Research Article pubs.acs.org/acscatalysis
Selective Catalytic Oxidation of Unprotected Carbohydrates Kevin Chung and Robert M. Waymouth* Department of Chemistry, Stanford University, Stanford, 94305 California, United States S Supporting Information *
ABSTRACT: The development of new strategies for the direct catalytic functionalization of unprotected carbohydrates would be an enabling advance for glycoscience. Herein we report that the catalytic oxidation of unprotected carbohydrates can be carried out selectively with [(neocuproine)Pd(OAc)]2(OTf)2 (1) to generate the 3-ketoses. Catalytic aerobic oxidation can be carried out with Pd loadings as low as 1% in the presence of phenolic additives. Catalytic oxidation of a variety of unprotected pyranosides in acetonitrile or acetonitrile/water with Pd catalyst 1 with either oxygen or benzoquinone selectively generates the 3-ketoses. Minor amounts of the 4-ketoses are formed competitively, particularly in the case of pyranosides bearing axial substituents at C4 of the pyranoside. Catalytic oxidations can also be carried out in trifluorethanol, but for pyranosides bearing axial substituents at C2 or C4, selective oxidation to the 3-ketose is accompanied by epimerization to afford the equatorial 3-ketoses. Catalytic oxidation of unprotected hexafuranosides or sialic acid derivatives occurs selectively at the exocyclic diol or triol in trifluoroethanol to generate exocyclic hydroxyketones. KEYWORDS: selective oxidation, homogeneous catalysis, carbohydrates, palladium, solvent effects, aerobic, alcohol oxidation
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substrates to yield the hyroxyketones.25,26 Early attempts in our laboratories to oxidize glucose were unselective, but de Vries and Minaard reported that oxidation of equatorial alkyl glycosides with Pd catalyst 1 was highly chemoselective to afford the 3-ketoses.22 Herein we describe improved catalytic methods for the selective catalytic oxidation of unprotected pyranosides with O2, studies to assess the influence of pyranose structure and solvents on the stereo- and chemoselectivity, and selective oxidations of exocyclic alcohols in hexofuranosides and sialic acid derivatives.
INTRODUCTION Carbohydrates are the most abundant renewal carbon-based resources on the planet,1 are key components of renewable materials,2 and exhibit a diverse, critical role in life processes.3,4 The utilization of carbohydrates for bulk and fine chemical synthesis is well-known, but is limited by the formidable challenges for the selective synthesis of regiochemically and stereochemically defined sugars. Carbohydrate synthesis has historically relied on sophisticated protecting group gymnastics to isolate particular hydroxyl groups for synthetic elaboration. Exquisite stoichiometric5−8 and catalytic strategies9,10 have been developed for the selective introduction and removal of protecting groups; these methods have proven critical for the development of sophisticated automated syntheses of oligosaccharides.11 The development of new strategies for the direct catalytic functionalization of unprotected12 carbohydrates would be an enabling advance for glycoscience.3,13−15 Notable advances include the chemoselective catalytic acylation, alkylation, silylation, phosphorylation, or sulfation of hydroxyl groups.9,13,15,16 Selective catalytic oxidations of unprotected carbohydrates have historically been restricted to the enzymes,17 but recent advances in the selective catalytic oxidations of primary C6 hydroxyls utilizing TEMPO,14,18 or secondary hydroxyls by Sn19−21 or Pd complexes,22 provide exciting opportunities for the direct synthesis of natural and non-natural aldoses and ketoses. We have developed a cationic palladium catalyst [(neocuproine)Pd(OAc)]2(OTf)2 (1) (neocuproine = 2,9dimethyl-1,10-phenanthroline)23,24 that exhibits high chemoselectivity for the oxidation of vicinal diols in polyhydroxylated © XXXX American Chemical Society
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RESULTS AND DISCUSSION Improved Methods for the Oxidation of Glycosides with O 2 . The cationic complex [(neocuproine)Pd(OAc)]2(OTf)2 (1) is an effective catalyst for the selective oxidation of primary and secondary alcohols, vicinal diols, polyols,23−30 and carbohydrates.22 Both air or quinones can be used as terminal oxidants, but aerobic oxidations require Pd loadings of 10 mol %, due to the oxidative degradation of the ligand.23,26,28,29 Mechanistic studies23,30,31 had implied that the competitive oxidative degradation of the neocuproine ligand was mediated by peroxides generated in the course of oxygen reduction/hydrogen peroxide disproportionation.31 These studies inspired us to investigate the addition of sacrificial reductants to improve the catalyst lifetimes; these studies, to be reported in detail elsewhere,32 revealed that the addition of Received: May 28, 2016
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DOI: 10.1021/acscatal.6b01501 ACS Catal. 2016, 6, 4653−4659
Research Article
ACS Catalysis Table 1. Preparative Scale Oxidation of Glycosides in Acetonitrilea
a
Acetonitrile solvent, unless otherwise noted. b1 atm O2. c1.5 equiv benzoquinone. dSolvent = 10:1 CH3CN:H2O.
