Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
2,4-Dinitrobenzenesulfonamide-Directed SN2‑Type Displacement Reaction Enables Synthesis of β‑D‑Glycosaminosides Xianyang Wang, Peng Wang, Dongwei Li, and Ming Li*
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School of Medicine and Pharmacy, Ocean University of China, Key Laboratory of Marine Medicine, Chinese Ministry of Education, Qingdao 266003 China Laboratory for Marine Drugs and Bioproducts, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237 China S Supporting Information *
ABSTRACT: An efficient protocol to construct β-D-gluco-/ galactosaminosyl linkages was established using nonparticipating and strong electron-withdrawing C-2-2,4-dinitrobenzenesulfonamide (DNsNH)-directed SN2-like glycosylation of glycosyl ortho-hexynylbenzoates. The reaction is applicable to a wide range of O-, N-, and C-nucleophiles and features convenient conversion of DNsNH into AcNH in high yield under mild conditions. Oligomerization-ready trisaccharide, composed of β-D-(1→3)-glucosamino residues, has been achieved, setting a solid foundation for the synthesis of oligosaccharides associated with Neisseria meningitidis capsular polysaccharide.
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triflimides or the corresponding contact ion pair (CIP) with a variety of acceptor alcohols with good nucleophilicity. SN2-type displacement of α-glycosyl triflates generated by activation of conformationally restricted 4,6-tethered C-2-azido and C-2dinitropyridone glucosyl thioglycosides was studied by Codée and co-workers, who demonstrated that β/α-selectivity of the reaction increases with increased acceptor nucleophilicity.7 Silver-mediated heterogeneous reaction between 2-benzenesulfonamido-2-deoxyglucosyl bromide and alcohols proceeds via an associative mechanism.8 Despite these significant advances, the concurrent methods suffer from shortcomings such as strong basic or acidic conditions required for conversion of PhthN or TrocNH to naturally occurring AcNH and low β/α ratios encountered with poor nucleophilic alcohols as the acceptors. As a continuation of our interest in gold(I)-catalyzed glycosylation,9 we describe an operationally simple and efficient protocol for constructing C-2-AcNH-β-D -gluco-/galactopyransides under mild conditions. The reaction relies on Yu’s goldcatalyzed glycosylation10 of C-2-2,4-dinitrobenzenesulfonamide (DNsNH)-glycosyl ortho-hexynylbenzoates and is amenable to a wide range of acceptor alcohols with various reactivities. The reaction is suggested to follow an SN2-type displacement pathway, stereocontrollably furnishing 1,2-trans-glycosides, and features ready conversion of sulfonamide to acetamides under mild conditions, which are orthogonal to azide, PhthN, CbzNH, and TrocNH commonly used as masking amino groups in glycosamide synthesis. Importantly, although DNs and its congeners such as ortho-/para-nitrobenzenesulfonyl (o-Ns/p-
tereoselective formation of glycosidic bonds is one of the cornerstones for the synthesis of biologically and medicinally important oligosaccharides and other glycoconjugates.1 To achieve stereocontrolled glycosylations, many strategies have been developed to change the equilibrium of glycosylating species involved in the continuum of reaction mechanisms that span the bimolecular stereoinvertive SN2 process to the stereorandomizing SN1 pathway.2 It is well appreciated that the disarming effects of protecting groups destabilize the oxocarbenium ions and favor the SN2 pathway relative to the SN1 reaction. As a case in point, the sulfonate substituent as a strong electron-withdrawing group at the C-2 position is capable of stabilizing anomeric α-mannopyranosyl and rhamnopyranosyl sulfates and favoring SN2-like displacement with Onucleophiles, leading to the challenging 1,2-cis-glycosides.3 Sulfonyl groups have also been employed at positions other than O-2 to modulate the stereoselectivity of forming glycosic bonds by retarding the formation of oxacarbonium ion intermediates.4 Most of the approaches5 available for incorporating C-2-acetamido (AcNH)-β-D-gluco-/galactopyranside into oligosaccharides, peptidoglycans, glycoproteins, and mucopolysaccharides of biological relevance turn to anchimeric assistance of C-2-substituted amino groups such as phthalimide (PhthN), benzyloxyl/trichloroethyloxy carbamate (CbzNH/ TrocNH), and AcNH, whereas an SN2-like reaction has only been observed in a few examples (Scheme 1). Hashimoto demonstrated that glycosylation of C-2-AcNH-glycosyl diethyl phosphite donors is a powerful method to synthesize β-Dglycosaminosides with stoichiometric activation of the triflic imide at −78 °C.6 The reaction is postulated to proceed following an SN2-type displacement of reactive α-glycosyl © XXXX American Chemical Society
Received: February 23, 2019
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DOI: 10.1021/acs.orglett.9b00688 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 1. Strategies for Constructing β-D-Glucosaminosides
Table 1. Glycosylation of DNs-Protected Donor 3 with Various Nucleophiles
a
The ratio based on isolated yield. bCHCl3 was used as solvent. cHB(C6F5)4 (0.1 equiv) in situ generated was added.
