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Glycobiology Pages 901-904  

1-Thio-[beta]-d-galactofuranosides: synthesis and evaluation as [beta]-d-galactofuranosidase inhibitors
Introduction
Results and discussion
Materials and methods
Acknowledgments
References

1-Thio-[beta]-d-galactofuranosides: synthesis and evaluation as [beta]-d-galactofuranosidase inhibitors

1-Thio-[beta]-d-galactofuranosides: synthesis and evaluation as [beta]-d-galactofuranosidase inhibitors

Carla Marino, Karina Mariño, Luiz Miletti1, M.Julia Manso Alves1, Walter Colli1, Rosa M.de Lederkremer2

CIHIDECAR, Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428, Buenos Aires, Argentina and 1Instituto de Química, Departamento de Bioquímica, Universidade de São Paulo, CP 260777, CEP05599-970, PP, Brazil

Received on December 23, 1997; revised on March 5, 1998; accepted on March 10, 1998

[beta]-d-galactofuranosidase is a good chemotherapeutic target for the design of inhibitors, since [beta]-d-galactofuranose is a constituent of important parasite glycoconjugates but is not present in the host mammals. With this aim, we have synthesized for the first time alkyl, benzyl and aryl 1-thio-[beta]-d-galactofuranosides by condensation of penta-O-benzoyl-[alpha],[beta]-d-galactofuranose with the corresponding thiols, in the presence of SnCl4 as catalyst. The complete chemical and spectroscopical characterization of these compounds showed that the reaction was stereoselective. Debenzoylation with sodium methoxide afforded the [beta]-S-galactofuranosides in high yield. The thioglycosides were tested as inhibitors of the [beta]-d-galactofuranosidase of Penicillium fellutanum, using for the first time 4-nitrophenyl-[beta]-d-galactofuranoside as chromogenic substrate. The 4-aminophenyl-1-thio-[beta]-d-galactofuranoside, obtained by catalytic hydrogenation of the nitrophenyl derivative, was the best inhibitor being then an adequate ligand for the preparation of an affinity phase aimed at the isolation of [beta]-d-galactofuranosidases from different sources. Also the inhibitory activity of d-galactono-1,4-lactone was shown.

Key words: affinity ligands/galactofuranose/ galactofuranosidase inhibitors/thio-[beta]-d-galactofuranosides

Introduction

Galactose in the furanoic configuration is a component of pathogenic bacteria, protozoa, and fungi (Lederkremer and Colli, 1995). For instance, internal [beta]-d-galactofuranosyl units are present in the cell wall arabinogalactan of Mycobacterium tuberculosis (Daffe et al., 1990), in the lipopolysaccharide of Klebsiella pneumoniae (Whitfield et al., 1991), and in the lipophosphoglycan of Leishmania (Turco and Descoteaux, 1992). Terminal nonreducing residues of Galf are found in the lipopeptidophosphoglycan (LPPG) (Lederkremer et al., 1980, 1991) and in the O-linked oligosaccharides from unique mucins of the parasite Trypanosoma cruzi (Previato et al., 1994; Acosta Serrano et al., 1995). Interestingly, the presence of [beta]-d-Galf units in the mucins is dependent on the strain (Previato et al., 1995), and also a microheterogeneity with respect to the galactofuranose content was determined in the LPPG (Lederkremer et al., 1991). In Penicillium fellutanum the percentage of galactofuranose in the extracellular peptidophosphogalactomannan decreases with the age of the culture (Rietschel-Berst et al., 1977). These structural variations in the glycoconjugates could be due to the action of a [beta]-d-galactofuranosidase. An exo-[beta]-d-galactofuranosidase was isolated from only two sources, Penicillium charlesii (now P.fellutanum; Rietschel-Berst et al., 1977) and Helminthosporium sacchari (Daley and Strobel, 1983). The absence of galactofuranose in mammal glycoconjugates suggests that the enzymes involved in the metabolism of this sugar could be a good target for the design of drugs.


Scheme 1.

It is known the importance of thioglycopyranosides as competitive inhibitors of glycosidases (Steers et al., 1971; Blanc-Muesser et al., 1978) and also as ligands for affinity chromatography (Pazur, 1981). We now describe for the first time the stereoselective synthesis of 1-thio-[beta]-d-galactofuranosides and their use in inhibition studies of the [beta]-d-galactofuranosidase of P. fellutanum. The use of 4-nitrophenyl-[beta]-d-galactofuranoside, previously synthesized in our laboratory (Varela et al., 1986), as substrate for the enzyme, is also reported for the first time.