terminal oxidant does not influence the chemoselectivity utilizing 1. The preparative scale oxidation of a variety of pyranosides was carried out both with O2/2,6-di-isopropylphenol (diPP) (Table 1, entries 1−4) and with benzoquinone (Table 1, entries 5−8).22,26 The mild conditions of this procedure are evident from the selective aerobic oxidation of the 1-azido-β-Dglucopyranoside 4 to afford the corresponding 3-ketose 4a in 60% isolated yield (Table 1, entry 3). This compound is rich in chemical handles: alkyne addition to azides generates triazoles, and azides serve as useful amine precursors.34−36 The oxidation of 4-methoxyphenyl-β-D-glucopyranoside 5 (Table 1, entry 4) afforded the corresponding 3-ketose 5a in 64% isolated yield. The high chemoselectivity for oxidation of 3-hydroxyl is also observed for the 6-deoxy glucopyranoside 6 (Table 1, entry 5) as well as methyl-α-D-xylopyranoside 7 to generate the 3-ketoses in 71−73% isolated yields. As described in detail below, these results imply that the selectivity for oxidation at the 3-position is not due to the presence or absence of a hydroxyl at the 6-position of the glycoside. Additional preparative scale oxidations were carried out with benzoquinone as the terminal oxidant. Oxidation of methyl-α-Lfucopyranoside 8, possessing an axial hydroxyl at C4, afforded a much lower isolated yield (31%, Table 1, entry 7) of the 3ketose 8a than that observed for the related all-equatorial isomer 6-deoxy glucopyranoside 6 (71%, Table 1, entry 3). Oxidation of methyl-α-L-rhamnopyranoside 9, containing an axial hydroxyl at C2, yielded the 3-ketose 9a in 63% isolated yield. In contrast, the related C2-axial methyl-α-D-mannopyranoside was reported to give a mixture of products.22
phenols enables catalytic oxidations with 1 to be carried out with O2 at room temperature with much lower Pd loadings. To test the effectiveness of this new protocol, we investigated the influence of phenolic additives on the oxidations of a variety of glycosides22,33 with O2 (Table 1). The catalytic oxidation of 0.107 g (0.36 mmol) of octyl-β-D-glucopyranoside 3 with 1 mol % Pd catalyst 1 and 0.172 mmol of 2,5-diisopropylphenol in acetonitrile at 60 °C occurred selectively at the 3-hydroxyl position to afford the 3-ketose octyl-β-D-ribo-hexapyranoside-3ulose 3a, which could be isolated in 74% yield (eq 1). Further
studies revealed that aerobic glycoside oxidation conducted with diisopropylphenol loadings as low as 0.1 equiv relative to substrate led to high conversion. This new oxidation protocol with Pd catalyst 1 provides an expedient method for the selective oxidation of a variety of pyranosides. Selective catalytic oxidations can be carried out with low Pd loadings; the use of O2 /phenol rather than quinones as terminal oxidants considerably simplifies the isolation of the products. The high regioselectivity for oxidation at the 3-position is similar to that observed by Jäger et.al.,22 indicating26 that the nature of the 4654
DOI: 10.1021/acscatal.6b01501 ACS Catal. 2016, 6, 4653−4659
Research Article
ACS Catalysis Table 2. Chemoselectivities for Oxidation of Glycosides in Acetonitrilea
a b
Reactions carried out at 50 °C in CD3CN:D2O 10:1, with 1.5 equiv of benzoquinone (vs substrate), with dimethylsulfone as internal standard. 0.15 mmol scale. c0.10 mmol scale. dNeat CD3CN solvent. e1 mol % Pd with 1 atm O2 and 0.05 mmol of 2,5-di-isopropylphenol.