B
DOI: 10.1021/acs.orglett.9b00688 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
achieved, attributable to intramolecular hydrogen bonding formation between the anomeric alkoxyl substituent and C-2DNsNH in α-isomers, which leads to polarity of α-glycosides that is much less than that of β-forms. To evaluate the structural effects of the electronic nature of substituents on the selectivity and efficiency of glycosylation, polar acetyl (Ac) groups, o-Ns, and p-Ns took the place of Bn groups and DNs in 3, resulting in 6−8 (Scheme 2). Significant
Ns) have been frequently exploited for the protection of amines in peptide and natural product synthesis because of easy removal under mild conditions and of ready conversion to other functionalities such as amides,11 their synthetic potential in glycosylation has not been well recognized.12 We initiated our investigations by preparing DNs-protected glucosaminosyl benzoate 3, which was stereoselectively achieved in 76% yield by selective sulfonation of 1,2-amino alcohol 1 and subsequent esterification of lactol 2 with ohexynylbenzoic acid (ABzOH) in the presence of DCC and DMAP. It was observed that the β-isomer of lactol 2 epimerized into the α-congener when applied to silica gel chromatography. Glycosylation of 3 (1.0 equiv) with diosgenin 4a (1.5 equiv) was executed as a model reaction for optimization. After scrupulous evaluation of various reaction parameters (see the Supporting Information (SI, Table S1), we found that a recipe, composed of (PhO)3PAuCl/AgB(C6F5)4 (0.15 equiv) as the catalyst in CH2Cl2, stood out to provide the desired saponin 5a in 93% yield with a β/α 20:1 ratio at 0 °C within 30 min (Table S1, entry 11). With the optimal conditions in hand, glycosylations between 3 and structurally diverse O-, N-, and C-nucleophiles were examined. The results are outlined in Table 1. In addition to diosgenin 4a, cholesterol 4b and oleanolic acid 4c as typical sapogenins were glycosylated with 3 to exclusively generate βglycosides in excellent yields. Importantly, it was found that not only primary sugar alcohols but also secondary ones were found to be competent acceptors to give disaccharides 5d−r in good to excellent yields with β/α ratio >8.6:1. These results indicate that although the stereoselectivity of glycosylations of 3 did appear to a certain extent to depend on the nucleophilicity of the acceptor alcohols (4e vs 4f),13 the reaction is amenable to both the sterically hindered axial 4-OH or 2-OH of acceptors such as 4i, 4j, and 4r as well as the less nucleophilic 4-OH such as uronic acid derivative 4k. Note that diol 4g was regioselectively glycosylated at 6-OH to furnish disaccharide 5g, leaving less reactive 4-OH untouched. Diol 4j was, however, preferably reacted at the axial 4-OH over the equatorial 3-OH, producing a regioisomeric mixture of 5j and 5j′, both with exclusive β stereoselectivity. Ready access to disaccharides 5m−p linked by β-(1→3)- and/or (1→4)-glycosidic bonds with two amino groups masked by different protecting groups offers opportunities for exploring the synthesis of structurally diverse glycosaminoglycans of importance. Other nucleophiles were also tested. Ethanol afforded ethyl βglycoside 5s as the sole product, and trifluoroethanol (TFE) gave trifluoroethyl glycoside 5t with β-isomer as the major product (β/α 3:1). Notably, due to the decreased nucleophilicity, TFE usually furnishes α-glycosides as the major isomer in glycosylation involving the glycosyl triflate intermediates.7,13c 1Adamantanol as a typical tertiary alcohol was extremely βselective, and phenol afforded a β/α 5.1:1 mixture in 79% yield. Amides are recognized to be challenging acceptors in the glycosylation.14 Asparagine 4w reacted with 3 in CHCl3 to smoothly provide β-glycopeptide 5w in 70% yield as the sole product. It is noteworthy that 1,3,5-trimethoxybenzene gave Cglycoside 5x in 94% yield with a β/α 3:1 ratio, indicating the potential for the synthesis of 2-amino-2-deoxyglycosyl aryl Cglycosides. It is usually difficult to separate an anomeric mixture when formed in a glycosylation; however, another bonus with DNsNH as the masked amino group is that when an anomeric mixture is produced, complete separation of α-isomers from βcongeners by silica gel column chromatography is easily
