Modular Synthesis of Nona-Decasaccharide Motif from Psidium guajava Polysaccharides: Orthogonal One-Pot Glycosylation Strategy

Total Synthesis of the Tridecasaccharide Motif from Angelica Sinensis APS-1 II Polysaccharide with Anti-Leukemia Activity and Structure-Activity Relationship Studies

A polysaccharide APS-1 II from a medicinal plant Angelica sinensis (Oliv.) Diels represents a potential therapeutic agent against leukemia. However, the synthetic accessibility of the highly branched and complex APS-1 II polysaccharide with multiple 1, 2-cis-glycosidic linkages remains a difficult task, impeding the in-depth structure-activity relationship biological studies and the development of carbohydrates-based therapeutics against leukemia. Here, we report the first chemical synthesis of tridecasaccharide repeating unit together with shorter sequences 4-mer, 6-mer and 9-mer from APS-1 II polysaccharide via one-pot orthogonal glycosylation strategy based on glycosyl ortho-(1-phenylvinyl)benzoates, which precluded the potential issues such as aglycone transfer associated with one-pot assembly with thioglycosides. The synthetic pathway also features the following aspects: 1) three contiguous and challenging 1, 2-cis-Fuc bonds were highly stereoselectively constructed via the newly developed stereoselective 1, 2-cis-fucosylation method; 2) several 1, 2-trans-glycosidic linkages were formed via neighboring group participation effect, while 1,2-cis-Glc linkage was stereoselectively assembled via N,N-dimethylformamide reagent modulation; 3) the final [1+1+2+9] one-pot assembly of the target tridecasaccharide via strategic utilizations of glycosyl N-phenyltrifluoroacetimidates, ortho-alkynylbenzoates and ortho-(1-phenylvinyl)benzoates. Biological studies revealed that human leukemia K562 and mouse L1210 cells could be effectively inhibited by tridecasaccharide repeating unit and substructure nonasaccharide.

Precision Synthesis and Antiliver Fibrosis Activity of a Highly Branched Acidic 63-Mer Pectin Polysaccharide

Natural polysaccharides possess various biological functions and have become increasingly important as drug candidates for biomedical development. However, the accessibility to multiple-branched and large-sized acidic polysaccharides with well-defined structures and the identification of related active glycan domains remain challenging. Here, we report the precision synthesis of a highly branched acidic pectin polysaccharide up to a 63-mer containing 10 different glycosidic linkages from Lycium barbarum. The synthetic strategy relies on highly stereoselective modular assembly of an orthogonally protected decasaccharide backbone, efficient synthesis of three side chain glycans by the integration of stereocontrolled one-pot chemoselective glycosylations and a hydrogen-bond-mediated aglycone delivery approach, and convergent assembly of the target polysaccharide in a branched site-specific glycosylation manner via flexible orthogonal protecting group manipulations. Structure-activity relationship studies of synthetic polysaccharide 63-mer and its short fragments (9-mer, 10-mer, 11-mer, and 33-mer) suggest that the decasaccharide as an active glycan domain exhibits better antiliver fibrosis activity.

Total Synthesis of Nona-decasaccharide Motif from Ganoderma sinense Polysaccharide Enabled by Modular and One-Pot Stereoselective Glycosylation Strategy

Polysaccharides from a medicinal fungus Ganoderma sinense represent important and adjunctive therapeutic agents for treating various diseases, including leucopenia and hematopoietic injury. However, the synthetic accessibility to long, branched, and complicated carbohydrates chains from Ganoderma sinense polysaccharides remains a challenging task in chemical synthesis. Here, we report the modular chemical synthesis of nona-decasaccharide motif from Ganoderma sinense polysaccharide GSPB70-S with diverse biological activities for the first time through one-pot stereoselective glycosylation strategy on the basis of glycosyl ortho-(1-phenyvinyl)benzoates, which not only sped up carbohydrates synthesis but also reduced chemical waste and avoided aglycones transfer issues inherent to one-pot glycosylation on the basis of thioglycosides. The synthetic route also highlights the following key steps: (1) preactivation-based one-pot glycosylation for highly stereoselective constructions of several 1,2-cis-glycosidic linkages, including three α-d-GlcN-(1 → 4) linkages and one α-d-Gal-(1 → 4) bond via the reagent N-methyl-N-phenylformamide modulation; (2) orthogonal one-pot assembly of 1,2-trans-glycosidic linkages in various linear and branched glycans fragments by strategic combinations of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl ortho-(1-phenyvinyl)benzoates; and (3) the final [1 × 4 + 15] Yu glycosylation for efficient assembly of nona-decasaccharide target. Additionally, shorter sequences of 4-mer, 5-mer, and 6-mer are also prepared for structure-activity relationship biological studies. The present work shows that this one-pot stereoselective glycosylation strategy can offer a reliable and effective means to streamline chemical synthesis of long, branched, and complex carbohydrates with many 1,2-cis-glycosidic bonds.