to spectra of products isolated from the preparative experiments, as well as those reported in the literature (unless otherwise noted, see Supporting Information). The oxidation of the equatorial glycosides methyl α-Dglucopyranoside 12, 6-deoxy glucopyranoside 6, and methyl-αD-xylopyranoside 7 in 10/1 CD3CN/D2O at 50 °C with benzoquinone as the terminal oxidant afforded the corresponding 3-ketoses with selectivities of 76−88%, but also generated the 4-ketoses as the predominant minor products (Table 2, entries 1−3). The oxidation of pyranosides containing an axial hydroxyl are less selective, unless constrained as the bicyclic 1,6anhydropyranoses. Oxidation of methyl-α-L-fucopyranoside 8, possessing an axial hydroxyl at C4, was unselective, affording a mixture of the 3-ketose 8a and 4-ketose 8b in almost equal yields (38% and 42%, respectively, Table 2, entry 7). In addition, a minor amount of the epimerized 3-ketose 8c (2%) could also be identified. Oxidation of methyl-β-D-arabinopyranoside 13, bearing an axial hydroxyl at C4 but lacking a substituent at C5, exhibited a modest but slightly higher
Oxidation of the conformationally constrained 1,6-anhydropyranoses 10 and 11, bearing an axial C3 hydroxyl, occurred selectivity at the 3-position to afford the corresponding 3ketoses 10a and 11a in 66% and 91% isolated yields, respectively. 1,6-Anhydropyranoses serve as useful synthetic intermediates.37−41 Oxidation of 1,6-anhydropyranoses thus provides a way to selectively oxidize mannose and galactose derivatives at C3, as the unconstrained glycosides methyl-α-Dmannopyranoside and methyl-α-D-galactopyranoside are not selectively oxidized with Pd catalyst 1.22 Chemoselectivity of Pyranoside Oxidation: Identification of Minor Products. To assess the influence of the structure of the pyranoside on the oxidation chemoselectivity, we carried out a series of experiments to identify the nature of the major as well as minor products formed in the catalytic oxidations of a variety of pyranosides with Pd catalyst 1. These experiments were carried out at 50 °C in CD3CN:D2O 10:1, at 3 mol % Pd with 1.5 equiv of benzoquinone. The identities of the major and minor products were determined by 2D NMR directly from the crude reaction mixtures, and were compared 4655
DOI: 10.1021/acscatal.6b01501 ACS Catal. 2016, 6, 4653−4659
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ACS Catalysis
also oxidized selectively at C3 implies that the substituents at C5 do not provide a steric bias against oxidation at C4. These results are complementary to the selectivities observed with catalytic Sn complexes, which typically generate the 4-ketose from oxidation of the 4-axial hydroxyl of the galactopyranosides.20 The conformationally constrained 1,6-anhydropyranoses 10 and 11 also oxidize selectively at the 3-position, even though the C3 hydroxyl is constrained in an axial orientation. The relatively modest range of pyranosides investigated (Tables 1 and 2) does not provide a clear picture of the factors which govern this remarkable selectivity for the regioselective oxidation of unprotected glycosides at the C3 position. Previous kinetic studies26 had implicated that the facile oxidation of vicinal diols was due to the formation of chelating Pd hydroxyalkoxides. The formation of chelating structures of the Pd with the vicinal diols at C3 and C4 (Figure 1) could
