One-Pot Relay Glycosylation

Glycosylation with sulfoxide-based glycosyl donors

Sulfoxides have emerged as pivotal constituents in modern carbohydrate chemistry. As anomeric leaving groups, sulfinyl moieties may occupy positions directly at the anomeric position or at a more remote site. This feature article is focused on the evolution and notable advancements of glycosyl sulfoxide donors in glycosylation reactions. Its objective is to elucidate the obstacles and prospects within this evolving research domain, with the aim of enhancing comprehension and progress in the field of carbohydrate chemistry.

Glycosylation of 2-(2-Propylsulfinyl)benzyl 1,2-Orthoester Glycosides Initiated by Sulfoxide Activation

We have developed a highly effective glycosylation method that involves the activation of 2-(2-propylsulfinyl)benzyl 1,2-orthoester glycosides using triflic anhydride (Tf2O). Our research indicates that half of the glycosyl donor is activated through Tf2O via an interrupted Pummerer reaction mechanism, while the remaining portion is activated by triflic acid (TfOH) generated in situ. As a result, as little as 0.5 equiv of Tf2O is adequate for activating the orthoester glycoside donors. This glycosylation procedure offers several benefits, such as high efficiency, wide applicability, and the utilization of a recyclable leaving group.

SFox imidates as versatile glycosyl donors for chemical glycosylation

Described herein is a continuation of our studies dedicated to the development of novel classes of leaving groups based on O- and S-imidates. The main focus of the study presented herein is the synthesis of novel 3,3-difluoro-3H-indol-2-ylthio (SFox) imidates and their application as glycosyl donors in chemical glycosylation. Being thioimidates, these compounds are more stable than O-imidates albeit much more reactive than conventional alkyl/arylthio glycosides. This study demonstrates that SFox imidates can be activated either with soft thiophilic reagents (N-iodosuccinimide or transition metal salts), typical for the activation of thioglycosides or thioimidates, or hard electrophilic reagents (protic or Lewis acids) common for the activation of O-imidates. Expectedly, complete β-selectivity was obtained from SFox donors equipped with 2-O-benzoyl group. Surprisingly, complete α-selectivity was obtained from 2-O-benzylated SFox imidates in all investigated cases.

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.

Stereoselective Synthesis of β-S-Glycosides via Palladium Catalysis

S-Glycosides are more resistant to enzymatic and chemical hydrolysis and exhibit higher metabolic stability than common O-glycosides, demonstrating their widespread application in biological research and drug development. In particular, β-S-glycosides are used as antirheumatic, anticancer, and antidiabetic drugs in clinical practice. However, the stereoselective synthesis of β-S-glycosides is still highly challenging. Herein, we report an effective β-S-glycosylation using 3-O-trichloroacetimidoyl glycal and thiols under mild conditions. The C3-imidate is designed to guide Pd to form a complex with glucal from the upper face, followed by Pd-S (thiols) coordination to realize β-stereoselectivity. This method demonstrates excellent compatibility with a broad scope of various thiol acceptors and glycal donors with yields up to 87% and a β/α ratio of up to 20:1. The present β-S-glycosylation strategy is used for late-stage functionalization of drugs/natural products such as estrone, zingerone, and thymol. Overall, this novel and simple operation approach provides a general and practical strategy for the construction of β-thioglycosides, which holds high potential in drug discovery and development.

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.

An Open-Air Palladium-Catalyzed Stereoselective O-Glycosylation of Glycals via in-situ Generation of gem-Disubstituted Methanols from p-Quinone Methides

We devised a palladium-catalyzed α-stereoselective glycosylation that incorporates oxygen via in-situ generation of gem-disubstituted methanols from p-quinone methides to access 2,3-unsaturated gem-diarylmethyl O-glycosides under open-air atmosphere at room temperature. Advantages of this environmentally friendly strategy include the absence of additives and ligands, using water as the green source of oxygen, mildest, operationally simple, exhibiting a wide functional group tolerance, and compatibility with a variety of glycal progenitors in appreciable yields. A mechanistic study has been verified via H2 18 O labeling, which validates that water (moisture) is a sole source of oxygen.

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.

