Palladium-Catalyzed O - and N -Glycosylation with Glycosyl Chlorides

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jul 2021Palladium-Catalyzed O- and N-Glycosylation with Glycosyl Chlorides Shuang An†, Quanquan Wang†, Wanjun Zhu, Qikai Sun, Gang He and Gong Chen Shuang An† State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Quanquan Wang† State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Wanjun Zhu State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Qikai Sun State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , Gang He *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 and Gong Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071 https://doi.org/10.31635/ccschem.020.202000445 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Despite the significant progress in carbohydrate chemistry, there remains a pressing need for efficient and practical glycosylation methods using simple glycosyl donors and with high atom economy. Herein, a new protocol for glycosylation with glycosyl chloride donors under palladium-catalyzed conditions is developed. PdII complex serves as a Lewis acid to promote the activation of glycosyl chloride for the formation of oxocarbenium ion intermediate. This new method is operationally simple, robust, and enables efficient synthesis of both O- and N-glycosides with a broad substrate scope. In particular, it offers an easy access to a range of N-ribonucleoside analogs. Download figure Download PowerPoint Introduction Glycosides, oligosaccharides, and glycoconjugates are widely found in nature and play a broad range of biological roles.1–6 Over the past century, synthetic carbohydrate chemistry has been greatly advanced to enable the formation of various O-, N-, C-, and S-linked glycosidic bonds in high efficiency and with excellent regio- and stereochemical control.7–13 A large number of carbohydrates of highly complex structures have been successfully constructed even in automated fashion. However, many difficult challenges in glycosylation chemistry remain to be addressed. In particular, the atom economy of glycosylation reactions needs to be significantly improved.14,15 The conventional methods typically require the use of stoichiometric amount of Lewis acid promoter and, sometimes, excess amount of acid scavenger. The modern glycosylation methods often involve the use of structurally sophisticated donors. Catalytic glycosylation reactions using simple glycosyl donors could provide more "ideal" and practical methods for carbohydrate synthesis.16–24 Glycosyl chlorides are one of the most prominent donors in glycosylation chemistry.25–29 They can be readily prepared from a variety of precursors.30–33 Since the pioneering studies by Michael,34 Koenigs and Knorr,35 a series of improvements and modifications have been made to glycosylation reactions with chloride donors. However, most of these classical methods use stoichiometric amounts and often toxic reagents such as silver and mercury salts (Helferich method) as promotors.25,36–39 Over time, the Koenigs–Knorr (KK) reaction has been gradually overshadowed by glycosylation reactions with higher reactivity and broader substrate scope. Recently, catalytic versions of KK O-glycosylation using chloride donors have emerged. Notably, the groups of Ye26 and Jacobsen27 showed that glycosyl chlorides can be activated under the promotion of urea or thiourea-based organocatalysts. The group of Demchenko29 reported a simple iron(III) chloride-catalyzed O-glycosylation method to prepare various disaccharides. Herein, we report the development of a new Pd-catalyzed O-glycosylation method using glycosyl chloride donors for synthesis of a variety of alkyl, aryl, and carboxyl glycosides. Moreover, the method can be readily applied for the synthesis of a variety of N-glycosides with arylamines, carboxamides, and N-heteroarenes (Schemes 1a–1c). Scheme 1 | (a–c) Catalytic Koenigs–Knorr (KK) glycosylation using glycosyl chloride donor. Download figure Download PowerPoint Experimental Section All commercial materials were used as received unless otherwise noted. Chloroform was dried by 4 Å molecular sieves (MS). All glycosyl chloride donors were prepared based on reported procedures. Thin-layer chromatography (TLC) was performed on silica gel Huanghai HSGF254 plates. Flash chromatography was performed using silica gel (200–300 mesh) purchased from Qingdao Haiyang Chemical Co (Qingdao, China). K2CO3 (99%, J&K Chemical, Beijing, China) and Pd(OAc)2 (98%, J&K Chemical) were used in all Pd-catalyzed glycosylation reactions. NMR spectra were recorded on Bruker AVANCE AV 400 instruments, and