Results and discussion

A wide variety of methods have been developed for the preparation of 1-thioglycopyranosides (Ferrier and Furneaux, 1980; Tropper et al., 1992). However, reports on the synthesis of thioglycofuranosides are scarce. The ethyl 1-thio-[alpha]-d-galactofuranoside has been prepared by partial hydrolysis of d-galactose diethyl dithioacetal (Wolfrom et al., 1959), and recently, the benzylated derivative was used as glycosyl donor for the preparation of [alpha]-galactofuranosides with partial stereoselectivity (Osumi et al., 1996).

In previous works from our laboratory, we have demonstrated the efficiency of SnCl4 as catalyst in the synthesis of 1,2-trans glycofuranosides from peracylated precursors (Marino et al., 1989, 1995, Lederkremer et al., 1990). We have now used this method for the preparation of 1-thio-[beta]-d-galactofuranosides with the same efficiency as in the case of the oxygenated analogues. Penta-O-benzoyl-[alpha],[beta]-d-galactofuranose (1) was S-glycosylated with alkyl, benzyl and aryl-thiols, in the presence of SnCl4 as catalyst. The thiogalactofuranosides 2-6 (Scheme 1) where thus synthesized with good yields (85-95%). As in the case of O-glycosylation of 1 (Marino et al., 1989), only the [beta]-anomers were obtained, owing to the anchimeric participation of the benzoyloxi group at C-2.

1H NMR spectra (Table I) of compounds 2-6 confirm the [beta]-configuration, showing the anomeric signals as broad singlets (J1,2 < 1 Hz). Also the other signals appear similarly as in the spectra of the O-glycoside analogs.

Deacylation of 2-6 with sodium methoxide afforded compounds 7-11 as crystalline products in good yields (85-94%). Catalytic reduction of the nitro group of 11 with hydrogen over Pd/C, afforded the 4-aminophenyl 1-thio-[beta]-d-galactofuranoside (12) in 96% yield.

The 13C NMR spectra of compounds 2-12 (Table II) showed the characteristic pattern of the analogue [beta]-d-galactofuranosyl-1-O-glycosides (Marino et al., 1989), with the signals corresponding to C-2 and C-4 appearing over 80 ppm. Signals of the anomeric carbons of the thioglycosides, when compared with those of the O-glycosides, appeared shifted upfield due to the shielding effect of the sulfur atom (Bock and Pedersen, 1983).

Comparison of the coupling constants (Table I) of benzoylated 1-thioglycosides (2-6) with the debenzoylated products (7-12) led us to the observation that a change of conformation took place. Benzoylated compounds (2-6) showed similar values for the vicinal coupling constants (J1,2 < 1; J2,3 < 1; J3,4 ~ 5) than the oxygenated glycosides, for example 4-methylumbelliferyl-[beta]-d-galactofuranoside (Marino et al., 1995). This indicated that they populate mainly the 1E = 1To = Eo segment of the pseudorotational itinerary, having the anomeric substituent in a quasi-axial position (Figure 1). Compounds 7-12 showed strikingly different vicinal coupling constants (J1,2 ~5.5; J2,3 ~5.5; J3,4 ~3.2), suggesting a conformational shift towards the 3E = 2T3= E3 region of the circle, with the anomeric sulfur atom in a quasi equatorial position. This fact may reflect the weaker anomeric effect of this atom (Zunszain and Varela, 1995).

For the inhibition assays the [beta]-d-galactofuranosidase of Penicillium fellutanum, and 4-nitrophenyl [beta]-d-galactofuranoside (13, Varela et al., 1986) as substrate, were used. Previous reports evaluated the enzymatic activity of galactofuranosidases measuring the amount of reducing sugar released from the natural peptidophosphogalactomannan (Rietschel-Berst et al., 1977) or from synthetic methyl [beta]-d-galactofuranoside (Daley and Strobel, 1983). The chromogenic substrate 13 proved to be efficient as in the case of other glycosidases (Wallenfels and Malhotra, 1961), showing a typical kinetic curve.

In order to study the inhibitory activity of compounds 7-12,enzymatic reactions were performed in their presence, at concentrations ranging from 0.05 to 1.25 mM (Figure 2). While compounds 7-11 showed only weak inhibitory activity (IC50 ~ 0.6 mM), [beta]-d-galactofuranosidase was very sensitive to inhibition by 12. More than 90% inhibition of activity was produced at concentrations < 0.2 mM with an IC50 of ~0.08 mM.