Recent chemical synthesis and immunological evaluation of glycans related to bacterial lipopolysaccharides

O-Antigens and core oligosaccharides from bacterial lipopolysaccharides (LPS) are often structurally unique and immunologically active, have become attractive targets in the development of antibacterial vaccines. Structurally well-defined and pure oligosaccharides can be used in identifying protective epitopes of the carbohydrate antigens, which is important for the design of an effective vaccine. Here, the recent progress on chemical synthesis and immunological evaluation of glycans related to O-antigens and core oligosaccharides from bacterial LPS are summarized.

Total Synthesis of the Purported Structure of Branched Resin Glycosides Merremoside G and H2

The first total synthesis of the purported structure of branched resin glycosides merremoside G and H2 is accomplished. A signature step is represented by the sequential transition-metal-catalyzed coupling of stable trisaccharide homoallylic alcohol and monosaccharide alkoxyallene to afford the pentasaccharide skeleton. This de novo strategy is conducted under mild conditions with no need of preactivation. In addition, it allows for efficient preparation of the target compounds in combination with late-stage functionalization.

Palladium catalysis enables cross-coupling-like SN2-glycosylation of phenols

Despite their importance in life and material sciences, the efficient construction of stereo-defined glycosides remains a challenge. Studies of carbohydrate functions would be advanced if glycosylation methods were as reliable and modular as palladium (Pd)-catalyzed cross-coupling. However, Pd-catalysis excels in forming sp2-hybridized carbon centers whereas glycosylation mostly builds sp3-hybridized C-O linkages. We report a glycosylation platform through Pd-catalyzed SN2 displacement from phenols toward bench-stable, aryl-iodide-containing glycosyl sulfides. The key Pd(II) oxidative addition intermediate diverges from an arylating agent (Csp2 electrophile) to a glycosylating agent (Csp3 electrophile). This method inherits many merits of cross-coupling reactions, including operational simplicity and functional group tolerance. It preserves the SN2 mechanism for various substrates and is amenable to late-stage glycosylation of commercial drugs and natural products.

Unraveling the promoter effect and the roles of counterion exchange in glycosylation reaction

The stereoselectivity of glycosidic bond formation continues to pose a noteworthy hurdle in synthesizing carbohydrates, primarily due to the simultaneous occurrence of SN1 and SN2 processes during the glycosylation reaction. Here, we applied an in-depth analysis of the glycosylation mechanism by using low-temperature nuclear magnetic resonance and statistical approaches. A pathway driven by counterion exchanges and reaction byproducts was first discovered to outline the stereocontributions of intermediates. Moreover, the relative reactivity values, acceptor nucleophilic constants, and Hammett substituent constants (σ values) provided a general index to indicate the mechanistic pathways. These results could allow building block tailoring and reaction condition optimization in carbohydrate synthesis to be greatly facilitated and simplified.

Base-Promoted Glycosylation Allows Protecting Group-Free and Stereoselective O-Glycosylation of Carboxylic Acids

Here we report a simple and general method to achieve fully unprotected, stereoselective glycosylation of carboxylic acids, employing bench-stable allyl glycosyl sulfones as donors. Running the glycosylation reaction under basic conditions was crucial for the efficiencies and selectivities. Both the donor activation stage and the glycosidic bond forming stage of the process are compatible with free hydroxyl groups, thereby allowing for the use of fully unprotected glycosyl donors. This transformation is stereoconvergent, occurs under mild and metal-free conditions at ambient temperature with visible light (455 nm) irradiation, and displays remarkable scope with respect to both reaction partners. Many natural products and commercial drugs, including an acid derived from the complex anticancer agent taxol, were efficiently glycosylated. Experimental studies provide insights into the origin of the stereochemical outcome.

Modular and Stereoselective One-Pot Total Synthesis of Icosasaccharide Motif from Cordyceps sinensis EPS-1A Glycan

The first stereoselective one-pot synthesis of the icosasaccharide motif of EPS-1A glycan from Cordyceps sinensis has been achieved. The synthetic approach highlights the following features: (1) merging reagent modulation and remote anchimeric assistance α-glycosylation strategy for the highly stereoselective formation of five and ten continuous 1,2-cis glucosidic bonds; (2) the strategic employment of glycosyl N-phenyltrifluoroacetimidates and glycosyl o-(1-phenylvinyl)benzoates as the matched pair for efficient orthogonal one-pot synthesis; and (3) [11 + 8 + 1] orthogonal one-pot glycosylation for the efficient assembly of the target icosasaccharide.

