Synthesis of C-Glycosides from S-Glycosyl Phosphorothioates

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Synthesis of C-Glycosides from S-Glycosyl Phosphorothioates

W. Kudelska

Institute of Chemistry, Faculty of Pharmacy, Medical University of Ło´dz´, 90Ð151 Ło´dz´, Muszyn´skiego 1, Poland

Reprint requests to Dr. W. Kudelska.

Fax: 48-42-678-83-98. E-mail: kudelska@ich.pharm.am.lodz.pl Z. Naturforsch.57b, 243Ð247 (2002); received October 16, 2001 S-Glycosyl Phosphorothioates,C-Glycosides, Synthesis

Treatment ofO-benzyl protectedS-glucosyl phosphorothioates with 1,3,5-trimethoxybenzene in the presence of iodine or boron trifluoride etherate led to appropriate arylC-β-D- glucosides. The reaction ofO-benzyl andO-acetyl-protected phosphorothioates of monosac- charides with allyltrimethylsilane, using boron trifluoride etherate as activator, gave mainly or exclusively, the corresponding 3-(α-D- glycopyranosyl)-1-propenes.C-Glucosidation of furan withO-benzyl protectedS- glucosyl phosphorothioate in the presence of boron trifluoride etherate afforded (2- furyl)-α-C-glucoside.

Introduction

Carbon-linked glycosides, stable analogues of naturally occurringO- andN-glycosides, have be- come the subject of considerable interest in bioor- ganic and medicinal chemistry[1Ð5].

Several approaches to the synthesis ofC-glycosides have been explored previously [2Ð5]. The most common method for the carbon-carbon bond formation at the anomeric center involves the coupling of carbon nucleophiles with carbohydrate- based electrophiles having diverse leaving groups e.g. lactols, lactones, anomeric esters, halides, glycosides, thioglycosides, imidates, glycals, enitols and 1,2-anhydro sugars. Furthermore, procedures employing transition metalsÐmediated couplings, anomeric anions and concerted reactions, such as [4+2] cycloadditions and sigmatropic rearrange- ment have also been used to synthesize C-glyco- sides. Recently, free radical chemistry has been ex- tended to this area and to O5C-glycoside rearrangements.

Here, we present a new efficient procedure for the synthesis of C-glycosides using sugar phos- phorothioates as the anomeric leaving group. In the previous papers [6,7], we have already re- ported the utility of S-glycosyl phosphorothioates for the efficient and rapid formation of isomeric 1,2-O-(1-cyanoethylidene)-D-glycoses and glycosyl cyanides.

0932Ð0776/2002/0200Ð0243 $ 06.00 ”2002 Verlag der Zeitschrift für Naturforschung, Tübingen · www.znaturforsch.com D

Results and Discussion

The O-benzyl-protected S-α-D-glycosyl phosphorothioates (1Ð3) andO-acetylatedS-(β-D-glycosyl)phosphorothioates (4Ð5), employed as electrophiles are listed in Fig. 1. The donors 1Ð3 were prepared from the corresponding 2,3,4,6- tetra-O-benzyl-D-glycopyranoses by Lewis acid- catalyzed reaction with ammonium salt of O,O- dialkylphosphorothioic acid [6,8], while donors4Ð 5 - by condensation of ammonium salt of O,O- dialkylphosphorothioic acid withO-acetylated glycosyl halides [9].

Fig. 1.S-Glycosyl phosphorothioates1Ð5applied as glycosyl donors andC-glycosides formed6,12.

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Reaction of glucosyl phosphorothioate 1 with electron rich aromatic compound such as 1,3,5-trimethoxybenzene in the presence of iodine was found to be an effective way to produce the steri- cally favoredβ-C-aryl-D-glucoside product6[10Ð 13] (Figure 1), exclusively. We obtained the same expedient result in the coupling of 1,3,5-trimethoxybenzene with1 in the presence of boron trifluoride etherate as an activator. Both reactions were performed in acetonitrile solution at room temperature for a few days with an excess (~2 equivalents) of acceptor and activator. After the usual work-up, product 6was obtained with good yield after column chromatography.