Table I. 1H NMR (200 MHz) chemical shifts of compounds 2-12
Comp. [delta] (ppm), J (Hz)
H-1 (J1,2) H-2 (J2,3) H-3 (J3,4) H-4 (J4,5) H-5 H-6,6[prime] Others
2a 5.63 (<1) 5.51 (1.5) 5.67 (4.5) 4.84 (4.4) 6.09 4.75* nPr: 2.67 (m); 1.70 (c); 0.99 (t)
3a 5.74 (<1) 5.53 (1.4) 5.68 (5.2) 4.86 (4.5) 6.11 4.76* iPr: 3.2 (m; J=6.8); 1.37 (2d)
4a 5.51 (<1) 5.51 (<1) 5.65 (4.8) 4.82 (4.5) 6.10 4.70* PhCH2: 3.96 (d);3.84 (d); Jgem = 3.7
5a 5.89 (<1) 5.72 (<1) 5.77 (5.2) 5.00 (4.5) 6.14 4.79*  
6a 6.01 (<1) 5.68 (<1) 5.78 (5.1) 4.92 (4.5) 6.11 4.77*  
7b 5.0 (5.0) 3.95 (5.5) 4.02 (7.2) 3.88 (3.2) 3.74 3.59*  
8b 5.16 (5.9) 3.90 (5.7) 4.11 (7.3) 3.84 (3.2) 3.81 3.68*  
9b 5.04 (4.8) 4.06*   3.59 3.82 3.59* PhCH2: 3.93 (s)
10b 5.25 (5.4) 4.02 (5.6) 4.10 (7.3) 3.88 (3.2) 3.77 3.59*  
11b 5.52 (5.1) 4.13*   4.00 3.81 3.60* Ph: 8.15(d), 7.63 (d)
12b 5.09 (4.7) 4.00 (5.5) 4.12 (7.2) 3.81*   3.6* Ph: 7.40(d), 6.85 (d)
aCl3CD.
bD2O; *center of a complex signal.

Table II. 13C NMR (25.3 MHz) Chemical shifts (ppm) of compounds 2-12
Comp. C-1 C-2 C-3 C-4 C-5 C-6 S-Substituent
2a 88.6 81.0 77.9 82.9 70.3 63.4 nPr: 33.3, 23.0, 13.4
3a 87.9 81.1 78.0 83.2 70.4 63.5 iPr: 35.9, 23.8, 23.6
4a 87.9 81.8 78.6 83.0 70.7 63.8 PhCH2: 35.4
5a 91.3 81.5 77.8 82.4 70.3 63.4  
6a 89.6 82.3 77.6 82.3 70.1 63.1  
7b 89.1 81.4 76.7 82.0 71.2 63.5 nPr: 33.5, 23.7, 13.5
8b 88.0 81.5 76.5 81.9 71.2 63.5 iPr: 36.5, 24.0, 23.7
9b 90.5 83.5 78.9 84.1 73.0 65.5 PhCH2: 37.2
10b 91.1 81.1 76.5 82.5 71.2 63.7  
11c 89.5 81.6 75.8 82.6 70.0 62.4  
12b 91.1 81.2 76.6 82.8 71.3 63.6  
aCl3CD.
bD2O.
cDMSO-d6.


Figure 1. Conformations of 1-O and 1-S-galactofuranosides on the basis of their coupling constants (3JH,H).


Figure 2. Effect of concentration of 1-thio-[beta]-d-galactofuranosides on the enzymatic activity of [beta]-d-galactofuranosidase from Penicillium fellutanum. Incubations were as described in the Material and methods section. The amount of 4-nitrophenol released from 4-nitrophenyl-[beta]-d-galactofuranoside was determined as a measure of galactofuranosidase activity. The numbers indicate the 1-thio-[beta]-d-galactofuranoside added.

Because of the related structure, aldonolactones have been tested as inhibitors of glycopyranosidases. For example, d-glucono-1,5-lactone has shown to be a good inhibitor for [beta]-glucosidases (Levvy et al., 1964). However, in the case of a human liver acid [beta]-d-galactopyranosidase, the d-galactono-1,4-lactone (14), with a different ring size, has been used for the elution in an affinity chromatography purification (Miller et al., 1977), suggesting interaction with the enzyme. These reports led us to test 14 as inhibitor of the [beta]-galactofuranosidase. The lactone showed a good activity, with IC50 0.02 mM, even lower than for compound 12.

