Concise synthesis of chafurosides A and B

Friedel-Crafts reactions for biomolecular chemistry

Chemical tools and principles have become central to biological and medical research/applications by leveraging a range of classical organic chemistry reactions. Friedel-Crafts alkylation and acylation are arguably some of the most well-known and used synthetic methods for the preparation of small molecules but their use in biological and medical fields is relatively less frequent than the other reactions, possibly owing to the notion of their plausible incompatibility with biological systems. This review demonstrates advances in Friedel-Crafts alkylation and acylation reactions in a variety of biomolecular chemistry fields. With the discoveries and applications of numerous biomolecule-catalyzed or -assisted processes, these reactions have garnered considerable interest in biochemistry, enzymology, and biocatalysis. Despite the challenges of reactivity and selectivity of biomolecular reactions, the alkylation and acylation reactions demonstrated their utility for the construction and functionalization of all the four major biomolecules (i.e., nucleosides, carbohydrates/saccharides, lipids/fatty acids, and amino acids/peptides/proteins), and their diverse applications in biological, medical, and material fields are discussed. As the alkylation and acylation reactions are often fundamental educational components of organic chemistry courses, this review is intended for both experts and nonexperts by discussing their basic reaction patterns (with the depiction of each reaction mechanism in the ESI) and relevant real-world impacts in order to enrich chemical research and education. The significant growth of biomolecular Friedel-Crafts reactions described here is a testament to their broad importance and utility, and further development and investigations of the reactions will surely be the focus in the organic biomolecular chemistry fields.

Triple Role of Proton Sponge (DMAN) in the Palladium-Catalyzed Direct Stereoselective Synthesis of C-Aryl Glycosides from Glycals

The triple role of 1,8-bis(dimethylamino)naphthalene (proton sponge) as a reductant, ligand precursor, and organic base in the palladium-catalyzed Heck-type coupling reaction of glycals with aryl iodides affords the rapid and stereoselective synthesis of 2',3'-unsaturated α-C-aryl glycosides in excellent yields. The role of the proton sponge in reducing palladium(II) to (0) has been studied using cyclic voltammetry, UV-vis, HRMS, and other spectroscopic techniques. This is the first example of a palladium proton sponge complex utilized in coupling reactions. The method is observed to be tolerant of various functional groups, as demonstrated by the huge substrate scope. Moreover, the 2',3'-unsaturated α-C-aryl glycosides were also converted to 3-keto-β-C-glycosides under sterically hindered pyridinium salt catalysis via a ring-opening and -closing mechanism.

Genome-wide identification of UDP-glycosyltransferases in the tea plant (Camellia sinensis) and their biochemical and physiological functions

Tea (Camellia sinensis) has been an immensely important commercially grown crop for decades. This is due to the presence of essential nutrients and plant secondary metabolites that exhibit beneficial health effects. UDP-glycosyltransferases (UGTs) play an important role in the diversity of such secondary metabolites by catalysing the transfer of an activated sugar donor to acceptor molecules, and thereby creating a huge variety of glycoconjugates. Only in recent years, thanks to the sequencing of the tea plant genome, have there been increased efforts to characterise the UGTs in C. sinensis to gain an understanding of their physiological role and biotechnological potential. Based on the conserved plant secondary product glycosyltransferase (PSPG) motif and the catalytically active histidine in the active site, UGTs of family 1 in C. sinensis are identified here, and shown to cluster into 21 groups in a phylogenetic tree. Building on this, our current understanding of recently characterised C. sinensis UGTs (CsUGTs) is highlighted and a discussion on future perspectives made.

Recent Advances on Natural Aryl-C-glycoside Scaffolds: Structure, Bioactivities, and Synthesis-A Comprehensive Review

Aryl-C-glycosides, of both synthetic and natural origin, are of great significance in medicinal chemistry owing to their unique structures and stability towards enzymatic and chemical hydrolysis as compared to O-glycosides. They are well-known antibiotics and potent enzyme inhibitors and possess a wide range of biological activities such as anticancer, antioxidant, antiviral, hypoglycemic effects, and so on. Currently, a number of aryl-C-glycoside drugs are on sale for the treatment of diabetes and related complications. This review summarizes the findings on aryl-C-glycoside scaffolds over the past 20 years, concerning new structures (over 200 molecules), their bioactivities-including anticancer, anti-inflammatory, antioxidant, antivirus, glycation inhibitory activities and other pharmacological effects-as well as their synthesis.