Highly stereoselective α-glycosylation with GalN3 donors enabled collective synthesis of mucin-related tumor associated carbohydrate antigens

Mucin-related tumor-associated carbohydrate antigens (TACAs) are important and interesting targets for cancer vaccine therapy. However, efficient access to a library of mucin-related TACAs remains a challenging task. One of the key issues is the challenging construction of α-GalNAc linkages. Here, we report highly stereoselective α-glycosylation with GalN3N-phenyl trifluoroacetimidate donors, which features excellent yields, outstanding stereoselectivities, broad substrate scope and mild reaction conditions. This method is successfully applied to highly stereoselective synthesis of GalN3-α-O-Ser, which served as the common intermediate for collective synthesis of a wide range of TACAs including TN antigen, STN antigen, 2,6 STF antigen, 2,3 STF antigen, glycophorin and cores 1-8 mucin-type O-glycans. In particular, the rationale for this highly stereoselective α-glycosylation is provided for the first time using DFT calculations and mechanistic studies, highlighting the crucial roles of reagent combinations (TMSI and Ph3PO) and the H-bonding directing effect of the N3 group.

One-Pot Assembly of Mannose-Capped Lipoarabinomannan Motifs up to 101-Mer from the Mycobacterium tuberculosis Cell Wall

Lipoarabinomannan (LAM) from the Mycobacterium tuberculosis cell envelope represents important targets for the development of new therapeutic agents against tuberculosis, which is a deadly disease that has plagued mankind for a long time. However, the accessibility of long, branched, and complex lipoarabinomannan over 100-mer remains a long-standing challenge. Herein, we report the modular synthesis of mannose-capped lipoarabinomannan 101-mer from the M. tuberculosis cell wall using a one-pot assembly strategy on the basis of glycosyl ortho-(1-phenylvinyl)benzoates (PVB), which not only accelerates the modular synthesis but also precludes the potential problems associated with one-pot glycosylation with thioglycosides. Shorter sequences including 18-mer, 19-mer, and 27-mer are also synthesized for in-depth structure-activity relationship biological studies. Current synthetic routes also highlight the following features: (1) streamlined synthesis of various linear and branched glycans using one-pot orthogonal glycosylation on the combination of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl PVB; (2) highly stereoselective construction of 10 1,2-cis-arabinofuranosyl linkages using 5-O-(2-quinolinecarbonyl)-directing 1,2-cis-arabinofuranosylation via a hydrogen-bond-mediated aglycone delivery strategy; and (3) convergent [(18 + 19) × 2 + 27] one-pot synthesis of the 101-mer LAM polysaccharide. The present work demonstrates that this orthogonal one-pot glycosylation strategy can highly streamline the chemical synthesis of long, branched, and complex polysaccharides.

Synthesis of Core M3 Matriglycan Constituents via an Additive-Controlled 1,2-cis-Xylopyranosylation with O-Xylosyl Imidates as Donors

A highly stereoselective strategy for 1,2-cis-xylopyranoside bond formation was established via a preactivation-based, additive-modulated trichloroacetimidate glycosidation strategy. The current protocol is mild, practical, and successful with various xylopyranosyl donors and glycosyl acceptors, including acceptors that are reported to be less reactive due to steric hindrance. The utility of this method was demonstrated with the facile assembly of matriglycan constituent tetra- and hexasaccharides.

Recent Advances in Chemical Synthesis of Structural Domains of Lipopolysaccharides from the Commensal Gut-Associated Microbiota

Lipopolysaccharides from the commensal gut-associated microbiota are interesting biomolecules for the treatment of various inflammatory diseases. Different from pathogenic lipopolysaccharides, commensal lipopolysaccharides have distinct chemical structures and mediate beneficial homeostasis with the immune system of the host. However, the accessibility issues of homogenous and pure commensal lipopolysaccharides hampered the in-depth studies of their functions. In this concept article, we highlight the recent synthesis of lipopolysaccharides from gut-associated lymphoid-tissue-resident Alcaligenes faecalis and Bacteroides vulgatus, which hopes to inspire the more efforts devoting to these fantastic biomolecules.

