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DOI: 10.1055/a-2348-2803
Highly Regio-/Stereoselective Synthesis of Carbohydrates with Unsaturated Glycosyl Donors under Mild Conditions
This work was partially supported by the National Natural Science Foundation of China (22207063), Higher Education Discipline Innovation Project (111 Project, D20015), the Natural Science Foundation of Hubei Province (2022CFB838), the Hubei Provincial Department of Education (D20221204, Q20221212), and the Natural Science Foundation of Yichang Municipality (A23-2-002).
Abstract
Carbohydrates and their conjugates play important roles in life activities and drug development. Our group was committed to the general and effective glycosylation methods and their application in chemical biology using unsaturated glycosyl donors. In the past five years, we have reported several synthetic strategies with high stereoselectivity and milder conditions compared with previous works. In particular, high chemo-/regio- and stereoselective O-glycosylation, C-glycosylation and S-glycosylation could be achieved via palladium catalysis under open-air conditions at room temperature. In this Account, we will introduce our research progress in constructing four types of glycosides.
1 Introduction
2 Stereoselective Synthesis of O-Glycosides
3 Stereoselective Synthesis of C-Glycosides
4 Stereoselective Synthesis of N-Glycosides
5 Stereoselective Synthesis of S-Glycosides
6 Conclusion
#
Key words
glycosylation - transition metal catalysis - unsaturated glycosyl donors - stereoselectivity - carbohydratesBiographical Sketches
Xinyu Gao obtained her bachelor’s degree from Wuhan University of Science and Technology, majored in inorganic nonmetallic materials engineering, and minored in English. Then she began her master’s degree in organic chemistry at China Three Gorges University (CTGU) and focused on glycosylation reactions and their application.
Keke Ren was born in Henan (China) and received her bachelor’s degree from Luoyang Normal University. Then she began her master’s degree at China Three Gorges University(CTGU) and focused on the development of glycosylation methods.
Lijuan Ma was born in Guizhou (China) in 2002. She majored in chemistry during her undergraduate studies at China Three Gorges University (CTGU). She is going to be a graduate student in the College of Biological and Pharmaceutical Sciences at China Three Gorges University. Her research interests focus on organic synthetic chemistry.
Nengzhong Wang obtained his PhD degree under the mentorship of Prof. Xuefeng Jiang at East China Normal University (ECNU) in 2018. From 2018 to 2022, he was a postdoc research fellow under the guidance of Prof. Yixin Lu at National University of Singapore (NUS) in the field of novel phosphine-catalyzed asymmetric reactions and their applications in the total synthesis of biologically active alkaloids. In 2022, he started his independent career as an associate professor at China Three Gorges University (CTGU). His research interest is devoted to asymmetric catalysis and natural product synthesis.
Nianyu Huang obtained his PhD degree in chemistry under Prof. Mingwu Ding at Central China Normal University in 2009. He studied at Nanyang Technological University as a visiting scholar under Prof. Xue-Wei Liu from 2015 to 2016. He is currently the director of Hubei Key Laboratory of Natural Products Research and Development at China Three Gorges University (CTGU). He is devoted to modification of natural products and organic synthesis in heterocyclic chemistry.
Hui Yao grew up in Anqing City Anhui Province, China. He obtained his BSc in pharmacy (2009) and MSc in medicinal chemistry (2012) supervised by Prof. Yuqiang Wang from Jinan University, Guangzhou, China. After working for two years at The Chinese University of Hong Kong under Prof. Tony K. M. Shing, he moved to Nanyang Technological University, Singapore to join Prof. Xue-Wei Liu’s group as a PhD candidate. After receiving his PhD degree in 2018, he started his independent career as a professor at China Three Gorges University. Dr. Yao’s major research interest is focused on the synthesis of carbohydrates and their conjugates and their application in chemical biology.