The present study indicates that 4-aminophenyl 1-thio-[beta]-d-galactofuranoside (12) with an adequate group for the coupling is a suitable ligand for affinity chromatography aimed to the isolation of [beta]-d-galactofuranosidases from different sources, and lactone 14 an adequate compound for the elution of the enzyme from the solid phase. Thioglycosides are also among the most useful glycosyl donors in convergent synthesis of oligosaccharides (Garegg, 1997); thus, benzoylated 1-thioglycofuranosides 2-6 would be interesting tools for the synthesis of galactofuranosyl oligosaccharides of biological significance.

Materials and methods

All reagents were analytical grade. Synthetic reactions were monitored by TLC on precoated aluminum plates of silica gel 60 F254 (Merck) with 9:1 toluene-EtOAc (solvent a) for compounds 2-6 and 9:1 EtOAc-MeOH (solvent b) for 7-12. The spots were visualized by exposure to UV light and by spraying the plates with 10% (v/v) H2SO4 in EtOH followed by heating. Column chromatography were performed on silica gel 60 (Merck). NMR Spectra were recorded with a Bruker AC 200 spectrometer. Melting points were determined with a Thomas-Hoover apparatus. Optical rotations were measured with a Perkin-Elmer 343 polarimeter. 4-Nitrophenyl-[beta]-d-galactofuranoside was synthesized as described previously (Varela et al., 1986). P.fellutanum (previously P.charlesii G. Smith NRRL 1987) was a kind gift from John Gander, Department of Microbiology and Cell Science, University of Florida, Gainesville, FL. P.fellutanum was grown as described previously (Preston and Gander, 1968). The filtered medium of a stationary culture (12 days)was used as the enzyme source (Rietschel-Berst et al., 1977). Protein content was determined using bovine serum albumin as standard (Bradford, 1976).

Thioglycosylation of per-O-benzoyl-[alpha],[beta]-d-galactofuranose. General Procedure:

To a solution of per-O-benzoyl-[alpha],[beta]-d-galactofuranose (1,1.40 g, 2.0 mmol; Varela et al., 1986) in dry CH2Cl2 (10 ml) cooled to 0°C, SnCl4 (0.25 ml, 2.1 mmol) was added. After 10 min of stirring, RSH (2.2 mmol) was added. Two hours later the mixture was diluted with CH2Cl2, extracted with NaHCO3 (sol. sat.), and dried (MgSO4); the solvent was evaporated, and the syrup was crystallized from EtOH. Analytical samples were obtained by column chromatography purification.

The following S-galactofuranosyl glycosides were obtained:

nPropyl 2,3,5,6-tetra-O-benzoyl-1-thio-[beta]-d-galactofuranoside (2): 1.11 g, 85%, RF 0.62, mp 101-102°C, [[alpha]]D -60° (c1, CHCl3). Anal. Calc. for C37H34O9S: C, 67.88; H, 5.23; S, 4.90. Found: C, 67.95; H, 5.07; S, 4.95.

iPropyl 2,3,5,6-tetra-O-benzoyl-1-thio-[beta]-d-galactofuranoside (3): 1.19 g, 91%, RF 0.72, m.p. 92-93°C, [[alpha]]D -48° (c 1, CHCl3). Anal. Calc. for C37H34O9S: C, 67.88; H, 5.23; S, 4.90. Found: C, 67.63; H, 4.98; S, 4.78.

Benzyl 2,3,5,6-tetra-O-benzoyl-1-thio-[beta]-d-galactofuranoside (4): 1.33 g, 95%, RF 0.61, m.p. 150-151°C, [[alpha]]D -96° (c 1, CHCl3). Anal. Calc. for C41H34O9S: C, 70.07; H, 4.88; S, 4.56. Found: C, 69.88; H, 4.99; S, 4.86.

Phenyl 2,3,5,6-tetra-O-benzoyl-1-thio-[beta]-d-galactofuranoside (5): 1.35 g, 98%, RF 0.63, m.p. 99-100°C, [[alpha]]D -72° (c1, CHCl3). Anal. Calc. for C40H32O9S: C, 69.75; H; 4.68; S, 4.65. Found: C, 69.83; H, 4.93; S, 4.73.