Recent chemical synthesis of plant polysaccharides

Here, chemical syntheses of long, branched and complex glycans over 10-mer from plants are summarized, which highlights amylopectin 20-mer from starch, 17-mer from carthamus tinctorius, α-glucan 30-mer from Longan, 19-mer from psidium guajava and 11-mer from dendrobium huoshanense. The glycans assembly strategies, protecting groups utilization and glycosylation methods discussed here will inspire the efficient synthesis of diverse complex glycans with many 1,2-cis glycosidic linkages.

Chemical approaches for the stereocontrolled synthesis of 1,2-cis-β-D-rhamnosides

In carbohydrate chemistry, the stereoselective synthesis of 1,2-cis-glycosides remains a formidable challenge. This complexity is comparable to the synthesis of 1,2-cis-β-D-mannosides, primarily due to the adverse anomeric and Δ-2 effects. Over the past decades, to attain β-stereoselectivity in D-rhamnosylation, researchers have devised numerous direct and indirect methodologies, including the hydrogen-bond-mediated aglycone delivery (HAD) method, the synthesis of β-D-mannoside paired with C6 deoxygenation, and the combined approach of 1,2-trans-glycosylation and C2 epimerization. This review elaborates on the advancements in β-D-rhamnosylation and its implications for the total synthesis of tiacumicin B and other physiologically relevant glycans.

One-Pot Assembly of the Highly Branched Tetradecasaccharide from Ganoderma lucidum Glycan GLSWA-1 with Immune-Enhancing Activities

The highly branched tetradecasaccharide repeating unit and shorter sequences of GLSWA-1 with immune-enhancing activities from Ganoderma lucidum have been prepared via a one-pot glycan assembly strategy. The synthetic route features (1) orthogonal one-pot glycosylation on the basis of PVB glycosylation to streamline glycan synthesis avoiding such issues as aglycone transfer, (2) one-pot assembly of oligosaccharides with up to four different glycosyl linkages, and (3) modular and convergent [4+5+5] one-pot assembly of the highly branched tetradecasaccharide.

Combination of 3-O-Levulinoyl and 6-O-Trifluorobenzoyl Groups Ensures α-Selectivity in Glucosylations: Synthesis of the Oligosaccharides Related to Aspergillus fumigatus α-(1 → 3)-d-Glucan

Stereospecific α-glucosylation of primary and secondary OH-group at carbohydrate acceptors is achieved using glucosyl N-phenyl-trifluoroacetimidate (PTFAI) donor protected with an electron-withdrawing 2,4,5-trifluorobenzoyl (TFB) group at O-6 and the participating levulinoyl (Lev) group at O-3. New factors have been revealed that might explain α-stereoselectivity in the case of TFB and pentafluorobenzoyl (PFB) groups at O-6. They are of conformational nature and confirmed by DFT calculations. The potential of this donor, as well as the orthogonality of TFB and Lev protecting groups, is showcased by the synthesis of α-(1 → 3)-linked pentaglucoside corresponding to Aspergillus fumigatus α-(1 → 3)-d-glucan and of its hexasaccharide derivative, bearing β-glucosamine residue at the non-reducing end.

Preparation, structural characterization, and bioactivities of polysaccharides from Psidium guajava: A review

Psidium guajava L. is one of the most pivotal members belong to the Myrtaceae family, and it is an important tropical fruit with highly nutritional, healthy, and pharmacological values prevailing in worldwide for decades. The polysaccharides of P. guajava (PGPs) are served as one of the most active constituents, which possess a variety of biofunctionalities including anti-inflammatory, antidiarrheic, antihypertension, and antidiabetic properties. Hence, a systematic review aimed to comprehensively summarize the recent research advances of PGPs is necessary for facilitating their better understanding. The present review discussed current research progress on the PGPs, including extraction and purification methods, structural features, biological activities, and potential pharmacological mechanism. In addition, this review may also provide some valuable insights for further development and potential value in affording functionally useful agents in food industry or therapeutically effective medicine in the fields of P. guajava polysaccharides.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

The total synthesis of rhynchosporosides via orthogonal one-pot glycosylation and stereoselective α-glycosylation strategies

The efficient and collective synthesis of rhynchosporosides causing scald diseases has been achieved, which features orthogonal one-pot glycosylation on the basis of PTFAI glycosylation, Yu glycosylation, and PVB glycosylation and merging reagent modulation and remote anchimeric assistance (RMRAA) α-glucosylation strategies. The issues inherent to the thioglycoside-based orthogonal one-pot glycosylation strategy, such as aglycone transfer, have been precluded by this orthogonal one-pot glycosylation strategy, which can streamline glycan chemical synthesis.