Introduction
Carbohydrates and their conjugates are widely existing in nature.[1] Glycosides can be divided into four main types according to atoms that are connected to the anomeric carbon, which are O-glycosides, N-glycosides, C-glycosides, and S-glycosides. They are playing important roles in life activities and drug development as shown in Figure [1].[2] For example, O-glycoside daunorubicin is applied as a drug for acute myeloid leukemia (AML) in clinical, and azithromycin is a widely used antibiotic.[3] Among natural O-glycosides, phlorizin can serve as an SGLT2 inhibitor to block glucose reabsorption,[4] and cardiac glycosides are a class of drugs for heart failure and arrhythmias.[5] Gastrodin is able to participate in osteogenesis and angiogenesis improvements by releasing PU/n-HA composite scaffolds to reprogram macrophages.[6] Russelioside B possesses a variety of pharmacological effects such as antidiabetic, antiobesity, antiulcer, and so on.[7] Regarding N-glycosides, the representative drug adenosine shows noticeable antineoplastic bioactivity and cytarabine can be used for childhood leukemia in combination with clofarabine.[8] Nikkomycin Z exhibits great antimycotic properties.[9] C-Glycosides dapagliflozin, canagliflozin, and empagliflozinis are commonly used to treat type 2 diabetes.[10] Mangiferin isolated from mango trees has been demonstrated high antioxidant effect and is expected to be
a potential anticancer agent.[11] Auranofin and lincomycin are two well-known S-glycoside drugs. The traditional antirheumatic agent auranofin has also been proven to possess synergistic anticancer activity in recent reports,[12] While lincomycin has been extensively used to treat Gram-positive bacterial infections for nearly half a century.[13]
Moreover, glycosylation plays an important role in the structural diversification modification of natural products. It affects the properties of parent molecules by improving the reactivity and solubility of the corresponding aglycones, thereby affecting cell localization and biological activity. Carbohydrates can also control the transmission of bioinformatics by specific recognition with biomolecules such as proteins.[14] Glycosylated natural products and drugs further exert various effects in vivo such as anti-inflammatory, antiviral, anticancer, and so on.[15] About one-fifth of the natural products are glycosylated.[16] Efficient glycosylation strategies have always been arguably the most important part of carbohydrate chemistry.[17]
Carbohydrate chemists have devoted much attention to the efficient construction of different types of glycosidic bonds.[18] As we know, there are two possible directions in stereochemistry when the glycosidic bond is formed, resulting in the α (C1–OR is on the opposite face to the –CH2OH) and the β (C1–OR is on the opposite face to the –CH2OH) configurations, respectively. Glycosides with different configurations play various roles in organisms due to their disparate three-dimensional structures. Due to anomeric effect, most of the conventional glycosylation gives α major products (mixtures).[19] How to construct an efficient stereoselective glycosylation strategy is still highly challenging and has always been the common goal of researchers.[20]
Classical glycosylation strategies are usually related to saturated glycosyl donors.[21] [22] A leaving group is attached to the anomeric carbon of the glycosyl donor, and then the nucleophiles attack the anomeric position to generate glycosides.[23] In the 1980s, Schmidt developed trichloroacetimidate as an effective leaving group to construct carbohydrates and glycoconjugates successfully mediated by stoichiometric Lewis acids in an anhydrous solvent under inert gas.[24] In 2008, Yu’s group reported glycosyl O-alkynyl benzoates as glycosyl donors, which could be applied to gain various glycosides mediated by catalytic gold species.[25] In recent years, unsaturated glycosyl donors have begun to attract the attention of carbohydrate chemists, and much progress has been made in carbohydrate synthesis (Scheme [1]). The double bond of the unsaturated glycosyl donors can coordinate with transition metals such as palladium, which can achieve stereodirected synthesis through the coordination effect. Further, the obtained unsaturated glycosides can undergo a subsequent functionalization to give quick access to various carbohydrates including rare sugars.