Per-O-Benzylated and per-O-acetylated phos- phorothioates1Ð5were evaluated as donors in the synthesis of C-allyl glycosides (Scheme 1). Reac- tions were conducted with allyltrimethylsilane in acetonitrile in the presence of boron trifluoride etherate. The amounts of the activator were ad- justed according to the nature of theO-protective groups of sugar. O-Benzyl protected phosphoro- thioates1Ð3were activated with 2 equivalents of BF₃·Et₂O and coupled with allyltrimethylsilane at room temperature to provide the corresponding 3-(O-benzyl-α-D-glycopyranosyl)prop-1-enes 7Ð9 [14Ð20], stereoselectively. Compunds 7Ð9 were isolated in high yield by preparative TLC (Ta- ble 1). After successful application of O-benzy- lated glycosyl donors1Ð3, synthesis ofC-allyl gly- cosides from O-acetylated galactosyl-4 and glucosyl-phosphorothioate5was also investigated.

The reaction of4or5with allyltrimethylsilane and BF3·Et2O (~1.5 equivalent) at room temperature did not proceed even after 10 days. Having increased the amount of BF₃·Et₂O to 10 equivalents, C-allylation with 4 and 5 occured at room temperature, but even after several days some starting material were still present in the reaction mixture. Treatment of glycosyl donors4or5with allyltrimethylsilane and 10 equivalents of BF3·Et2O at 80∞C for a few hours led to 3-(O- acetylated α- and β-glycopyranosyl)prop-1-enes

Scheme 1.

10,11[21Ð24] in 83% and 63% yield, respectively, after purification by column chromatography.

Analysis of ¹³C NMR spectra of 10 and 11 re- vealed that a ~7:1 mixture of α- and β-anomers was formed (Table 1) with preponderance of the thermodynamically more stable α-isomer, despite the presence of theC-2-O-acetyl groups. Recrys- tallization of theα/β mixture 11 gave pure α-anomer [21].

Table 1. Stereoselective synthesis of 3-glycopyranosyl- prop-1-enes Bn = -CH2C6H5, Ac = -COCH3a) theα/β ratio was determined by¹³C NMR, b) product isolated.

In expanding this methodology to other C-nu- cleophiles, furan, a reactive electron-rich aromatic, was used as acceptor in the reaction with the glucosyl donor1. In the presence of boron trifluoride etherate, in acetonitrile, the 2-(O-benzyl-α-D-glucopyranosyl)furan (12) [25] (Fig. 1) was isolated in

~60% yield, after column chromatography.

In summary, the application ofS-glycosyl phosphorothioates in the efficient synthesis of various C-aryl, C-allyl and C-heterocyclic glycosides has been demonstrated. Iodine was successfully employed as an activator in the conversion ofS-glycosyl phosphorothioates to C-glucosylarenes, widespread in nature and having interesting physi- ological properties (hedamycin [26], kyanamycin

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[26], bergenin [27Ð29], et.). Allylation of the glycosyl phosphorothioates donors allowed the formation of C-glycosylalkenes, useful carbohydrate mimetics commonly employed as enzyme inhibi- tors [19,20].

Experimental Section General methods

Melting points were determined with Boetius PHMK 05 apparatus and are uncorrected. IR spectra were obtained by using the Infinity MI-60 FT-IR spectrometer.¹H,¹³C and³¹P NMR spectra were measured in CDCl₃ solution on a Bruker DPX spectrometer operating at 250.13, 62.9 and 101.24 MHz, respectively. The following starting materials were purchased from Aldrich Co: 1,3,5- trimethoxybenzene, allyltrimethylsilane, furan and boron trifluoride etherate. Preparative TLC was performed on 20¥20 cm glass plates coated with 2 mm of silica gel 60 F254 (E. Merck); detection was effected by UV lamp or by exposure to iodine vapours. Column chromatography was performed on Silica Gel 60 (E. Merck) (70Ð230 mesh, ASTM).