Convergent Synthesis and Anti-Pancreatic Cancer Cell Growth Activity of a Highly Branched Heptadecasaccharide from Carthamus tinctorius

Bioactive polysaccharides from natural resources target various biological processes and are increasingly used as potential target molecules for drug development. However, the accessibility of branched and long complex polysaccharide active domains with well-defined structures remains a major challenge. Herein we describe an efficient first total synthesis of a highly branched heptadecasaccharide moiety of the native bioactive galectin-3-targeting polysaccharide from Carthamus tinctorius L. as well as shorter fragments of the heptadecasaccharide. The key feature of the approach is that a photo-assisted convergent [6+4+7] one-pot coupling strategy enables rapid assembly of the heptadecasaccharide, whereby a photoremovable o-nitrobenzyl protecting group is used to generate the corresponding acceptor for glycosylation in situ upon ultraviolet radiation. Biological activity tests suggest that the heptadecasaccharide can target galectin-3 and inhibit pancreatic cancer cell growth.

Total synthesis of Lentinus giganteus glycans with antitumor activities via stereoselective α-glycosylation and orthogonal one-pot glycosylation strategies

The accessibility to long, branched and complex glycans containing many 1,2-cis glycosidic linkages with precise structures remains a challenging task in chemical synthesis. Reported here is an efficient, stereoselective and orthogonal one-pot synthesis of a tetradecasaccharide and shorter sequences from Lentinus giganteus polysaccharides with antitumor activities. The synthetic strategy consists of: (1) newly developed merging reagent modulation and remote anchimeric assistance (RMRAA) α-(1→6)-galactosylation in a highly stereoselective manner, (2) DMF-modulated stereoselective α-(1→3)-glucosylation, (3) RMRAA stereoselective α-(1→6)-glucosylation, (4) several orthogonal one-pot glycosylations on the basis of N-phenyltrifluoroacetimidate (PTFAI) glycosylation, Yu glycosylation and ortho-(1-phenylvinyl)benzoate (PVB) glycosylation to streamline oligosaccharide synthesis, and (5) convergent [7 + 7] glycosylation for the final assembly of the target tetradecasaccharide. In particular, this new RMRAA α-galactosylation method has mild reaction conditions, broad substrate scopes and significantly shortened step counts for the heptasaccharide synthesis in comparison with 4,6-di-tert-butylsilyene (DTBS) directed α-galactosylation. Furthermore, DFT calculations shed light on the origins of remote anchimeric assistance effects (3,4-OBz > 3,4-OAc > 4-OBz > 3-OBz) of acyl groups.

Stereoselective gold(I)-catalyzed approach to the synthesis of complex α-glycosyl phosphosaccharides

Glycosyl phosphosaccharides represent a large and important family of complex glycans. Due to the distinct nature of these complex molecules, efficient approaches to access glycosyl phosphosaccharides are still in great demand. Here, we disclose a highly efficient and stereoselective approach to the synthesis of biologically important and complex α-glycosyl phosphosaccharides, employing direct gold(I)-catalyzed glycosylation of the weakly nucleophilic phosphoric acid acceptors. In this work, the broad substrate scope is demonstrated with more than 45 examples, including glucose, xylose, glucuronate, galactose, mannose, rhamnose, fucose, 2-N3-2-deoxymannose, 2-N3-2-deoxyglucose, 2-N3-2-deoxygalactose and unnatural carbohydrates. Here, we show the glycosyl phosphotriester prepared herein was successfully applied to the one-pot synthesis of a phosphosaccharide from Leishmania donovani, and an effective preparation of a trisaccharide diphosphate of phosphosaccharide fragments from Hansenula capsulate via iterative elongation strategy is realized.