[21] [26] In 1914, Fisher found that 2,3-unsaturated glycoside was obtained by allylic rearrangement when heating tri-O-acetyl-d-glucal in water. In the 1960s, tri-O-acetyl-d-glucal was employed by Ferrier to react alcohols to generate the corresponding O-glycosides in the presence of a stoichiometric Lewis acid in anhydrous solvent under inert gas.[27] In this reaction, the leaving group would be cleaved from C3 to generate an allylic oxocarbenium ion intermediate, followed by the nucleophilic attack to produce the corresponding glycosides, which was named as Ferrier-type-I reaction. Besides Lewis acids, Takemoto found that a tricyclic borinic acid catalyst as a Brønsted acid could also catalyze the formation of glycosidic bonds.[28] With the development of transition-metal chemistry, the coordination effect between metal and the olefin of glycal made promising progress in stereocontrolled glycosylation.[29] In 2004, Lee’s group developed a stereoselective synthetic method of O-glycosides with unsaturated glycosyl donors promoted by catalytic transition metal catalyst Pd(OAc)2 with Et2Zn as an additive.[30] In 2009, Ye and coworkers developed an oxidative Heck-type C-glycosylation of glycals with various arylboronic acids catalyzed by Pd(OAc)2 in the presence of oxidants.[31] In 2014, Xue-Wei Liu’s group reported a stereoselective O-glycosylation method using 3-O-picoloyl d-glucal as the donor at 60 ℃ catalyzed by Pd(PPh3)4 without the requirement of adding sensitive additive Et2Zn.[32] In 2017, Galan found that gold assisted by AgOTf can also catalyze α-O-deoxyglycosylation.[33] Recently, Hong Liu and co-workers reported that the cobalt catalyst could catalyze the formation of 2-deoxy β-C-glycosides from unsaturated pyranosyl and furanosyl donors in the presence of triethoxysilane.[34] Almost at the same time, Qiang Liu’s group also reported a similar strategy for 2-deoxy β-C-glycosides by C(sp3)–C(sp3) coupling under cobalt catalysis.[35] These strategies have received a lot of praise for their high efficiency and high stereoselectivity but generally, they were supposed to be under anhydrous solvent and inert gas atmosphere. Our group was committed to the general and effective glycosylation methods and their application in chemical biology using unsaturated glycosyl donors. In the past five years, we have reported some synthetic strategies with high stereoselectivity and milder conditions compared with previous works. In particular, high chemo-/regio- and stereoselective O-glycosylation, C-glycosylation, and S-glycosylation could be achieved via palladium catalysis under open-air conditions at room temperature. In this Account, we will introduce our research progress in constructing four types of glycosides: O-, C-, N-, and S-glycosides.
# 2
Stereoselective Synthesis of O-Glycosides
Among all the types of glycosides, O-glycosides are arguably naturally occurring most broadly. As mentioned, the chemical synthesis of O-glycosides was generally conducted under anhydrous solvents and inert atmosphere, because water in air exhibits stronger nucleophilicity than the common glycosyl acceptors alcohols and phenols. Furthermore, generally, when boric acid was used as a coupling reagent, C–C bond-coupling products were obtained. Our group first reported a strategy for synthesizing O-glycosides in an open-air system using glycals as glycosyl donors.[36] Under the combined action of White catalyst, ligands, and base additive, β-O-glycosides were synthesized with excellent stereoselectivity using aryl boronic acids as glycosyl acceptors in a common solvent (commercially available solvents without further processing) under open-air conditions at room temperature (Scheme [2a]). The subsequent mechanistic studies revealed that the oxygen in the C–O glycosidic bond originates from atmospheric oxygen. This strategy is not only high yielding under mild conditions, but also has good substrate adaptability. We explored the substrate scopes using various substituted aryl boronic acids and glycosyl donors like l-fucal, 3,4-O-carbonate-l-arabinal, and 3,4-O-carbonate-d-galactal with different protective groups at C6, giving around 30 targeted O-glycosides with good to excellent yields. The reaction also tolerated the electron-withdrawing groups, for example, using p-fluorophenylboronic acid as an acceptor, the yield was obtained as high as 91%.