selectivity for the 3-ketose 13a (51%) than 8, but also generated a significant amount of the 4-ketose 13b (21%). We have previously reported26 that conformationally locked cyclohexane-1,2-diols bearing both an axial and an equatorial alcohol were preferentially oxidized at the axial alcohol. Thus, the lower selectivity observed for the oxidation of C4 axial pyranosides (8, 13) may be a consequence of a competitive selectivity for oxidation of axial hydroxyls and what appears to be an intrinsic preference for oxidation at the 3-position. The low selectivity observed for the C4-axial pyranosides 8 and 13 correlates to observations by de Vries and Minnaard22 who had reported that oxidation of methyl-α-D-galactopyranoside, bearing an axial C4 hydroxyl group, was oxidized unselectively with Pd catalyst 1. In contrast, pyranosides such as 9, bearing an axial hydroxyl at C2, do not undergo oxidation at the 2-position, but generate a mixture of the 3-ketose 9a (61%) and the 4-ketose 9b (19%, Table 2, entry 6). de Vries and Minnaard reported that methylα-D-mannopyranoside, bearing an axial C2 hydroxyl group, oxidizes unselectively to unidentified products.22 We propose that the oxidation at the 2-position is disfavored electronically. Previous studies had shown that the Pd catalyst 1 oxidizes polyols selectively to hydroxy ketones,24−26 but the resulting hydroxyketones are resistant to further oxidation. This we attribute to the reticence of Pd alkoxides α to a ketone or acetal to undergo β-H elimination with Pd catalyst 1. While oxidation of the conformationally constrained 1,6anhydropyranoses 10 and 11, bearing an axial C3 hydroxyl, occurred selectivity at the 3-position with high selectivity (80% and 90%, respectively, see Table S2 of Supporting Information), the oxidation of 1,6-anhydro-β-D-glucopyranoside 14, bearing axial hydroxyls at C2, C3, and C4, yielded the 3-ketose in lower selectivity (58% NMR yield), and approximately 15% of another product that could not be identified. The lower selectivities observed for the all-axial substrate 14 may be a consequence of all-trans arrangements of the vicinal diols, which would preclude chelation of the vicinal diols to the Pd center. We had previously proposed that chelation of vicinal diols was responsible for the faster rate of vicinal diol oxidation relative to that of alcohols lacking a vicinal OH group;26 nevertheless, the Pd catalyst 1 is capable of oxidizing trans diaxial vicinal diols, albeit with lower selectivity than axial− equatorial vicinal diols.26 The results of Tables 1 and 2 reveal that oxidation of pyranosides bearing equatorial hydroxyls occurs selectively to generate the 3-ketoses in isolated yields of 60−74%. The high selectivity for oxidation of equatorial pyranosides at the 3position is accompanied by a minor amount of oxidation at the 4-position, which depends on the substituent pattern of the pyranoside, but does not appear to be influenced by the nature of the terminal oxidant (O2 vs benzoquinone) or by the presence or absence of water in CH3CN (see Supporting Information). For unprotected alkyl or azido pyranosides containing equatorial hydroxyl substituents, the selectivities for the 3-ketose are high (70−88%); for pyranosides containing axial hydroxyl substituents at C2 or C4, the selectivity for the 3ketoses are lower, and in the case of methyl-α-L-fucopyranoside 8, bearing an axial hydroxyl at C4, there is a slight preference for the 4-ketose. The high selectivity for oxidation to the 3ketose is not restricted to equatorial glucopyranosides or 2desoxyglucopyranosides,22 but extends to the 6-deoxypyranosides 2 and xylopyranosides 7 as well. The observation that 6deoxy glucopyranoside 6 and methyl-α-D-xylopyranoside 7 are
Figure 1. Proposed chelation and β-H elimination to generate 3ketose (a) or 4-ketose (b).