Ni-catalyzed domino transformation of enopyranoses and 2-iodo phenols/anilines to pyrano cis fused dihydro-benzofurans/indoles

A Ni-catalyzed direct access to various pyrano cis-fused dihydro benzofurans and indoles from unsaturated enopyranoses and o-iodo phenols/anilines is developed. The domino synthesis of pyrano C2-C1 and C3-C2 cis-fused heteroarynes were achieved both from glycals and pseudo glycals in which heteroatoms are linked at C2 and C3 positions, respectively, with excellent chemo-selectivity.

Facile preparation and conversion of 4,4,4-trifluorobut-2-yn-1-ones to aromatic and heteroaromatic compounds

The concise preparation of 4,4,4-trifluorobut-2-yn-1-ones by the oxidation of the readily accessible corresponding propargylic alcohols as well as their utilization as Michael acceptors for the construction of aromatic and heteroaromatic compounds are reported.

Improved preparation of vitexin from hot water extract of Basella alba, the commercially available vegetable Malabar spinach ("Tsurumurasaki" in Japanese) and the application to semisynthesis of chafuroside B

Hot water extraction of D-arabinofuranosylvitexin from the raw leaves of commercially available Basella alba "Tsurumurasaki" and subsequent acidic hydrolysis was improved to be a procedure using a high-pressure steam sterilizer to afford vitexin. The amount was estimated to be 14.1 mg from 1 g of dry weight of the raw leaves, whose recovery was calculated to be 95% based on the estimated content of D-arabinofuranosylvitexin in B. alba raw leaves. The product was dehydratively cyclized between hydroxy groups on the carbohydrate and flavone skeletons under modified Mitsunobu reaction conditions in N,N-dimethylformamide to give chafuroside B, which is known to be a bioactive Oolong tea polyphenol. Through these transformations, 10.2 mg of chafuroside B could be semisynthesized from 1 g of dry weight of the raw leaves, and the efficiency was improved compared to that from the extraction from Oolong tea (3.4 μg from 1 g of dry weight).

Versatility of glycals in synthetic organic chemistry: coupling reactions, diversity oriented synthesis and natural product synthesis

Glycals, 1,2-unsaturated sugar derivatives, are versatile starting materials for the synthesis of natural products and the generation of novel structural features in Diversity Oriented Synthesis (DOS). The versatility of glycals in synthesis emanates, among others, from the presence of the ring oxygen and the enol-ether type unsaturation, the different types of stable conformations they can adopt depending on the nature of the protecting groups present and the ease with which the protecting groups of the three hydroxy groups could be tailored to suite for a desired manipulation. This review summarizes the literature on the different transformations of the endo glycals into biologically relevant compounds such as chromans, thiochromans, chromenes, thiochromenes, peptidomimetics, bridged benzopyrans etc., as well as on the use of glycals as chiral building blocks for the synthesis of various natural products such as aspicilin, reblastatin, diospongins, decytospolides, osmundalactones, paclitaxel, isatisine, d-fagomine, and spliceostatin, reported post 2014.

Synthesis of bicyclic ethers by a palladium-catalyzed oxidative cyclization-redox relay-π-allyl-Pd cyclization cascade reaction

Bicyclic ether scaffolds are found in a variety of natural products and are of interest in probe and drug discovery. A palladium-catalyzed cascade reaction has been developed to provide efficient access to these scaffolds from readily available linear diene-diol substrates. A Pd redox-relay process is used strategically to transmit reactivity between an initial oxypalladative cyclization and a subsequent π-allyl-Pd cyclization at remote sites. The reaction affords a variety of bicyclic ether scaffolds with complete diastereoselectivity for cis-ring fusion.

Synthesis of trilobatin from naringin via prunin as the key intermediate: acidic hydrolysis of the α-rhamnosidic linkage in naringin under improved conditions

Trilobatin [4'-(β-D-glucopyranosyloxy)-2',4",6'-trihydroxydihydrochalcone] was synthesized from commercially available naringin in three steps with an overall yield of 30%. The key step was the acid-catalyzed site-selective hydrolysis of terminal α-rhamnopyranosidic linkage in neohesperidose involved in naringin under controlled conditions, by applying a high-pressure steam sterilizer.