Scheme 2. Glycosylation of Glucosaminosyl Donors 6−8
attenuation of the stereochemical outcome for 9 arising from coupling of 6 with diosgenin 4a underpinned the ancillary role of the peripheral Bn groups in governing selectivity. Coupling of 7 and 8 with 4a and 4r provided glycosides 10−13 in 78−92% yields with excellent stereoselectivity, comparable with that of 3. These results demonstrate the potential of o- and p-Nsprotected donors for stereocontrolled glycosylations. Although it has been observed that C-2-ortho-nitrobenzyloxyl donors principally provide β-glycosides by blocking the α-face through nitro group participation,15 stereocontrolled glycosylations of 8 indicate that the o-nitro group is not necessary for stereoselective glycosylation, thus highlighting the importance of electronic effects on the glycosylations in the present case. This method could be extended to selective β-galactosaminosylation. As shown in Scheme 3, coupling of galactosaminosyl donor 14 to diosgenin 4a and primary and secondary sugar alcohols 4e and 4m afforded products 15−17 in excellent yields with a β/α ratio >8.5:1. Scheme 3. Glycosylation of Galactosaminosyl Donor 14
Variable-temperature NMR experiments16 were carried out to probe the reaction mechanism (see SI, Figure S1). It was found that 3 was completely converted to glycosyloxyisochromenylium intermediate A within 10 min by activation of stoichiometric amounts of (PhO)3PAuCl/AgB(C6F5)4 (1.4 equiv) at −50 °C in CD2Cl2. The structure of species A was C
DOI: 10.1021/acs.orglett.9b00688 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters characterized by extensive 1D and 2D NMR experiments, which revealed the anomeric chemical shifts of δH 6.87 and δC 102.9 ppm, respectively (see SI, Figures S2−S5). The anomeric proton of intermediate A resonates downfield compared to that of glycosyl triflates (δH 5.95−6.30)16 and the glycosyloxyisocoumarinium ions Yu reported17b,c (δH 6.61), implying that the anomeric center of ion A is more electron-deficient than that of glycosyl triflates and should facilitate the ensuing displacement reaction. It was observed that intermediate A was stable below −40 °C in CD2Cl2, at which point it began to hydrolyze by reaction with adventitious water in the reaction mixture and completely transformed to β-oriented lactol 2β within 45 min when warmed to −30 °C. The structure of 2β was fully characterized by NMR spectroscopy (see SI, Figures S6−S8). Based on these results and the related literature,17 we suggest a plausible mechanism. As shown in Scheme 4, the formed species
Scheme 5. One-Step Conversion of Sulfonamides into Acetamides
transform saponin 5a into 18 in 80% yield. These conditions, however, provided disaccharide 19 from 5m in 50% yield due to uncompleted acetylation of the intermediate amine. The transformation could be readily achieved by treating DNsNH with AcSH in the presence of mild base DMAP in CH2Cl2. Thus 5a, 5m, and glycopeptide 5w smoothly furnished the corresponding acetamides 18−20 in 80−94% yield at room temperature with azide and CbzNH substituents intact. This novel protocol featured by mildness and excellent chemoselectivity offers a one-step and high-yielding approach to converting DNsNH into AcNH. Neisseria meningitidis is a Gram-negative bacteria responsible for outbreaks of meningitis. Development of a well-defined glycoconjugate vaccine is greatly desired to prevent N. meningitidis infection. As a final endeavor, we set out to synthesize the repeating unit of N. meningitidis (serogroup L) capsular polysaccharide (NMCPS) as the O-antigen, which is a trisaccharide