rationalize the competitive oxidation at the C3 and C4 hydroxyls. Nevertheless, the stereoelectronic factors which favor β-H elimination at the C3 C−H bond (Figure 1a) remain unclear and are the subject of ongoing studies. Oxidation in Fluorinated Alcohol Solvents. The high solubility of glycosides in fluorinated alcohols, the resistance of these solvents to oxidation, and their H-bonding ability prompted us to investigate these solvents for the oxidation of unprotected carbohydrates. Catalytic oxidation of the glucopyranoside 12, 6-deoxy-α-D-glucopyranoside 6, and methyl-αxylopyranoside 7 in 2,2,2-trifluoroethanol (TFE) afforded a slightly higher selectivity (3−6% improvement) for C3 oxidation relative to the analogous oxidation22 in acetonitrile/ water (Tables 2 and 3). In contrast, oxidation of methyl-α-Lfucopyranoside 8 in TFE afforded, as the major product, the epimerized 3-ketose 8c (73% NMR yield) along with the 4ketose 8b (11%), and only a minor amount of the nonepimerized 3-ketose 8a (3%). Preparative oxidation of 8 on a 1.7 mmol scale with 3 mol % Pd in trifluoroethanol afforded the epimerized ketose 8c as a crystalline solid in 64% isolated yield after 14 h at 50 °C (Table 3, entry 4). A similar selectivity for 8c was observed when the oxidation was carried out in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). Preliminary studies reveal that when the catalytic oxidation of the 4-axial fucopyranoside 8 is carried out in deuteriated DOCH2CF3 (TFE-d1), no deuterium is incorporated into the epimerized ketose 8c. This latter observation would imply that epimerization is not a consequence of keto−enol equilibration of the 3-ketose, but further studies are underway to address the mechanism of this oxidation−epimerization pathway. The one-step epimerization and oxidation of 8 provides an attractive method of generating 8c, the equatorial ketoglycoside of the rare sugar 6-deoxy-L-glucose (L-quinovose).42 Treatment of 8c with hydroxylamine produced the oxime 8d as 4656
DOI: 10.1021/acscatal.6b01501 ACS Catal. 2016, 6, 4653−4659
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ACS Catalysis Table 3. Chemoselectivities for Oxidation of Glycosides in Fluorinated Alcohol Solventsa
Reactions carried out at 50 °C in TFE with 0.1 mmol of substrate, at 3 mol % Pd with 1.5 equiv of benzoquinone (vs substrate), with dimethylsulfone as internal standard.
a
a mixture of E/Z isomers in 79.6% yield. Similar oximes have been used to form N−O glycosidic linkages, motifs present in the antitumor agents calicheamicin and esperamicin.43−45 Addition of methoxylamine to 8c generated the methyl oxime 8e as a mixture of E/Z isomers, which was cleanly hydrogenated to the amine 8f. The peracetylated aminosugar 8g was also efficiently formed (eq 2).
substituted glycoside 18 preferentially generated the 3-ketose 18a, but upon silica gel chromatography, partial elimination of the azide occurred to afford a mixture of 18a and α,βunsaturated pyrone 18b. In trifluoroethanol, oxidation of a variety of pyranosides bearing axial hydroxyls occurs selectively to give the epimerized 3-ketoses (Table 4, entries 5−7). Oxidation of methyl-αgalactopyranoside 20 in TFE at 50 °C affords methyl-α-D-ribohexanpyranoside-3-ulose 12a in 70% isolated yield. In acetonitrile/water, oxidation of methyl-α-galactopyranoside 19 was reported to be unselective22 yielding a uncharacterized mixture of products, whereas oxidation in TFE affords 12a in excellent yield, albeit as the epimerized ketose. The ketose 12a can also be generated selectively by oxidation of methyl α-Dglucopyranoside 12 with catalyst 1 in acetonitrile/water22 (see also Table 2, entry 1). Similarly, oxidation of other pyranosides bearing axial C4 hydroxyls in TFE, such as methyl-α-L-fucopyranoside 8 (eq 2) or methyl-β-D-arabinopyranoside 13, selectively yields the epimerized ketoses 8c and 13a in isolated yields of 64% and 56%, respectively. Under similar conditions, methyl-α-Lrhamnopyranoside 9, a glycoside bearing a C2 axial hydroxyl, was also preferentially converted to the epimerized ketose 8c in 45% isolated yield (Table 3, entry 7). The formation of the ketose 8c from oxidation of either 8 or 9 with Pd catalyst 1 in TFE indicates that the oxidation of pyranosides bearing axial substitutuents at the C2 and C4 positions is selective for the 3ketose, but is accompanied by epimerization to generate the equatorial ketoses. The selective oxidation/epimerization of either L-fucopyranoside 8 or L-rhamnopyranoside 9 provides a selective synthesis of 8c, the 3-ketose of 6-deoxy-L-glucose (L-quinovose).42 Moreover, selective oxidation of methyl-β-D-arabinopyranoside 13 in TFE affords 13a, the 3-ketose of L-xylopyranoside. Thus, this oxidation, epimerization sequence provides a strategy for generating the rare 3-ketoses of the L-sugars.42 The high selectivity of Pd catalyst 1 is also manifested in the selective oxidation of exocyclic polyols in hexafuranosides and sialic acid derivatives (Table 4, entries 8−10). Oxidation of the unprotected octyl-β-D-galactofuranoside 21 at 50 °C with Pd catalyst 1 in TFE occurred selectively at the exocyclic secondary alcohol to provide 21a in 73% isolated yield. While C5 hydroxyl oxidations of hexofuranosides have been demonstrated on