Isotope-labeling experiments were carried out to investigate the reaction mechanism (Scheme [3a]). Isotope-labeled H2 18O and 18O2 were submitted to participate in the reaction, respectively, to figure out the source of the oxygen in the C–O glycosidic bond and then we detected the isotopically labeled O-glycosides only in reactions involving 18O2, confirming that oxygen atoms in the O-glycosidic bonds were from the air. Based on it, we proposed a mechanism as shown in Scheme [2b]. White catalyst was converted into a Pd(0) complex with the ligand at first, which was then oxidized by atmospheric oxygen to a peroxy palladium complex. This complex reacted with aryl boronic acid to form an intermediate, which underwent in situ elimination to produce phenol, and Pd(0) continued to coordinate with phenol. As the direction of 3,4-O-cyclic carbonate from the upper face of the sugar ring, Pd(0)–phenol complex preferred to coordinate the olefin bond of glycal from the β side, followed by decarboxylation to form an η3-π-allyl-Pd complex. Finally, after intramolecular nucleophilic attack and reduction elimination, the required O-glycosides were obtained stereoselectively, and the Pd(0) complex was released to continue the catalytic cycle.
Besides, reactions of glycosyl donors with aryl boronic acids containing competitive reaction sites like amino or hydroxymethyl groups exclusively yielded α-phenolic O-glycosides, demonstrating high chemoselectivity and good compatibility with amino groups. Later, the dihydroxylation and hydrogenation reduction reaction were successfully implemented on the O-glycosides we obtained (Scheme [3b]). We also successfully synthesized natural products with anticancer and antiviral activities such as diphyllin O-glycoside (Scheme [3c]), further proving the practicality of this strategy.
Transition-metal-catalyzed glycosylation has been a hot topic in carbohydrate research. Among these precious metals like palladium, yttrium, rubidium, and rhodium are known for their high reactivity and effectiveness. But their higher costs also exist to be a drawback, hence, the other transition metals. Fortunately, the iron catalyst was found to enable catalyzing glycal and alcohols/phenol to form 2-deoxy sugars by our group recently.[37]
The donor 3,4-O-carbonate-d-galactal was applied to react with phenols or alcohols under the catalysis of Fe(OTf)3, generating the target O-glycosides at room temperature (Scheme [4a]). This strategy demonstrated high stereoselectivity and was suitable for various phenols and alcohols. Based on these results and previous literature, a possible mechanism is shown in Scheme [4b]. Fe(OTf)3 first coordinated with the olefin bond of the unsaturated glycosyl donors through the spatially favored α-face, polarizing the double bond, and leading to proton transfer to the less hindered C2 position. The π-complex formed a crucial semichair oxocarbenium ion intermediate, which can transform into the intermediate β-CIP via solvent-separated ion-pair conversion. The activated nucleophilic acceptor (RO–) attacked the intermediate from the less hindered α-position, stereoselectively producing α-O-glycosides.
To gain some insights into the mechanism, some competitive experiments were conducted. After adding a mixture of α- and β-methyl 2-deoxy glycosides to the reaction system, highly stereoselective O-glycoside products could still be obtained, indicating that the β-product can undergo a reversible conversion into the oxocarbenium ion intermediate, which then stereoselectively produces α-2-deoxy glycosides. The proposed oxocarbenium ion intermediate was also successfully captured by benzyl alcohol.
The broad substrate scope of this method was demonstrated not only by the glycosylation of common alcohols/phenols but also applied to the late-stage functionalization of natural products and pharmaceutical molecules. As shown in Figure [2], cholesterol, diosgenin, estrone, podophyllotoxin, and menthol were all glycosylated successfully in high yields with only α-selectivity. In addition, a tetrasaccharide was also synthesized smoothly, indicating the practicality of this reaction.