Hypoglycemic and Hepatoprotective Effects of Dried and Rice-Fried Psidium guajava Leaves in Diabetic Rats

Psidium guajava leaves (PGL) have been long used as an adjuvant therapy for diabetics. The present study evaluated the in vivo hypoglycemic and hepatoprotective effects of dried and the rice-fried PGL decoctions (PGLD and RPGLD). Our results indicated that both PGLD and RPGLD could significantly decrease the contents of fasting blood glucose (FBG) and haemoglobin A1c (HbA1c) in diabetic rats. Compared with the HFD/STZ (high-fat diet with streptozotocin) group, the PGLD and RPGLD-treated diabetic rats showed different degrees of recovery against the liver pathological changes. The upregulated expressions of glucokinase (GK), glucose transporter 2 (GLUT2), insulin growth factor-1 (IGF-1), insulin receptor substrate-1 (IRS-1), and insulin receptor substrate-2 (IRS-2) in PGLD and RPGLD-treated groups were observed. In general, RPGLD exhibited a much better antidiabetic effect than PGLD, which was further verified by the comprehensive evaluation with the TOPSIS method. Besides, HPLC (high-performance liquid chromatography) and UPLC-MS/MS (ultra-performance liquid chromatography-tandem mass spectrometry) analyses revealed that the contents of the primary constituents (ellagic acid, hyperoside, isoquercitroside, reynoutrin, guaijaverin, auicularin, and quercetin) in RPGLD increased obviously compared with PGLD. These results shed new light on the antidiabetic potential and mechanism of PGL, as well as the "higher efficacy" of the rice-fried processing method in traditional Chinese medicine.

A One-Pot Synthesis of Glycans and Nucleosides Based on ortho-(1-Phenylvinyl)benzyl Glycosides

One-pot synthesis of both glycans and nucleosides remains rare and challenging. Herein, we report a one-pot glycosylation strategy for glycans and nucleosides synthesis based on ortho-(1-phenylvinyl)benzyl glycosides, which has several advantages, including no aglycon transfers, no undesired interference of departing species, no unpleasant odor, and up to the construction of four different glycosidic linkages.

Merging Reagent Modulation and Remote Anchimeric Assistance for Glycosylation: Highly Stereoselective Synthesis of α-Glycans up to a 30-mer

The efficient synthesis of long, branched, and complex carbohydrates containing multiple 1,2-cis glycosidic linkages is a long-standing challenge. Here, we report a merging reagent modulation and 6-O-levulinoyl remote anchimeric assistance glycosylation strategy, which is successfully applied to the first highly stereoselective synthesis of the branched Dendrobium Huoshanense glycans and the linear Longan glycans containing up to 30 contiguous 1,2-cis glucosidic bonds. DFT calculations shed light on the origin of the much higher stereoselectivities of 1,2-cis glucosylation with 6-O-levulinoyl group than 6-O-acetyl or 6-O-benzoyl groups. Orthogonal one-pot glycosylation strategy based on glycosyl ortho-alkynylbenzoates and ortho-(1-phenylvinyl)benzoates has been demonstrated in the efficient synthesis of complex glycans, precluding such issues as aglycon transfer inherent to orthogonal one-pot synthesis based on thioglycosides.

An orthogonal and reactivity-based one-pot glycosylation strategy for both glycan and nucleoside synthesis: access to TMG-chitotriomycin, lipochitooligosaccharides and capuramycin

Both glycans (O-glycosides) and nucleosides (N-glycosides) play important roles in numerous biological processes. Chemical synthesis is a reliable and effective means to solve the attainability issues of these essential biomolecules. However, due to the stereo- and regiochemical issues during glycan assembly, together with problems including the poor solubility and nucleophilicity of nucleobases in nucleoside synthesis, the development of one-pot glycosylation strategies toward efficient synthesis of both glycans and nucleosides remains poor and challenging. Here, we report the first orthogonal and reactivity-based one-pot glycosylation strategy suitable for both glycan and nucleoside synthesis on the basis of glycosyl ortho-(1-phenylvinyl)benzoates. This one-pot glycosylation strategy not only inherits the advantages including no aglycon transfers, no undesired interference of departing species, and no unpleasant odors associated with the previously developed orthogonal one-pot glycosylation strategy based on glycosyl ortho-alkynylbenzoates, but also highly expands the scope (glycans and nucleosides) and increases the number of leaving groups that could be employed for the multistep one-pot synthesis (up to the formation of four different glycosidic bonds). In particular, the current one-pot glycosylation strategy is successfully applied to the total synthesis of a promising tuberculosis drug lead capuramycin and the divergent and formal synthesis of TMG-chitotriomycin with potent and specific inhibition activities toward β-N-acetylglucosaminidases and important endosymbiotic lipochitooligosaccharides including the Nod factor and the Myc factor, which represents one of the most efficient and straightforward synthetic routes toward these biologically salient molecules.