2,4,6-Trimethoxy-1-(1⬘-deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O- benzyl-β-D-glucopyranosyl) benzene(6)

(A). To a solution of 1 (211 mg, 0.3 mmol) in acetonitrile (5 ml) 1,3,5-trimethoxybenzene (101 mg, 0.6 mmol) molecular sieves 4 A˚ , and sub- limated iodine (152 mg, 0.6 mmol) were added.

The reaction mixture was stirred for 4 d at r.t. The reaction mixture was diluted with CH2Cl2(50 ml) and filtered through Celite. The filtrate was washed with Na2S2O3(1 ¥30 ml) and water (1 ¥ 30 ml), dried (MgSO4) and evaporatedin vacuoto obtain crude product: after column chromatography product 6 was obtained as colourless syrup (141 mg, 68.1%). Ð ¹³C NMR: δ = 55.5 (OCH3), 56.0 (OCH3), 56.3 (OCH3), 69.3, 72.8, 73.3, 74.6, 75.3, 76.0, 78.6, 79.8, 80.1, 87.6, 90.9, 91.8 (pyranose-C, 4¥CH2Ph), 107.7 (C-1), 127.3Ð128.5 (4Ph), 138.4, 138.6, 138.9, 139.0 (4C, ipso Ph), 158.8, 160.9, 161.4 [3C aromatic from C6H2(OCH3)3]. -¹H NMR (selected): δ= 3.70 (s, 3 H, OCH3), 3.78 (s, 3 H, OCH3), 3.81 (s, 3 H, OCH3), 6.10 (d,J= 2.25 Hz, 1 H, C6H2), 6.16 (d, J =2.0 Hz, 1 H, C6H2).¹H and¹³C NMR data are in agreement with the lit. data [10Ð12].

(B). To a solution of 1 (211 mg, 0.3 mmol) in acetonitrile (5 ml), 1,3,5-trimethoxybenzene (101 mg, 0.6 mmol), molecular sieves 4 A˚ and BF3·Et2O (0.11 ml, 0.128 g, 0.9 mmol) were added.

The reaction mixture was stirred for 4 d at r.t. The reaction mixture was diluted with CH2Cl2(50 ml) and filtered through Celite. The filtrate was washed with satd. NaHCO3(2 ¥ 30 ml), water (1

¥30 ml), dried (MgSO4) and evaporatedin vacuo to obtain crude product. The oily residue was purified on silica gel column chromatography petro- leum ether Ð ethyl acetate (4:1, v/v) to give 6 (0.149 g, 72%) as colourless syrup.

General procedure. 3-(1⬘-Deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O- benzyl-α-D-glycopyranosyl)prop-1-enes(7Ð9)

To the solution of thiophosphate 1Ð3 (0.211 g, 0.3 mmol), in dry acetonitrile (3 ml), allyltrimethylsilane (1.5 mmol, 0.171 g, 0.24 ml) was added, then molecular sieves 4 A˚ and finally BF3·Et₂O (0.085 g, 0.08 ml, 0.6 mmol). The resulting solution was stirred for the time indicated in Table 1. The reaction mixture was diluted with CH₂Cl₂(50 ml) and filtered through Celite. The filtrate was washed with satd. NaHCO₃(2 x 30 ml), water (1 x 20 ml), dried (MgSO₄) and evaporatedin vacuo to obtain the crude product. This product was dis- solved with AcOEt and purified by preparative thin layer chromatography using benzeneÐEt₂O (4:1, v/v) as the eluent.