After successfully designing an alcohol-based glycosylation strategy, we turned our attention to borate esters as glycosyl acceptors (Scheme [5]).[38] Borate esters are generally widely used as coupling reagents in organic synthesis, yet their roles in glycosylation have been seldom reported. Then the borate esters were examined as glycosyl acceptors with unsaturated glycosyl donors under transition-metal catalysis, successfully giving the targeted O-glycosides. It was noted that the selectivity of this glycosylation could be controlled by the type of catalysts. With the copper triflate, the reaction yielded the 2-deoxy α-glycosides as the predominant product, while palladium catalyst and ligand cooperation led to the exclusive formation of 2,3-unsaturated β-O-glycosides. This might be due to the mechanism involving Cu(OTf)2 being similar to the previously reported Fe(OTf)3 catalysis, where there is a coordination and ligand-exchange process between borate esters and Cu catalyst; whereas the Pd catalyst does not undergo this process. First, the Pd(II) precursor was reduced to Pd(0) by phosphine ligands.[39] The complex opted to attack the olefin of glycals from the bottom side influenced by the steric hindrance of the 3,4-O-cyclic carbonate and the subsequent decarboxylation happened, ended up with an oxygen anion intermediate with exposed α orientation at C4. The boron atom quickly coordinates with this oxyanion from the upper face. Subsequently, due to the influence of this coordinate bond and the spatial hindrance brought by the palladium catalyst on the other side of the glycosyl donor, the borate ester can only perform a nucleophilic attack from the upper side, resulting in the stereoselective β-O-glycosides.
# 3
Stereoselective Synthesis of C-Glycosides
Compared with O-glycosides, C-glycosides are more resistant to chemical hydrolysis and enzymatic degradation than O-glycosides, so they are widely used as carbohydrate mimics. Hence, the highly regio-/stereoselective synthesis of C-glycosides under mild conditions has also been a hot topic and challenge in carbohydrate research.[40] Additionally, besides using borate esters as acceptors, boric acids are also classical coupling reagents in Suzuki coupling reaction. Our group utilized aryl boronic acids as acceptors and the White catalyst which is stable in the air to successfully design a strategy for stereoselectivity C-glycosides under open-air conditions (Scheme [6a]).[41] This strategy did not require strict anhydrous solvents and inert gas conditions, nor the involvement of ligands, and it showed good adaptability to different glycosyl donors and various substituted borate esters. Competitive experiments also demonstrated its tolerance for exposed hydroxyl and amino groups on the acceptors.
The mechanism was proposed as shown in Scheme [6b], the White catalyst first underwent a transmetalation with the boric acid and formed an intermediate. Due to the steric hindrance of the cyclic carbonate, it then coordinated with the π-bond in the sugar ring from the below site and inserted into the double bond to form the four-membered-ring transition state, generating a key intermediate coupled with palladium at the C2 position. Subsequently, this intermediate underwent reductive elimination to produce the target C-glycoside, releasing the White catalyst to continue the catalytic cycle, significantly saving on catalyst usage.
After the success of the synthesis of α-C-glycosides, our subsequent research explored the stereoselective synthesis of β-C-glycosides.[42] Oxazol-5(4H)-ones were employed as the glycosyl acceptors, which were also direct precursors to amino acids. Under the palladium catalyst and bulky phosphine ligands, oxazol-5(4H)-ones reacted with 3,4-O-carbonate-d-galactal to form β-C-glycosides stereoselectively. This reaction demonstrated good yields and excellent chemoselectivity and stereoselectivity, as other reactive sites of acceptors barely participate in the highly β-selective glycosylation. Additionally, we used nitroalkanes as glycosyl acceptors to synthesize C-glycosides following the Tsuji–Trost reaction mechanism, with all products being β-configured.[43]
# 4
Stereoselective Synthesis of N-Glycosides
N-Glycosides are an important class of carbohydrates found widely in natural products and pharmaceuticals, with nucleosides and glycopeptides being the most significant two types. Nucleosides are not only fundamental in the structure of genetic materials such as DNA and RNA but are also known for their antitumor and antiviral properties, as well as their role in immunomodulation.[44] Dipeptides often function in biological activities as enzymes, hormones, and antibodies. However, due to the unique basicity of the nitrogen atom, it preferentially binds with acids in the presence of protic acids, forming ammonium ions and reducing its nucleophilicity. According to previous reports, the strategies for N-glycosides often face limitations in substrate scope and stereoselectivity, leading to slower research progress compared to other glycosides.