all NMR experiments are reported in units of parts per million (ppm), using residual solvent peaks [chloroform (δ = 7.26 ppm) or tetramethylsilane (TMS) (δ = 0.00 ppm) for 1H NMR, chloroform (δ = 77.16 ppm) for 13C NMR] as internal reference. Multiplicities are recorded as: s = singlet, d = doublet, t = triplet, dd = doublet of doublets, td = triplet of doublets, br s = broad singlet, m = multiplet. High-resolution electrospray ionization (ESI) mass experiments were operated on a Q Exactive Focus Hybrid Quadrupole-Orbitrap Mass Spectrometer instrument. A representative procedure of Pd-catalyzed O-glycosylation reaction under general conditions A: A mixture of compound 1 (12.2 mg, 0.1 mmol, 1 equiv), perbenzyl mannosyl chloride donor a (84 mg, 0.15 mmol, 1.5 equiv), and Pd(OAc)2 (0.5 mg, 0.002 mmol, 0.02 equiv) in 1 mL of dry CHCl3 in an 8 mL glass vial [purged with N2, sealed with poly(tetrafluoroethylene) (PTFE) cap] was stirred at room temperature for 24 h. The reaction mixture was concentrated in vacuo and the resulting residue was purified by silica gel flash chromatography using 10% ethyl acetate in hexanes as eluents to give the O-glycosylation product 1-a as a colorless oil (57 mg, 88% yield). See Supporting Information for details. Results and Discussion Recently, we discovered a new strategy for synthesis of C-aryl glycosides via palladium-catalyzed auxiliary-directed C–H glycosylation of arenes.40 Arylcarboxamide or 2-arylacetamide substrates equipped with a N,N-bidentate 8-aminoquinoline (AQ) auxiliary react with various glycosyl chloride donors to give the corresponding aryl C-glycosides in good to excellent yield and with excellent diastereoselectivity under the catalysis of Pd(OAc)2. Mechanistic studies showed that the PdII palladacycle intermediate generated via C–H palladation can undergo oxidative addition with glycosyl chlorides and reductive elimination to give the C-glycosylated product. Our mechanistic studies also revealed that Pd(OAc)2 can act as a Lewis acid to activate glycosyl chlorides to generate glycosyl oxocarbenium intermediates as a side reaction pathway in this reaction system.41,42 Encouraged by these results, we questioned whether the Pd(II)-catalyzed conditions can be further improved to provide a general method for synthesis of O-glycosides. As shown in Scheme 2, we were pleased to find that reaction of primary alcohol 2-phenylethanol (1 equiv) with tetrabenzyl protected mannose chloride donor a (1.5 equiv) in the presence of 2 mol % of Pd(OAc)2 catalyst in CHCl3 at room temperature (rt) under N2 atmosphere for 24 h gave the desired product 1-a in excellent yield and exclusive α diastereoselectivity. Use of other carboxylate Pd catalysts such as Pd(OPiv)2 gave slightly reduced yield (56%). Solvents such as toluene, CH2Cl2, and tetrahydrofuran gave reduced yield, whereas CH3CN and dimethylformamide (DMF) gave much lower yield (see Supporting Information for optimization of reaction conditions). Use of MS did not further improve the reaction. Reaction under air atmosphere gave very similar results. Scheme 2 | Pd-catalyzed O-glycosylation of alkyl alcohols with chloride donors. Isolated yield on a 0.1 mmol scale. a2.5 mmol scale. ND, not detected. Download figure Download PowerPoint Reactions of a variety of primary alcohols with donor a proceeded well under the optimized conditions A to give the corresponding O-mannosides in exclusive α selectivity. Functional groups including aryl bromide ( 2-a), alkenes ( 3-a and 4-a), and tert-butoxycarbonyl (Boc) ( 7-a) were tolerated. Reaction of secondary alcohols such as protected threonine ( 7-a), steroid ( 8-a), and menthol ( 9-a) requires higher temperature (60 or 110 °C) and/or higher loading of catalyst (5 mol %). As shown in compounds 10-a– 12-a, reactions of mannose substrates with 1 gave the corresponding disaccharides in good yield and exclusive α selectivity. In contrast, reaction at the C3-OH of mannose acceptor did not give the desired product ( 13-a), probably due to the diminished reactivity caused by steric hindrance. Reaction of mannose donor b bearing a C2-OAc group or tetraallyl protected donor c gave similar results ( 1-b, 9-b, and 1-c). Tetra-O-acetate protected mannose donor d showed little reactivity ( 1-d). Reactions of tetrabenzyl protected glucose donor f and galactose donor h with primary alcohol gave a mixture of diastereomers ( 1-f and 1-h) under conditions A. In comparison, reactions of donors g and i bearing a C2-OAc or C2-OBz group exhibited relatively lower reactivity at rt but proceeded well to give products 1-g and 1-i in good yield and with excellent β