Preparative Oxidations in Trifluoroethanol. The different selectivities observed in TFE, coupled with the high solubility of a series of sugars in this solvent, encouraged us to investigate a series of preparative-scale oxidations in TFE (Table 4). Initially, aerobic conditions were investigated; while the use of 2,6-diisopropylphenol as an additive was similarly able to increase the yields of the C3 oxidized ketose, the overall yields using aerobic conditions in TFE were moderate at best (see Table S3 in the Supporting Information). The benzoquinoneassisted oxidation of methyl 2-acetamido-2-deoxy-α-D-glucopyranoside 16 or methyl 2-acetamido-2-deoxy-β-D-glucopyranoside 17 in TFE afforded the corresponding 3-ketoses 16a and 17a in 63% or 69% isolated yields, respectively. The oxidation of 16 in acetonitrile/water has previously been reported,22 but we were unable to effect this oxidation efficiently in this solvent mixture. However, oxidation of 16 in TFE at 60 °C gave 16a in 77% by 1H NMR after just 1 h, and a 63% isolated yield when carried out for 3.5 h (Table 4, entry 1). The β-anomer 17 behaved similarly and could be oxidized to the 3-ketose in 69% isolated yield, which slowly dimerized.20,26 Acylation immediately after oxidation with acetic anhydride affords the stable peracylated derivative of ketose 17a in solid form, with 37% overall yield over two steps. Oxidation of the acetamido-azido4657
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ACS Catalysis Table 4. Preparative Scale Oxidation of Glycosides in Trifluoroethanola
a
1.5 equiv of benzoquinone unless otherwise stated. bCombined yield of mixture.
acetonide protected systems (Table 3, entry 8),47,48 the selective oxidation of the exocyclic alcohol in the presence of the unprotected vicinal diol of 21 is noteworthy. Oxidation of N-acetylneuraminic acid methyl ester 22 occurs selectively at C8 to afford the exocyclic hydroxyketone 22a in 49% isolated yield. The selective oxidation of 22 is remarkable, given that it contains multiple exocyclic secondary alcohols as well as a C3 hydroxyl, all of which are known to be rapidly oxidized in other contexts.25,26 The neuraminic (sialic) acids exhibit diverse biological roles and are important components of the oligosaccharide chains of mucins, glycoproteins and glycolipids.46 In addition, natural and artificial sialic acid derivatives have shown antiviral activity through neuraminidase inhibition.46−48
at C4 of the pyranoside. In trifluoroethanol solvent, catalytic oxidation of pyranosides bearing axial substituents at the C2 and C4 positions is selective for the 3-ketose, but is accompanied by epimerization to generate the equatorial ketoses. Catalytic oxidation of unprotected hexafuranosides or sialic acid derivatives occurs selectively at the exocyclic diol or triol to generate exocyclic hydroxyketones. These results reveal that catalytic oxidations of unprotected carbohydrates, long restricted to the enzymes, can now be carried out efficiently with appropriate homogeneous catalysts.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01501. Materials and methods, synthetic procedures, characterization of products, NMR spectra, and chromatograms (PDF)
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CONCLUSION In summary, we describe a method for the selective catalytic oxidation of unprotected carbohydrates with Pd loadings as low as 1−2% for Pd catalyst 1. Catalytic oxidation of a variety of unprotected pyranosides in acetonitrile or acetonitrile/water with Pd catalyst 1 with either oxygen or benzoquinone selectively generates the 3-ketoses. The selectivity for the 3ketoses is highest for equatorial glycosides; the 4-ketoses are formed competitively, particularly in the case of axial hydroxyls
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 4658
DOI: 10.1021/acscatal.6b01501 ACS Catal. 2016, 6, 4653−4659
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ACS Catalysis
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ACKNOWLEDGMENTS This work was supported by the Department of Energy (DE SC0005430). K.C. acknowledges a Stanford Graduate Fellowship.
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DOI: 10.1021/acscatal.6b01501 ACS Catal. 2016, 6, 4653−4659