Due to the low nucleophilicity of amides, they are rarely applied in glycosylation directly as acceptors. Takemoto’s team has previously reported a direct strategy for synthesizing glycosyl amides catalyzed by Brønsted salts using saturated glycosyl as donors.[45] As our group focuses on unsaturated glycosyl donors, the direct N-glycosylation of amides using unsaturated glycosyl donors was developed in 2020 (Scheme [7]).[46] We achieved direct glycosylation of amides and N-heterocycles under the catalysis of palladium catalyst and the ligand with 3,4-O-carbonate-d-galactal being the glycosyl donors.
This strategy exhibited excellent stereoselectivity with a broad substrate scope. N-Glycosides from 8 amides were successfully formed. This approach was also applied effectively to 15 N-heterocycles, including imidazole, carbazole, indole, and so on. Among them, three products show surprising cytotoxicity to cancer cells in activity tests, demonstrating the potential of this strategy in medicinal chemistry. However, the scope of this strategy was limited to amides and N-heterocycles. As most N-glycosides in nature are saturated, the unsaturated glycosides usually require dihydroxylation by OsO4.[47] After the establishment of the C–N glycosidic bonds between primary and secondar amines by palladium catalysis, K2OsO4·2H2O and NMO were directly added to the system for dihydroxylation reaction, so the deprotected glycosyl primary amine was originated by the one-pot method. The deprotected glycosides, with exposed hydroxyl groups, can exhibit higher biological activity. This method achieved moderate to good yields for various primary and secondary amines with diastereomeric ratios always exceeding 30:1. Additionally, we used this strategy to modify the drugs piperazine and equinox with glycosylation, and successfully synthesized amphimedoside A analogues, which exhibited cytotoxicity towards mouse leukemia cells. This strategy was also based on the Tsuji–Trost reaction mechanism and featured high stereoselectivity. However, its scope was limited to aromatic amines, and we plan to develop more strategies for N-glycoside synthesis to overcome the limitations of existing methods.
# 5
Stereoselective Synthesis of S-Glycosides
As one of the essential life-sustaining elements, sulfur can also participate in the formation of glycosides, which is also more stable to natural O-glycosides.[48] Sulfur nucleophiles can couple with unsaturated glycosyl donors at either the C1 or C3 position of the sugar ring and the formation of C–S glycosidic bonds also face the challenges in regioselectivity. S-Glycosides can serve as lead compounds in drug development, and they are more easily chemically degraded or enzymatic in the human body compared to O-glycosides. Certain S-glycosides with excellent antirheumatic and antimicrobial activities have been developed into clinical drugs, such as auranofin and lincomycin. Besides S-glycosides, 3-S-glycosides also hold significant potential as drugs and multifunctional intermediates. However, controlling regioselectivity and chemoselectivity in S-glycosylation also remains a great challenge.
Our group successfully regulated the regioselectivity of S-glycosides through the type of transition-metal catalyst (Scheme [8]).[49] 3,4-O-Carbonate-d-galactal and thiols were chosen as the starting materials yielding S-glycosides with β only stereoselectivity and high regioselectivity catalyzed by palladium in a common solvent at room temperature under open-air conditions, while cobalt catalysts resulted in products coupled at the C3 position under anhydrous conditions, with only 3S-configured products in high yields.
To evaluate the chemoselectivity of this strategy, we introduced highly active functional groups such as alcohol hydroxyl, phenol hydroxyl, amide, and aniline into the thiol acceptors. The strategy showed high functional group tolerance, as the presence of highly reactive groups in the reaction did not affect the formation of S-glycosides as well as the control of catalyst over product regioselectivity and stereoselectivity.