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.

Sulfonium-based precise alkyl transposition reactions

S-adenosyl-L-methionine (SAM), a sulfonium-based cofactor, plays an important role in numerous biological processes as methyl donor. Inspired by the function of sulfonium motif in this nature's synthetic toolkit, we here present an aryne-activation strategy that the sulfonium intermediates in situ generated from thioethers display unique reactivity toward alkyl group transposition. Experimental and theoretical studies indicate that the reaction occurs in an intermolecular fashion where the TfO--incorporated [K(18-crown-6)] complex acts as a key promoter for this thermodynamically favored process. Next, a series of robust, easy-to-prepare sulfonium salts are designed and developed as electrophilic alkylation reagents accordingly. Both systems feature for broad scope, excellent selectivity, and simple operation. Moreover, we highlight the synthetic value through molecular editing and late-stage modification of complex scaffolds or even active pharmaceutical ingredients.

Stereoselective Synthesis of O-Glycosides with Borate Acceptors

Borate esters have been applied widely as coupling partners in organic synthesis. However, the direct utilization of borate acceptors in O-glycosylation with glycal donors remains underexplored. Herein, we describe a novel O-glycosylation resulting in the formation of 2,3-unsaturated O-glycosides and 2-deoxy O-glycosides mediated by palladium and copper catalysis, respectively. This O-glycosylation method tolerated a broad scope of trialkyl/triaryl borates and various glycals with exclusive stereoselectivities in high yields. All the desired aliphatic/aromatic O-glycosides and 2-deoxy O-glycosides were generated successfully, without the hemiacetal byproducts and O→C rearrangement because of the nature of borate esters. The utility of this strategy was demonstrated by functionalizing the 2,3-unsaturated glycoside products to form saturated β-O-glycosides, 2,3-deoxy O-glycosides, and 2,3-epoxy O-glycosides.

Empowering boronic acids as hydroxyl synthons for aryne induced three-component coupling reactions

Boronic acids have become one of the most prevalent classes of reagents in modern organic synthesis, displaying various reactivity profiles via C-B bond cleavage. Herein, we describe the utilization of a readily available boronic acid as an efficient surrogate of hydroxide upon activation via fluoride complexation. The hitherto unknown aryne induced ring-opening reaction of cyclic sulfides and three-component coupling of fluoro-azaarenes are developed to exemplify the application value. Different from metal hydroxides or water, this novel hydroxy source displays mild activation conditions, great functionality tolerance and structural tunability, which shall engender a new synthetic paradigm and in a broad context offer new blueprints for organoboron chemistry. Detailed computational studies also recognize the fluoride activation mode, provide in-depth insights into the unprecedented mechanistic pathway and elucidate the reactivity difference of ArB(OH) x F y complexes, which fully support the experimental data.

Kukhtin-Ramirez-Reaction-Inspired Deprotection of Sulfamidates for the Synthesis of Amino Sugars

Herein, we present a mild strategy for deprotecting cyclic sulfamidates via the Kukhtin-Ramirez reaction to access amino sugars. The method features the removal of the sulfonic group of cyclic sulfamidates, which occurs through an N-H insertion reaction that implicates the Kukhtin-Ramirez adducts, followed by a base-promoted reductive N-S bond cleavage. The mild reaction conditions of the protocol enable the formation of amino alcohols including analogs that bear multiple functional groups.

Progress in the total synthesis of resin glycosides

Resin glycosides, mainly distributed in plants of the family Convolvulaceae, are a class of novel and complex glycolipids. Their structural complexity and significant biological activities have received much attention from synthetic chemists, and a number of interesting resin glycosides have been synthesized. The synthesized resin glycosides and their analogues not only helped in structural verification, structural modification, and further biological activity exploration but also provided enlightenment for the synthesis of glycoside compounds. Herein, the present review summarizes the application of various efforts toward the synthesis of resin glycosides in the last decade.

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.