composed of a C-2-AcNH-glucopyranosyl unit joined by β-1,3-linkages.19 As shown in Scheme 6, our synthesis commenced with coupling of glucosaminosyl acceptor 21 and donor 22. Under the activation of gold catalyst, the reaction worked well to afford disaccharide 23 in 91% yield as the single product. Treatment of 23 with HF·pyridine effected the removal of the TBS group to give alcohol 24 in 89% yield without affecting sulfonamides. Compound 24 was coupled with a second aliquot of benzoate donor 21 to generate the desired trisaccharide 25 in 70% yield. For future conjugation purposes, we attempted to install a 6-azidohexanol spacer. Glycosylation of trisaccharide 25 with 6-azidohexanol was promoted by NIS/AgOTf to give glycoside 26 in an excellent yield of 98% as an anomeric β/α mixture of 6.3:1 without the conditions optimized. This experiment indicates that DNs-protected thioglycosaminosides have the potential to be attractive glycosylating agents for stereoselective glycosylations. Installation of the orthogonal TBS group in 25−27 sets a solid foundation for assembly of phosphate-linked oligomers of NMCPS. Simultaneous conversion of three sulfonamides into the acetamides was successfully achieved by treatment with AcSH and DMAP, giving trisaccharide glycoside 27 in 70% yield. In summary, we have established an efficient protocol for synthesis of β-glycosaminosides that uses C-2-DNsNH as the stereodirecting group to favor SN2-type displacement. The reaction proceeds under mild conditions and enjoys a broad substrate scope to give structurally diverse coupling products. A novel method has also been established for transforming DNsNH into naturally occurring AcNH in high yield by one step, which is compatible with the commonly used amino protecting groups such as azide, CbzNH, and TrocNH, thus offering opportunities for diversely N-substituted glycosamino-
Scheme 4. Proposed Mechanism for Glycosylation of 3
A equilibrates with transient CIP B and oxocarbenium ion C. The equilibrium heavily shifts toward associative species because C-2-DNsNH as the powerful electron-withdrawing group destabilizes oxocarbenium C. Nucleophiles preferentially attack intermediates A and/or B to stereocontrollably result in β-glucosaminosides in an SN2-type fashion. To obtain more experimental evidence for the mechanism, the below reactions were carried out. In the presence of 4 Å molecular sieves, diosgenin 4a was found to couple to the preformed intermediate A at −40 °C to stereocontrollably afford 5a in 88% yield; meanwhile, intermediate A was treated with less nucleophilic TFE at the higher temperature of −28 °C to give β-configured 5t as the sole product. These results demonstrate that glycosyloxyisocoumarinium ions A and/or CIP B are reactive enough to engage in an SN2-like displacement with nucleophiles. Additionally, we also found that TFE reacted with 3 in the presence of (PhO)3PAuCl/AgB(C6F5)4 (1.4 equiv) at 0 °C to give 5t with β-isomer as the single product in 72% yield. This result is an improvement compared to that obtained in the catalytic conditions (leading to 5u, β/α 3:1, Table 1), indicating that the intermediacy of oxocarbenium ion C does not govern the glycosylation stereoselectivity of 3, although we cannot entirely exclude the possibility of its presence in the reaction. With the DNsNH-substituted glycosides in hand, we attempted to convert DNsNH into naturally occurring AcNH (Scheme 5). Initially, conventional conditions18 involving thioacetic acid (AcSH) and Cs2CO3 in DMF was applied to D
DOI: 10.1021/acs.orglett.9b00688 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 6. Assembly of the Repeating Unit of Trisaccharide Corresponding to NMCPS