selectivity at 60 °C (conditions B), indicating the involvement of neighboring group participation (NGP) effect. Reactions of g and i with menthol required higher temperature and proceeded with eroded diastereoselectivity (110 °C, conditions D), indicating diminished NGP effect at high temperature. L-Rhamnose donor j and diacetonide-protected D-mannofuranose donor l showed good reactivity and excellent α diastereoselectivity under the standard conditions. To our delight, reactions of D-ribofuranose donor k also proceed with high reactivity and excellent β diastereoselectivity under the Pd-catalyzed conditions for most substrates. Notably, its reaction with sterically more challenging secondary alcohol acceptors gave diminished diastereoselectivity (see 12-k). In addition to alkyl alcohols, phenols, and carboxylic acids are also glycosylated to give the corresponding aryl O-glycosides and glycosyl esters under similar conditions (Schemes 3a and 3b).43–51 Reaction of phenol with mannose donor a at rt gave compound 14-a in about 25% yield and with excellent α diastereoselectivity. The yield was increased to 78% at 60 °C (conditions C). Reaction of more electron-rich phenol (see 15-a) can take place in good yield at rt (conditions A). Reaction of protected tyrosine or tyrosine–proline dipeptide gave 16-a and 17-a in good yield. Interestingly, reactions of phenol with perbenzyl protected glucose and galactose donors f and h did not give any desired product under the standard conditions. While a large portion of the donors were unconsumed, a small portion of the donors decomposed to give the hydrolyzed byproduct bearing a C1-OH group and a trace amount of esterification by-product bearing a C1-OAc group. In contrast to donors f and h, glucose and galactose donors g and i bearing a C2-OAc group worked well to give the O-glycosylation products 14-g and 14-i in moderate to good yield and with excellent α diastereoselectivity at 110 °C (conditions D). Notably, no NGP effect was observed. L-Rhamnose donor j, D-mannofuranose l, and D-ribofuranose k reacted well with phenol to give the corresponding aryl O-glycosides in good yield and excellent diastereoselectivity under conditions C or D. The structure of 14-l was confirmed by X-ray crystallography. Scheme 3 | (a and b) Pd-catalyzed O-glycosylation of phenols and carboxylic acids with chloride donors. Isolated yield on a 0.1 mmol scale. ND, not detected. Download figure Download PowerPoint As shown Scheme 3b, the reaction of 3-methylbenzoic acid with mannose donor a in the presence of 5 mol % of Pd(OAc)2 and 2 equiv of K2CO3 base at 60 °C (conditions E) gave the glycosyl ester 20-a in 85% yield and with exclusive α selectivity. In comparison, reaction in the absence of K2CO3 gave little product (conditions C). As shown by 21-a and 22-a, reactions of alkyl carboxylic acids also worked well. Reaction of 3-methylbenzoic acid with glucose donors f and g gave 20-f and 20-g in 68% and 80% yield respectively and with exclusive β selectivity at 110 °C (conditions F). Reactions of 3-phenylpropionic acid with f and g gave 21-f and 21-g as a diastereomeric mixture respectively with opposite stereochemical preferences. Reactions of D-mannofuranose l and rhamnose j with both aryl and alkyl carboxylic acid worked well under condition F. The structure of 21-l was confirmed by X-ray crystallographic analysis. Reaction of D-ribofuranose k with 3-methylbenzoic acid gave 20-k as a diastereomeric mixture in 83% yield under condition F. The reason for the diminished diastereoselectivity is unclear at the moment. Reaction of mannose donor a with ortho-alkynylbenzoic acid did not give any desired product (see 24-a). The analogous reaction with toluenesulfonic acid and dibenzyl phosphoric acid also failed to give the corresponding O-glycosylated product 25-a and 26-a under similar conditions. Like O-glycosides, N-glycosides bearing all kinds of sugar and amine moieties are also prevalent in nature and play important roles in drug design. For example, a number of nucleotide analogs have been approved as anticancer and antiviral drugs.52,53N-glycosylation featuring an N-glycosyl carboxamide of asparagine represents an important mode of posttranslational modification of proteins.2,54N-glycosyl indoles, arginines, ureas, and arylamines have been found in many natural products such as akashin C, mannopeptimycins, and ansacarbamitocin A.55–57 In comparison with O-glycosides, the chemistry for N-glycoside synthesis is considerably less developed.58–64 The strong Lewis basicity of the corresponding amines can interfere with the common Lewis acid promoters, requiring