Beyond GalNAc! Drug delivery systems comprising complex oligosaccharides for targeted use of nucleic acid therapeutics

Nucleic Acid Therapeutics (NATs) are establishing a leading role for the management and treatment of genetic diseases following FDA approval of nusinersen, patisiran, and givosiran in the last 5 years, the breakthrough of milasen, with more approvals undoubtedly on the way. Givosiran takes advantage of the known interaction between the hepatocyte specific asialoglycoprotein receptor (ASGPR) and N-acetyl galactosamine (GalNAc) ligands to deliver a therapeutic effect, underscoring the value of targeting moieties. In this review, we explore the history of GalNAc as a ligand, and the paradigm it has set for the delivery of NATs through precise targeting to the liver, overcoming common hindrances faced with this type of therapy. We describe various complex oligosaccharides (OSs) and ask what others could be used to target receptors for NAT delivery and the opportunities awaiting exploration of this chemical space.

Solvent free, catalytic and diastereoselective preparation of aryl and alkyl thioglycosides as key components for oligosaccharide synthesis

A new and environmentally friendly protocol for the conversion of sugar per-acetates into thioglycosides under solvent free and catalytic conditions is presented. The procedure involves heating in the presence of InCl₃ and various aryl thiols. For alkyl thioglycoside synthesis, cyclohexane thiol was found to give good results and yield a glycosyl donor with reactivity similar to a thioethyl congener. The established optimum reaction conditions were found to provide the desired thioglycoside products in an easy and highly diastereoselective manner even when conducted on a multigram scale.

A general electron donor-acceptor complex for photoactivation of arenes via thianthrenation

General photoactivation of electron donor-acceptor (EDA) complexes between arylsulfonium salts and 1,4-diazabicyclo[2.2.2]octane with visible light or natural sunlight was discovered. This practical and efficient mode enables the production of aryl radicals under mild conditions, providing an unrealized opportunity for two-step para-selective C-H functionalization of complex arenes. The novel mode for generating aryl radicals via an EDA complex was well supported by UV-vis absorbance measurements, nuclear magnetic resonance titration experiments, and density functional theory (DFT) calculations. The method was applied to the regio- and stereo-selective arylation of various N-heterocycles under mild conditions, yielding an assembly of challengingly linked heteroaryl-(hetero)aryl products. Remarkably, the meaningful couplings of bioactive molecules with structurally complex drugs or agricultural pharmaceuticals were achieved to display favorable in vitro antitumor activities, which will be of great value in academia or industry.

Boron-Promoted Umpolung Reaction of Sulfonyl Chlorides for the Stereospecific Synthesis of Thioglycosides via Reductive Deoxygenation Coupling Reactions

S-Glycosides have broad biological activities and serve as stable mimics of natural O-glycoside counterparts and thus are of great therapeutic potential. Herein we disclose an efficient method for the stereospecific synthesis of 1-thioglycosides via a boron-promoted reductive deoxygenation coupling reaction from readily accessible sulfonyl chlorides and glycosyl bromides. Our protocol features mild conditions and excellent functional group tolerance and stereoselectivity. The translational potential of this metal-free approach is demonstrated by the late-stage glycodiversification of natural products and drug molecules.

Photolabile 2-(2-Nitrophenyl)-propyloxycarbonyl (NPPOC) for Stereoselective Glycosylation and Its Application in Consecutive Assembly of Oligosaccharides

A photolabile protecting group (PPG) 2-(2-nitrophenyl)-propyloxycarbonyl (NPPOC) was explored in glycosylation and applied in the consecutive synthesis of oligosaccharides. NPPOC displays a strong neighboring group participation (NGP) effect to facilitate the construction of 1,2-trans glycosides in excellent yield. Notably, NPPOC could be efficiently removed by photolysis, and the deprotection conditions are friendly to typical protecting groups. A branched and asymmetric oligomannose Man6 was rapidly prepared, and the consecutive assembly of oligosaccharides without intermediate purification was further investigated owing to the compatibility conditions between NPPPOC's photolysis and glycosylation.

Thiocarbonyl as a Switchable Relay-Auxiliary Group in Carbohydrate Synthesis

A multifunctional O-phenyl thiocarbonyl (O(C═S)OPh) group was introduced in glycosylation reactions. This auxiliary group exhibits three features (1) C6-long-range participation effect, (2) relay activation, and (3) switchable promoter-controlled carbonylation, which enables the facile synthesis of both 6-deoxy glucoside and 6-alcohol glucoside. In addition, we successfully quantified the extent of the C6-acyl participation effect and developed its application toward the α-trisaccharide motif.