3-(1⬘-Deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O-benzyl-α-D- glucopyranosyl)prop-1-ene(7)

7 was obtained as colourless crystalline residue (0.142 g, 84.2%). M. p. 64Ð65∞C (after crystallization from hexane), (lit. [16] 64Ð65 ∞C). Ð ¹³C NMR: δ = 29.7 (C-3), 68.8, 71.0, 73.0, 73.4, 73.6, 75.0, 75.4, 78.0, 80.0, 82.3, (pyranose-C, 4¥CH2Ph ), 116.8 (C-1), 127.5Ð128.3 (4¥Ph), 134.7 (C-2), 138.0, 138.4, 138.7,(4 C, ipso Ph).Ð¹H NMR:δ = 2.47 (m, 2 H, 3-H), 3.59Ð3.82 (m, 6 H, 2⬘, 3⬘, 4⬘, 5⬘, 6⬘. 6⬙-H), 4.14 (m, 1 H, 1⬘-H), 4.44Ð4.95 (m, 8 H, 4¥CH2Ph), 5.09 (m, 2 H, 1a-H, 1b-H), 5.81 (m, 1 H, 2-H), 7.01Ð7.31 (m, 20 H, 4¥Ph). ¹H NMR spectra are in agreement with lit. [15,16,18,19],¹³C NMR spectra are in agreement with lit. [30].

3-(1⬘,3⬘,4⬘,6⬘-tetra-O-benzyl-α-D- galactopyranosyl)prop-1-ene(8)

8 was obtained as a colourless syrup (0.125 g, 74%).Ð¹³C NMR:δ= 29.6 (C-3), 67.1, 72.8, 72.9, 73.0, 74.1,( pyranose-C, 4¥CH2Ph), 76.3 (C-1⬘), 116.6 (C-1), 127.3Ð128.2 (4¥Ph), 135.0 (C-2), 138.1, 138.3, 138.4, 138.5, (4C, ipso Ph),-¹H NMR:

δ= 2.37 (m, 2 H, 3-H), 3.75Ð4.05 (m, 6 H, 2⬘, 3⬘, 4⬘, 5⬘, 6⬘, 6⬙-H), 4.11 (m, 1H, 1⬘-H), 4.49Ð4.71 (m, 8 H, 4¥PhCH2), 5.1 (m, 2 H, 1a-H, 1b-H), 5.74 (m,

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1 H, 2-H), 7.25Ð7.36 (m, 20 H, 4¥Ph).¹H and¹³C NMR spectra are in agreement with the lit. [14].

3-(1⬘-Deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O-benzyl-α-D- mannopyranosyl)prop-1-ene(9)

9 was obtained as colourless syrup (0.144 g, 85.2%). Ð¹³C NMR: δ = 34.6 (C-3), 69.1, 71.4, 72.0, 72.2, 73.2, 73.6, 73.7, 74.8, 75.1, 76.8, (pyranose-C, 4¥ CH2Ph), 117.1 (C-1), 127.4Ð128.3 (4¥Ph), 134.2 (C-2), 138.2, 138.3, (4C, ipso Ph),Ð

1H NMR:δ= 2.40 (m, 2 H, 3-H), 3.70Ð3.92 (m, 6 H, 2⬘, 3⬘, 4⬘, 5⬘, 6⬘, 6⬙-H), 4.11 (m, 1 H, 1⬘-H), 4.57Ð 4.77 (m, 8 H, 4¥CH2Ph), 5.01 (m, 2 H, 1a-H, 1b- H), 5.82 (m, 1 H, 2-H), 7.26Ð7.41 (m, 20 H, 4¥Ph).

1H and ¹³C NMR spectra are in agreement with the lit. [18,20].

General procedure. 3-(1⬘-Deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O- acetyl-D-glycopyranosyl)prop-1-enes(10,11)

To the solution of thiophosphate4or5(0.307 g, 0.6 mmol) in acetonitrile (10 ml), allyltrimethylsilane (0.95 ml, 0.685 g, 6 mmol), molecular sieves 4 A˚ , and finally BF3·Et2O (0.76 ml, 0.85 g, 6 mmol) were added. The resulting solution was refluxed (4,5) for the times indicated in Table 1. The reac- tion mixture was cooled, diluted with CH2Cl2

(50 ml), filtered (Celite), and washed with CH2Cl2. The organic layer was washed with satd. NaHCO3

(2 x 30 ml) water (1 x 30 ml), and dried (MgSO4).