more forced conditions for N-glycosylation reaction. We were pleased to find that this Pd-catalyzed protocol provides a simple and efficient solution for the synthesis of a variety of N-glycosides. As shown in Scheme 4, reaction of aniline with 2 equiv of mannose chloride a in the presence of 5 mol % of Pd(OAc)2 and 2 equiv of K2CO3 in CHCl3 at 60 °C for 24 h (conditions E) gave the desired product 27-a in good yield. The use of base is critical, as the control experiments without it gave only trace amount of product. No glycosylation occurred below 45 °C. In contrast to the α stereoselectivity in O-glycosylation using the same donor, 26-a was obtained in exclusive β selectivity. Such selectivity can be explained by the weak anomeric effect (nO-σ*C–N interaction) of N-glycosides and the favorable sterics of β anomer.65,66 As shown by 27-b, reaction of aniline with mannose donor b bearing a 2-OAc group also proceeded with exclusive β selectivity under the same conditions, showing no NGP effect. Scheme 4 | Pd-catalyzed N-glycosylation of amines and N-heteroarenes. Yields are based on isolated yields on a 0.1 mmol scale. a2 equiv of BSA was added. Download figure Download PowerPoint As shown in 27-a and 28-a, arylamine bearing electron-withdrawing substituents showed lower reactivity than arylamines bearing electron-donating groups. The structure of 27-a was confirmed by X-ray crystallographic analysis. Heteroaryl amine bearing multiple basic N atoms reacted well ( 29-a). The reactions of mannose donor a with other amine nucleophiles including indoline ( 30-a), primary benzamide ( 32-a), N-methoxy benzamide ( 33-a), benzylamine ( 38-a), and p-toluenesulfonamide ( 40-a) worked well under conditions E or F. In comparison, indole ( 37-a), N-methyl aniline ( 31-a), and alkyl carboxamide ( 39-a) gave little products. Moreover, reaction of N-heteroarenes including 1,3-benzodiazole ( 34-a), 1,2,3-benzotrizaole ( 35-a), 1,2,4-triazole ( 36-a), and imidazole ( 41-a) gave the corresponding N-heteroaryl N-mannosides in good yield and with excellent β selectivity under conditions E. In comparison with mannose chloride donors, other pyranose chlorides such as D-glucose f, d-galactose i, and L-rhamnose j are much less reactive for N-glycosylation (see 26-f, 26-i, and 26-j).a Their reactions with more reactive N-heteroarene accepters such as 1,3-benzodiazole and 1,2,3-benzotrizaole gave the corresponding products in moderate to good yield and with excellent stereoselectivity under slightly more forced conditions (see 34-f, 34-i, and 35-j). Notably, D-ribofuranose chloride donor k exhibits excellent reactivity for N-glycosylation under the standard conditions with 10 mol % of Pd catalyst, forming various N-ribonucleoside analogs.52,53,67–70 For example, its reactions with arylamines gave products 26-k– 28-k in good yield and with exclusive β stereoselectivity. Product 42-k was obtained in good yield under slightly modified conditions with 2 equiv of N,O-bis(trimethylsilyl)acetamide (BSA) additive. Reaction of p-toluenesulfonamide ( 43-k) and 1,3-benzodiazole ( 34-k) required more forced conditions at 110 °C and proceeded with reduced diastereoselectivity. Reactions of benzamide ( 32-k) and benzylamine ( 38-k) failed under the standard conditions.b The detailed mechanisms of these Pd-catalyzed O- and N-glycosylation reactions with chloride donors remain unclear at the moment. Control experiments suggest that cationic PdII species derived from Pd(OAc)2 serve as a Lewis acid to activate the glycosyl chloride to form a glycosyl oxocarbenium ion intermediate, which is then attacked by various nucleophiles to give the glycosylation products. In comparison with bromide and iodide donors, chloride donors are much more stable and can be easily prepared. The stability can tolerate relatively high reaction temperature without rapid decomposition. The chloride ion has lower binding affinity to PdII than bromide and iodide and can be easily displaced from Pd complexes, thus allowing facile catalyst turn over. Conclusion We have developed a new protocol for glycosylation with glycosyl chloride donors under palladium-catalyzed conditions. PdII complex serves as a Lewis acid to promote the activation of glycosyl chloride for the formation of oxocarbenium ion intermediate. This new method is simple, robust, and enables efficient synthesis of both O- and N-glycosides with a remarkably broad substrate scope. In particular, it offers an easy access to a wide range of N-ribonucleoside analogs, which are important in drug discovery and biological studies. 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