure

Chalcones are a type of molecule that can be considered as easily synthesizable through aldol condensation or that can be readily purchased from habitual commercial vendors. However, on reviewing the literature, one realizes that there are no standard procedures for such aldol condensations, that there exists a wide range of alternative methods for the aldol condensation (indicating that such a condensation is not always simple), and that, in many cases, low yields are obtained that involve purifications by recrystallization or column chromatography. To develop a robust standard protocol independent of the nature of the substituents present on the acetophenone or the benzaldehyde involved in the aldol condensation leading to the chalcone, we made a comparison between an aldol condensation in KOH/EtOH and a Wittig reaction between the corresponding ylide and benzaldehyde in water. We describe an improved procedure for the Wittig reaction and a protocol for the elimination of the Ph3P=O byproduct (and the excess of ylide used) by filtration of the crude reaction product through a silica gel plug. We thus demonstrate that such an improved procedure can be a general method for the synthesis of chalcones in high yield and excellent purity and is clearly an improvement on the classical aldol condensation.

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.

Copper ferrite nanoparticles catalyzed the challenging Diels-Alder reaction of aromatic chalcones with cyclopentadiene

Copper ferrite nanoparticles efficiently transformed the unreactive aromatic chalcones into activated dienophiles for a Diels-Alder reaction with cyclopentadiene/isoprene as dienes. The best results in terms of rate of reaction and product yields were obtained in an eco-friendly solvent i.e. ethanol using 5 mol% catalytic loading. Substrate scope was also investigated for a number of chalcone derivatives, and all the reactions proceeded smoothly to provide the corresponding DA adducts in high yields (up to 89%) and good diastereoselectivities (up to >99%) with endo-preference.

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.

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.

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.

Chalcones, a Privileged Scaffold: Highly Versatile Molecules in [4+2] Cycloadditions

Chalcones are aromatic ketones found in nature as the central core of many biological compounds. They have a wide range of biological activity and are biogenetic precursors of other important molecules such as flavonoids. Their pharmacological relevance makes them a privileged scaffold, advantageous for seeking alternative therapies in medicinal chemistry. Due to their structural diversity and ease of synthesis, they are often employed as building blocks for chemical transformations. Chalcones have a carbonyl conjugated system with two electrophilic centers that are commonly used for nucleophilic additions, as described in numerous articles. They can also participate in Diels-Alder reactions, which are [4+2] cycloadditions between a diene and a dienophile. This microreview presents a chronological survey of studies on chalcones as dienes and dienophiles in Diels-Alder cycloadditions. Although these reactions occur in nature, isolation of chalcones from plants yields very small quantities. Contrarily, synthesis leads to large quantities at a low cost. Hence, novel methodologies have been developed for [4+2] cycloadditions, with chalcones serving as a 2π or 4π electron system.

Total synthesis of (-)-panduratin D

Herein, an enantioselective total synthesis of (-)-panduratin D, a novel secondary metabolite against human pancreatic PANC-1 cancer cell, from commercially available 3-methoxyphenol is reported. The synthesis was completed in nine steps and the key features include Sonogashira coupling, anionic Snieckus-Fries rearrangement, directed ortho metalation, tandem Si → C Alkyl rearrangement/Claisen-Schmidt condensation, and chiral boron complex-promoted asymmetric Diels-Alder cycloaddition. These endeavors could facilitate the biological studies of (-)-panduratin D and its analogs.

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.

Taxodisones A and B: bioactive C30-terpenes with new skeletons from Taxodium distichum and their biosynthetic origin

Herein, taxodisones A and B were isolated from the seeds of T. distichum, and the structures, configurations, bioactivities and origin of these compounds were determined.

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.

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.

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.

First enantioselective total synthesis of altersolanol A

The first enantioselective total synthesis of altersolanol A was accomplished by Diels–Alder cycloaddition promoted by (R)-3,3′-BINOL/boron Lewis acid.

Total Synthesis of (±)-Kuwanol E

The total synthesis of the Diels-Alder-type adducts (±)-kuwanol E and the heptamethyl ether derivative of (±)-kuwanon Y has been accomplished via a convergent strategy involving 2'-hydroxychalcone 6 or 9 and dehydroprenylstilbene 7, in nine steps. The synthesis features, as a key step, a Lewis acid-mediated biomimetic intermolecular Diels-Alder [4+2] cycloaddition for the construction of the cyclohexene skeleton with three stereogenic centers. Notably, the endo/exo diastereoselectivity of the reaction proved to be temperature-controlled.

Silanediol-Catalyzed Chromenone Functionalization

Promising levels of enantiocontrol are observed in the silanediol-catalyzed addition of silyl ketene acetals to benzopyrylium triflates. This rare example of enantioselective, intermolecular chromenone functionalization with carbonyl-containing nucleophiles has potential applications in the synthesis of bioactive chromanones and tetrahydroxanthones.

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

Metal-catalyzed, double Claisen rearrangement of a bis-allyloxyflavone has been utilized to enable a concise synthesis of the hydrobenzofuro[3,2-b]chromenone core structure of the natural products sanggenon A and sanggenol F. In addition, catalytic, enantioselective [4+2] cycloadditions of 2'-hydroxychalcones have been accomplished using B(OPh)3/BINOL complexes. Asymmetric syntheses of the flavonoid Diels-Alder natural products sanggenons C and O have been achieved employing a stereodivergent reaction of a racemic mixture (stereodivergent RRM) involving [4+2] cycloaddition.