The solvent was removed under reduced pressure, and the crude product was purified by column chromatography.

3-(1⬘-Deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O-acetyl-D- galactopyranosyl)prop-1-ene(10)

Column chromatography, gradient toluene Ð AcOEt, (1:053:1, v/v).10was obtained as a mixture (α/β= ~ 7:1).ÐIR (film):ν = 2961 (=CH2), 1746 (C=O), 1643 (C=C), 1435, 1372, 1233, 1102, 1053, 984, 912 (=CH2), 756, 590, 500 cm^Ð1. -¹³C NMR (α Ð anomer): δ = 20.2, 20.3, 20.4 (OAc), 30.5 (C-3), 61.1 (C-6⬘), 71.1 (C-1⬘), 67.3, 67.6, 67.8, 67.9 (C2⬘ÐC5⬘), 117.2 (C-1), 133.2 (C-2), 169.4, 169.5, 169.7, 170.1 (C=O); β-anomer: δ = ~ 20.0 (OAc), 35.6 (C-3) 61.3 (C-6⬘), 67.4, 68.8, 71.8, 73.7, 77.2 (C-1⬘), 117.0 (C-1), 133.0 (C-2), 169.3, 169.7, 169.9 (C=O), in agreement with the lit. [22,23]. -

1H NMR:δ= 2.08, 2.05, 2.04 (OAc), 2.29 (m, 1 H, 3-H), 2.46 (m, 1 H, 3-H), 4.09 (m, 2 H, 5⬘,6⬘-H), 4.21 (dd,J_5⬘,6⬘ = 7.2 Hz,J_6⬘,6⬘ = 12.6 Hz, 1 H, 6⬘- H), 4.31 (ddd, J1’,2⬘ = 5.1 Hz, J_1⬘.3’ = 10.5 Hz, J_1⬘.3⬘= 5.4 Hz, 1 H, 1⬘-H), 5.10Ð5.16 (m, 2 H, 1a- H, 1b-H), 5.22 (dd,J_3⬘,4⬘= 3.0 Hz,J_2⬘,3⬘= 9.3 Hz,

1 H, 3⬘-H), 5.28 (dd,J2,1= 4.8 Hz, J2,3= 9.3 Hz, 1 H, 2⬘-H), 5.42 (t, J_3⬘,4⬘= 3.0 Hz, J_4⬘,5⬘= 2.4 Hz, 1 H, 4⬘- H), 5.76 (m, 1 H, 2-H), in agreement with the lit. [22,23]¹H NMR spectra.

3-(1⬘-Deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O-acetyl- glucopyranosyl)prop-1-ene(11)

Column chromatography, toluene Ð AcOEt (4:1, v/v). 11 was obtained as a mixture (α/β =

⬃ 7:1). Ð¹³C NMR (α-anomer): δ = 20.8, 20.9 (OAc), 30.7 (C-3), 62.3 (C-6⬘), 68.9 (C-4⬘, 5⬘), 70.3 (C-2⬘), 70.4 (C-3⬘) 72.0 (C-1⬘), 117.8 (C-1), 133.0 (C-2), 169.0, 169.6, 170.0, 170.6 (C=O);δ-anomer:

δ = ~ 20 (OAc), 36.0 (C-3), 62.4 (C-6⬘), 68.3 (C- 4⬘), 71.7 (C-2⬘), 74.5 (C-3⬘), 75.7 (C-5⬘), 77.4 (C- 1⬘), 117.4 (C-1), 133.0 (C-2), 169.4, 170.3, 170.8 (C=O), in agreement with the lit. [21]¹³C NMR spectra.