4-NO2-Phenyl-2,3,5,6-tetra-O-benzoyl-1-thio-[beta]-d-galactofuranoside (6): 1.29 g, 88%, RF 0.68, m.p. 121°C, [[alpha]]D -113° (c 1, CHCl3). Anal. Calc. for C40H31NO11S: C, 65.48; H, 4.26; N, 1.21; S, 4.36. Found: C, 65.33; H, 4.40; N, 1.12; S, 4.49.

Debenzoylation: general procedure

Compounds 2-6 were suspended in NaOMe/MeOH 0.5N, and stirred at room temperature during 2 h. The solution was passed through a column (1.5 cm × 5.0 cm) containing Dowex 50 W (H+) resin. The solvent was removed under vacuum and the remaining methyl benzoate was eliminated by several coevaporations with water.

The following compounds were obtained:

nPropyl 1-thio-[beta]-d-galactofuranoside (7): 91%, RF 0.45, [[alpha]]D -122° (c 1, H2O). Anal. Calc. for C9H18O5S: C, 45.36; H, 7.61; S, 13.45. Found: C, 45.50; H, 7.49; S, 13.16.

iPropyl 1-thio-[beta]-d-galactofuranoside (8): 90%, RF 0.35, [[alpha]]D -135° (c1, H2O). Anal. Calc. for C9H18O5S: C, 45.36; H, 7.61; S, 13.45. Found: C, 45.31; H, 7.32; S, 13.05.

Benzyl 1-thio-[beta]-d-galactofuranoside (9): 87%, RF 0.52, [[alpha]]d-185°, (c 1, H2O). Anal. Calc. for C13H18O5S: C,55.71; H, 4.31; S, 11.44. Found: C, 56.00; H, 4.45; S, 11.30.

Phenyl 1-thio-[beta]-d-galactofuranoside (10): 87%, RF 0.45, m.p. 73-74°C, [[alpha]]D -160° (c 1, H2O). Anal. Calc. for C12H16O5S: C, 52.93; H, 5.92; S, 11.77. Found: C, 52.69; H, 6.18; S, 12.00.

4-Nitrophenyl 1-thio-[beta]-d-galactofuranoside (11): 85%, RF 0.48 , m.p. 165°C, [[alpha]]D -328° (c 1, MeOH). Anal. Calc. for C12H15NO7S: C, 45.42; H, 4.77; S, 10.11. Found: C, 45.24; H, 4.56; S, 10.50.

4-Aminophenyl 1-thio-[beta]-d-galactofuranoside (12): Compound 11 (0.37 g, 0.5 mmol), dissolved in MeOH (15.0 ml) was hydrogenated with 10% Pd/C (0.12 g) at 15 psi (1 atm) for 2 h. The catalyst was filtered and the filtrate was evaporated to afford a syrup (0.32 g, 98%), which gave RF 0.29, [[alpha]]D -200° (c 1, MeOH). Anal. Calc. for C12H17O5NS: C, 50.16; H, 5.96; N, 4.88; S, 11,16. Found: C, 50.35; H, 5.20; N, 4.76; S, 10.93.

Assays of [beta]-d-galactofuranosidase inhibition

The enzymatic activity was assayed using 4-nitrophenyl-[beta]-d-galactofuranoside as substrate. The standard assay was done with 100 µl of 66 mM sodium acetate buffer (pH 4.0), 62 µl of a 5 mM solution of the substrate and 100 µl (20 µg protein) of the enzyme medium, in a final volume of 500 µl. The inhibitors were incorporated in various amounts to give a final concentration range of 0.05-1.25 mM. The enzymatic reaction was started with the addition of the enzyme, and after 1.5 h at 37°C it was stopped with 1 ml of 0.1 M sodium carbonate buffer (pH 9.0). The released 4-nitrophenol was measured spectrophotometrically at 410 nm.

Acknowledgments

We thank Dr. Oscar Varela (Departamento de Química Orgánica, FCEyN, UBA, Argentina) for helpful discussions. This work was financially supported by UNDP/World Bank/World Health Organization Special Program for Research and Training in Tropical Diseases, CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas) and University of Buenos Aires to R.M. de L., Fundacion Antorchas/Vitae (to R.M.de L. and W.C.); Fundaçao de Amparo à Pesquisa do Estado de São Paulo (thematic project 95/4562-3) (to W.C.). R.M. de L. is research member of CONICET.

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2To whom correspondence should be addressed

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