Mangiferin: a natural miracle bioactive compound against lifestyle related disorders

The current review article is an attempt to explain the therapeutic potential of mangiferin, a bioactive compound of the mango, against lifestyle-related disorders. Mangiferin (2-β-D-glucopyranosyl-1,3,6,7-tetrahydroxy-9H-xanthen-9-one) can be isolated from higher plants as well as the mango fruit and their byproducts (i.e. peel, seed, and kernel). It possesses several health endorsing properties such as antioxidant, antimicrobial, antidiabetic, antiallergic, anticancer, hypocholesterolemic, and immunomodulatory. It suppresses the activation of peroxisome proliferator activated receptor isoforms by changing the transcription process. Mangiferin protects against different human cancers, including lung, colon, breast, and neuronal cancers, through the suppression of tumor necrosis factor α expression, inducible nitric oxide synthase potential, and proliferation and induction of apoptosis. It also protects against neural and breast cancers by suppressing the expression of matrix metalloproteinase (MMP)-9 and MMP-7 and inhibiting enzymatic activity, metastatic potential, and activation of the β-catenin pathway. It has the capacity to block lipid peroxidation, in order to provide a shielding effect against physiological threats. Additionally, mangiferin enhances the capacity of the monocyte-macrophage system and possesses antibacterial activity against gram-positive and gram-negative bacteria. This review summarizes the literature pertaining to mangiferin and its associated health claims.

C-Glycopyranosyl Arenes and Hetarenes: Synthetic Methods and Bioactivity Focused on Antidiabetic Potential

This Review summarizes close to 500 primary publications and surveys published since 2000 about the syntheses and diverse bioactivities of C-glycopyranosyl (het)arenes. A classification of the preparative routes to these synthetic targets according to methodologies and compound categories is provided. Several of these compounds, regardless of their natural or synthetic origin, display antidiabetic properties due to enzyme inhibition (glycogen phosphorylase, protein tyrosine phosphatase 1B) or by inhibiting renal sodium-dependent glucose cotransporter 2 (SGLT2). The latter class of synthetic inhibitors, very recently approved as antihyperglycemic drugs, opens new perspectives in the pharmacological treatment of type 2 diabetes. Various compounds with the C-glycopyranosyl (het)arene motif were subjected to biological studies displaying among others antioxidant, antiviral, antibiotic, antiadhesive, cytotoxic, and glycoenzyme inhibitory effects.

Synthesis of Effective Food Constituents toward the Development of Chemical Biology Investigations

This article describes the development of various probes and immunogens for chemical-biological investigations of food flavonoids. We accomplished a large (gram)-scale asymmetric synthesis of a key intermediate, 5-aminopentyl deoxy epigallocatechin-3-gallate (APDOEGCg; 3), an analogue of green tea polyphenol EGCg, in which the key step was cationic cyclization utilizing neighboring group participation of the gallate carbonyl group. The synthetic APDOEGCg (3) was efficiently converted to a fluorescent probe 18 and an immunogen 19 by utilizing the high reactivity of the amine functional group. We confirmed the usefulness of these probes for imaging studies and the generation of antibodies, respectively. We also describe the efficient synthesis of a positron emission tomography (PET) probe [¹¹C]20 by incorporation of ¹¹C into EGCg (1), for which synthetic 4″-Me-EGCg (20) was utilized as an authentic sample. Our synthetic strategy was also applied for the practical synthesis of nobiletin (21), a polymethoxylated flavone from citrus. Synthetic nobiletin was readily converted to various probes by selective demethylation and incorporation of fluorescein, biotin or ¹¹C. These probes should be useful for a range of biological applications. Detailed examination of the mechanisms and further applications are in progress.