Recrystalization (chloroform-hexane) gave 11 α, colorless needles. M. p. 107Ð108 ∞C (lit. [21]

108∞C). Ð¹H NMR: δ = 2.03, 2.04, 2.05, 2.08, (OAc), 2.62 (m, 1 H, 3-H), 2.29 (m, 1 H, 3-H), 3.86 (ddd, J_5⬘,6⬘ = 2.7 Hz, J_4⬘,5⬘ = 9.4 Hz, 1 H, 5⬘- H), 4.08 (dd, 1 H, 6⬘-H), 4.21 (dd,J_5⬘,6⬘= 5.4 Hz,J_6⬘,6⬙= 12.1 Hz, 1 H, 6⬘-H), 4.28 (ddd,J_3,1⬘= 4.6 Hz,J_3,1⬘= 10.8 Hz, 1 H, 1⬘-H), 4.98 (t, 1 H,J_3⬘,4⬘= 9.4 Hz, 4⬘- H), 5.09 (dd,J_1⬘,2⬘= 5.7 Hz,J_2⬘.3⬘= 9.5 Hz, 1 H, 2⬘- H), 5.12 (ddd,J= 1.3 Hz,J= 2.7 Hz,J= 10.2 Hz, 1 H, 1-H), 5.15 (ddd, J= 1.5 Hz, J= 2.7 Hz, J= 17.1 Hz, 1 H, 1-H), 5.34 (t,J= 9.1 Hz, 1 H, 3⬘-H), 5.75 (m, 1 H, H-2), in agreement with the lit.

[21,24]¹H NMR spectra.

2-(1⬘-Deoxy-2⬘,3⬘,4⬘,6⬘-tetra-O-benzyl-α-D- glucopyranosyl)furan(12)

To the solution of thiophosphate 1 (0.211 g, 0.3 mmol) in dry acetonitrile (5 ml) furan (0.11 ml, 0.102 g, 1.5 mmol) was added, then molecular sieves 4 A˚ and finally BF3·Et2O (0.11 ml, 0.128 g, 0.9 mmol). The mixture was allowed to react for 24 h at r.t. The reaction mixture was diluted with CH2Cl2 (50 ml) and filtered through Celite The filtrate was washed with satd. NaHCO3 (2 ¥ 30 ml), water (1 ¥ 20 ml), dried (MgSO4) and evaporated in vacuo to obtain crude product, which was purified by column chromatography pe- troleum ether Ð AcOEt (19:1, v/v). 12 was obtained as colourless crystals (0.105 g, 59.3%). M.

p. 92Ð3∞C, [after crystallization (hexane)], lit. [25]

92Ð3 ∞C. Ð ¹³C NMR: δ = 68.8, 70.0, 72.8, 73.2, 73.4, 75.0, 75.4, 75.6, 78.0, 79.4, 82.8 (pyranose-C, 4¥CH2Ph), 110.0, 116.6 (C-3, C-4), 127.4Ð128.2 (4Ph), 138.1, 138.5, 138.8 (4-C, ipso Ph), 142.6 (C- 5), 150.7 (C-2). Ð¹H NMR: δ = 3.45Ð3.69 (m, 4

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H, 4⬘, 5⬘, 6⬘, 6⬙-H), 3.88 (dd,J_1⬘,2⬘= 6.6 Hz,J_2⬘,3⬘= 9.5 Hz, 1 H, 2⬘-H), 4.15 (dd,J_2⬘,3⬘= 9.5 Hz,J_3⬘,4⬘= 9.2 Hz, 1 H, 3⬘-H), 4.94Ð4.36 (m, 8 H, 4¥CH2Ph), 5.05 (d,J_1⬘2⬘= 6.6 Hz, 1 H, 1⬘-H), 6.28 (dd,J= 1.8 Hz,J= 3.3 Hz, 1 H, 4-H), 6.46 (d,J= 3.3 Hz, 1 H, 3-H), 7.03Ð7.27 (m, 20 H, 4C6H5), 7.36 (dd,J= 1 Hz, 1 H, 5-H).¹H and¹³C NMR are in agreement with the lit. data [25].

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Acknowledgements

Financial support from the Medical University of Ło´dz´ (502Ð13Ð682) is gratefully acknowledged.

Author is greatly indebted to Prof. M. Michalska for her kind interest in this work.