In the palladium-catalyzed system, the reaction follows the mechanism of Tsuji–Trost reaction. The thiol preferred to attack the anomeric carbon from the below side due to the hydrogen bonding between the oxygen anion at the C4 position of the glycosyl donor and the thiol, resulting in remarkable stereoselectivity (Scheme [9]). In contrast, the mechanism in the cobalt-catalyzed system might be partly different. After coordinating with phosphine and then being reduced by zinc from divalent to monovalent, Co(II) catalyst followed a process similar to the palladium system, including coordination with the double bond, oxidative addition, and decarboxylation. But the paths diverged after that. The thiol was more likely to coordinate with the cobalt center to form a branched allylic intermediate. Finally, the thiol launched a nucleophilic attack to the C3 position from the below side of the glycosyl donors, producing (S)-3-S-glycosides.
The intermediate of the cobalt complex was attempted to be captured to research the mechanism by several experiments. The corresponding peaks of the intermediate dppbzCo(II)(BF4)2 and its Co(I) complex reduced by sodium borohydride were successfully detected by the NMR phosphorus spectrum. To verify the necessity of zinc in the reaction, we also carried out a control experiment, and the target product was not detected without the zinc being added. After adding 10 mol% Zn powder, the reaction then was initiated, indicating Zn played a critical role in reducing the cobalt species.
Additionally, we performed DFT calculations on four possible intermediates. In the palladium-catalyzed system, if the thiophenol was coordinated with palladium from the below side, the energy of the resulting intermediate would be higher, so it was unlikely to form. Thus, the thiophenol was more inclined to coordinate with the oxygen anion at the C4 position from the upper side. Moreover, the Pd–C1 bond occurs a longer bond length and higher bond energy than the Pd–C3 bond, making it more susceptible to break when nucleophilic attacked by S, leading to the coupling product at the C1 position, thus forming the β-S-glycosides. In contrast, the cobalt system shows the opposite behavior, thus resulting in different chemoselectivity and stereoselectivity.
Besides regulating the selectivity by the type of transition-metal catalysts, we also tried to use synthetic glycosyl donors with different anomeric carbon configurations for stereospecific reaction studies (Scheme [10]).[50] In 2003, Feringa’s group[51a] and O’Doherty’s team[51b] synthesized 2,3-unsaturated-4-keto glycosyl donors, which were applied widely in O-glycosylation and natural carbohydrate syntheses. However, these de novo glycosyl donors were still not explored in the synthesis of thioglycosides. Inspired by the previous work, then the stereospecific S-glycosylation method was developed by our group, and both α- and β-S-glycosides could be gained successfully from α- and β-O-Boc unsaturated glycosyl donors, respectively.
The mechanism was proposed that the bulky phosphine ligands formed complexes with palladium catalyst firstly in the reaction. Due to the steric effect of the Boc group at the C1 position, the complex only coordinated with the anomeric carbon from the opposite side of Boc, and the complex then underwent the oxidative addition and decarboxylation to form a π-allyl-Pd intermediate. Last, thiol chose to attack the π-allyl-Pd intermediate from the opposite side of the phosphine–palladium complex due to steric reason, and the target S-glycosides are produced with high stereospecificity, releasing Pd(0) to continue the catalytic cycle.
# 6
Conclusion
In summary, our group has developed a series of glycosylation strategies with high regio-/stereoselectivity using unsaturated glycosyl through transition-metal catalysis under very mild conditions. Chemical synthetic methods of various types of carbohydrates were discussed, including 2,3-unsaturated O-, C-, N-, S-glycosides, 2-deoxy sugars, and 3-substituted sugars. Notably, O-, C-, and S-glycosylation reactions could be achieved under open-air conditions at room temperature, which is very easy to operate. Most of these approaches were demonstrated by broad substrate scopes with excellent chemo-, regio-, and stereoselectivity in high yields. The practicality was proven by functionalization and late-stage modification of natural products and drugs. In the future, more general, effective and milder glycosylation strategies are still required to develop for the autosynthesis of oligosaccharides and polysaccharide conjugates.
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Conflict of Interest
The authors declare no conflict of interest.
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Corresponding Authors
Publication History
Received: 30 April 2024
Accepted after revision: 18 June 2024
Accepted Manuscript online:
19 June 2024
Article published online:
10 July 2024
© 2024. Thieme. All rights reserved
Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany
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