Total synthesis of tricolorin A via interrupted Pummerer reaction-mediated glycosylation and one-pot relay glycosylation

Tricolorin A, a bioactive resin glycoside, was synthesized stepwise or in one pot based on interrupted Pummerer reaction-mediated (IPRm) glycosylation. The stepwise synthesis adopted a [2 + 2] assembly sequence, and all of the glycosidic bonds were constructed efficiently by IPRm glycosylation. The one-pot synthesis employed our recently developed one-pot relay glycosylation strategy, in which two different glycosidic bonds were sequentially connected with only one equivalent of external activator.

Synthesis of chimera oligopeptide including furanoid β-sugar amino acid derivatives with free OHs: mild but successful removal of the 1,2-O-isopropylidene from the building block

Complementary to hydrophobic five membered ring β-amino acids (e.g. ACPC), β-sugar amino acids (β-SAAs) have found increasing application as hydrophilic building blocks of foldamers and α/β chimeric peptides. Fmoc-protected β-SAAs [e.g. Fmoc-RibAFU(ip)-OH] are indeed useful Lego elements, ready to use for SPPS. The removal of 1,2-OH isopropylidene protecting group increasing the hydrophilicity of such SAA is presented here. We first used N3-RibAFU(ip)-OH model compound to optimize mild deprotection conditions. The formation of the 1,2-OH free product N3-RibAFU-OH and its methyl glycoside methyl ester, N3-RibAFU(Me)-OMe were monitored by RP-HPLC and found that either 50% TFA or 8 eqv. Amberlite IR-120 H+ resin in MeOH are optimal reagents for the effective deprotection. These conditions were then successfully applied for the synthesis of chimeric oligopeptide: -GG-X-GG- [X=RibAFU(ip)]. We found the established conditions to be effective and-at the same time-sufficiently mild to remove 1,2-O-isopropylidene protection and thus, it is proposed to be used in the synthesis of oligo- and polypeptides of complex sequence combination.

Electrochemical Oxidative Carbon-Atom Difunctionalization: Towards Multisubstituted Imino Sulfide Ethers

Ethers (C-O/S) are ubiquitously found in a wide array of functional molecules and natural products. Nonetheless, the synthesis of imino sulfide ethers, containing an N(sp² )=C(sp² )-O/S fragment, still remains a challenge because of its sensitivity to acid. Developed here in is an unprecedented electrochemical oxidative carbon-atom difunctionalization of isocyanides, providing a series of novel multisubstituted imino sulfide ethers. Under metal-free and external oxidant-free conditions, isocyanides react smoothly with simple and readily available mercaptans and alcohols. Importantly, the procedure exhibited high stereoselectivities, excellent functional-group tolerance, and good efficiency on large-scale synthesis, as well as further derivatization of the products.

Sequential activation of thioglycosides enables one-pot glycosylation

This review describes recent developments in relative reactivity value (RRV) controlled sequential glycosylation, pre-activation based iterative glycosylation, and sulfoxide activation initiated one-pot glycosylation.

Chemoselective activation of ethyl vs. phenyl thioglycosides: one-pot synthesis of oligosaccharides

Ethyl and phenyl thioglycosides are the two most common types of thioglycoside donors in carbohydrate chemistry. However, the chemoselective activation of ethyl vs. phenyl thioglycosides is very rare in the literature. In this work, ethyl thioglycosides could be readily activated with an N-trifluoromethylthiosaccharin/TMSOTf system in the presence of phenyl thioglycosides carrying the same or even more armed protecting group pattern. Both armed and disarmed thioglycosides exhibited high chemoselectivity towards the promoter system. Chemoselective glycosylation was subsequently applied to one-pot synthesis, thus providing an efficient means to oligosaccharides.

Palladium-Catalyzed ortho-C-H Glycosylation/ipso-Alkenylation of Aryl Iodides

This report describes the first example of palladium-catalyzed ortho-C-H glycosylation/ipso-alkenylation of aryl iodides, and the easily accessible glycosyl chlorides are used as a glycosylation reagent. The reaction is compatible with the functional groups of the substrates, and a series of C-aryl glycosides have been synthesized in good to excellent yield and with excellent diastereoselectivity. It is found that a cheap 5-norbornene-2-carbonitrile as a transient mediator can effectively promote this reaction. In addition, ipso-arylation and cyanation were also realized by the strategy.