Biosynthesis of natural and novel C-glycosylflavones utilising recombinant Oryza sativa C-glycosyltransferase (OsCGT) and Desmodium incanum root proteins

The rice C-glycosyltransferase (OsCGT) is one of only a small number of characterised plant C-glycosyltransferases (CGT) known. The enzyme C-glucosylates a 2-hydroxyflavanone substrate with UDP-glucose as the sugar donor to produce C-glucosyl-2-hydroxyflavanones. We tested substrate specificity of the enzyme, using synthetic 2-hydroxyflavanones, and showed it has the potential to generate known natural CGFs that have been isolated from rice and also other plants. In addition, we synthesised novel, unnatural 2-hydroxyflavanone substrates to test the B-ring chemical space of substrate accepted by the OsCGT and demonstrated the OsCGT capacity as a synthetic reagent to generate significant quantities of known and novel CGFs. Many B-ring analogues are tolerated within a confined steric limit. Finally the OsCGT was used to generate novel mono-C-glucosyl-2-hydroxyflavanones as putative biosynthetic intermediates to examine the potential of Desmodium incanum biosynthetic CGTs to produce novel di-C-glycosylflavones, compounds implicated in the allelopathic biological activity of Desmodium against parasitic weeds from the Striga genus.

Stereoselective construction of 2-vinyl 3-hydroxybenzopyran rings: total syntheses of teadenols A and B

Total syntheses of teadenols A and B, isolated from fermented tea, were accomplished in a highly stereocontrolled manner.

Diastereoselective and regiodivergent oxa-[3 + 2] cycloaddition of Achmatowicz products and cyclic 1,3-dicarbonyl compounds

Development of two novel oxa-[3 + 2] cycloaddition reactions of Achmatowicz products with 1,3-dicarbonyl compounds for rapid and highly efficient assembly of polycyclic furopyranones is described. Plausible mechanisms were proposed.

Total synthesis of mangiferin, homomangiferin, and neomangiferin

Efficient synthesis of natural C-glycosyl xanthone mangiferin, homomangiferin and neomangiferin based on their biosynthetic pathway.

Practical synthesis of natural plant-growth regulator 2-azahypoxanthine, its derivatives, and biotin-labeled probes

AHX derivatives and their probe molecules.

A concise synthesis of 3-aroylflavones via Lewis base 9-azajulolidine-catalyzed tandem acyl transfer-cyclization

Lewis base-catalyzed tandem acyl transfer-cyclization of acylated o-alkynoylphenols leading to 3-aroylflavones was developed. 9-Azajulolidine smoothly promoted the reaction of the aroyl derivatives at ambient temperature, and the structure-diversed synthesis of 3-aroylflavones with distinct substituents was achieved in moderate to excellent yields.

3,3',4',5,5'-Pentahydroxyflavone is a potent inhibitor of amyloid β fibril formation

The natural flavonoid fisetin (3,3',4',7-tetrahydroxyflavone) is neurotrophic and prevents fibril formation of amyloid β protein (Aβ). It is a promising lead compound for the development of therapeutic drugs for Alzheimer's disease. To find even more effective drugs based on the structure of fisetin, we synthesized a series of fisetin analogues lacking the 7-hydroxyl group and compared their effects on Aβ fibril formation determined by the thioflavin T fluorescence assay. 3,3',4'-Trihydroxyflavone and 3',4'-dihydroxyflavone inhibited Aβ fibril formation more potently than fisetin or 3',4',7-trihydroxyflavone, suggesting that the 7-hydroxy group is not necessary for anti-amyloidogenic activity. 3,3',4',5'-Tetrahydroxyflavone and 3',4',5'-trihydroxyflavone inhibited Aβ fibril formation far more potently than 3,3',4'-trihydroxyflavone and 3',4'-dihydroxyflavone, suggesting that 3',4',5'-trihydroxyl group of the B ring is crucial for the anti-amyloidogenic activity of flavonoids. Based on the structure-activity relationship, we synthesized 3,3',4',5,5'-pentahydroxyflavone, and confirmed that this compound is the most potent inhibitor of Aβ fibril formation among fisetin analogues that have been tested. Cytotoxicity assay using rat hippocampal neuron cultures demonstrated that Aβ preincubated with 3,3',4',5,5'-pentahydroxyflavone was significantly less toxic than Aβ preincubated with vehicle. 3,3',4',5,5'-Pentahydroxyflavone could be a new therapeutic drug candidate for the treatment of Alzheimer's disease.