(4) (a) Tanaka, H.; Yoshizawa, A.; Takahashi, T. Direct and Stereoselective Synthesis of β-Linked 2,6-Deoxyoligosaccharides. Angew. Chem., Int. Ed. 2007, 46, 2505−2507. (b) Baek, J. Y.; Lee, B.Y.; Jo, M. G.; Kim, K. S. β-Directing Effect of Electron-Withdraw-ing Groups at O-3, O-4, and O-6 Positions and α-Directing Effect by Remote Participation of 3-O-Acyl and 6-O-Acetyl Groups of Donors in Mannopyranosylations. J. Am. Chem. Soc. 2009, 131, 17705−17713. (c) Baek, J. Y.; Kwon, H.-W.; Myung, S. J.; Park, J. J.; Kim, M. Y.; Rathwell, D. C. K.; Jeon, H. B.; Seeberger, P. H.; Kim, K. S. Directing Effect by Remote Electron-Withdrawing Protecting Groups at O-3 or O-4 Position of Donors in Glucosylations and Galactosylations. Tetrahedron 2015, 71, 5315−5320. (5) (a) Bongat, A. F. G.; Demchenko, A. V. Recent Trends in the Synthesis of O-glycosides of 2-Amino-2-deoxysugars. Carbohydr. Res. 2007, 342, 374−406. (b) Enugala, R.; Carvalho, L. C. R.; Dias Pires, M. J.; Marques, M. M. B. Stereoselective Glycosylation of Glucosamine: The Role of the N-Protecting Group. Chem. - Asian J. 2012, 7, 2482− 2501. (c) El Sayed Aly, M. R.; El Ashry, E. S. H. Recent Advances toward Robust N-Protecting Groups for Glucosamine as Required for Glycosylation Strategies. Advances in Carbohydrate Chemistry and Biochemistry; Elsevier, 2016; Vol. 73, pp 117−224. (d) Beau, J.-M.; Boyer, F.-D.; Norsikian, S.; Urban, D.; Vauzeilles, B.; Xolin, A. Glycosylation: The Direct Synthesis of 2-Acetamido-2-deoxy-sugar Glycosides. Eur. J. Org. Chem. 2018, 2018, 5795−5814. (6) (a) Arihara, R. S.; Nakamura, S.; Hashimoto, S. Direct and Stereoselective Synthesis of 2-Acetamido-2-deoxy-β-D-glycopyranosides by Using the Phosphite Method. Angew. Chem., Int. Ed. 2005, 44, 2245−2249. (b) Arihara, R.; Kakita, K.; Suzuki, N.; Naka mura, S.; Hashimoto, S. Glycosylation with 2-Acetamido-2-deoxyglycosyl Donors at a Low Temperature: Scope of the Non-Oxazoline Method. J. Org. Chem. 2015, 80, 4259−4277. (c) Arihara, R.; Kakita, K.; Yamada, K.; Nakamura, S.; Hashimoto, S. Synthesis of the Tetrasaccharide Repeating Unit from Acinetobacter baumannii Serogroup O18 Capitalizing on Phosphorus-Containing Leaving Groups. J. Org. Chem. 2015, 80, 4278−4288. (7) van der Vorm, S.; Overkleeft, H. S.; van der Marel, G.; Codée, D. C. Stereoselectivity of Conformationally Restricted Glucosazide Donors. J. Org. Chem. 2017, 82, 4793−4811. (8) Onodera, K.; Kitaoka, S.; Ochiai, H. Synthesis of 2-Amino-2deoxy-β-D-glucosides via 3,4,6-Tri-O-acetyl-2-benzyl-sulfonamido-2deoxy-α-D-glucopyranosyl Bromide. J. Org. Chem. 1962, 27, 156−159. (9) (a) Sun, P.; Wang, P.; Zhang, Y.; Zhang, X.; Wang, C.; Liu, S.; Lu, J.; Li, M. Construction of β-Mannosidic Bonds via Gold(I)-Catalyzed Glycosylations with Mannopyranosyl ortho-Hexynylbenzoates and Its Application in Synthesis of Acremomannolipin A. J. Org. Chem. 2015, 80, 4164−4175. (b) Zhang, L.; Li, L.; Bai, S.; Zhou, X.; Wang, P.; Li, M. Access to Diosgenyl Glycoconjugates via Gold(I)-Catalyzed Etherification of Diosgen-3-yl ortho-Hexynylbenzoate. Org. Lett. 2016, 18, 6030−6033. (10) (a) Li, W.; Yu, B. Gold-Catalyzed Glycosylation in the Synthesis of Complex Carbohydrate-Containing Natural Products. Chem. Soc. Rev. 2018, 47, 7954−7984. (b) Yu, B. Gold(I)-Catalyzed Glycosylation
glycans. The method has been applied to assemble the repeating trisaccharide unit of NMCPS, ready for oligomerization. Given the abundance of β-gluco- and galactosaminosidic linkages in nature, the present work should be useful in the synthesis of oligosaccharides and glycoconjugates.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00688. Experimental details and spectroscopic data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ming Li: 0000-0003-2719-5429 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (21672194 and 21272220), the National Natural Science Foundation of China−Shandong Joint Fund (U1606403), and the Shandong Provincial National Natural Science Foundation (ZR2018MB015). We also thank Dr. Blaine Pfeifer at the State University of New York at Buffalo for his revision of this manuscript.
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REFERENCES
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