Thiazolylketol Acetates as Glycosyl Donors: Stereoselective Synthesis of a C-Ketoside

Abstract

We have already proven that thiazolylketol acetates, synthetised by addition of 2-lithiothiazole to sugar lactones followed by acetylation, are efficient glycosyl donors in the presence of O-, N-, and P-nucleophiles. We describe here their first use in the C-glycosidation using trimetylsilyl cyanide as the acceptor in order to prepare, after thiazole-to-formyl unmasking and reduction, the corresponding C-ketosides.

1. Introduction

The C-glycosides are carbohydrate mimics featuring a carbon instead of an oxygen atom at the anomeric position. Since a strong carbon–carbon bond replaces the labile (acetalic) C-O bond, these chemically and enzymatically stable sugar analogues are valuable biological and pharmacological tools because they can be employed as probes for the carbohydrate–protein interaction studies or as inhibitors of glycosidases and glycosyltransferases. Given the importance of the C-glycosides, many synthetic approaches to these compounds have been proposed over the last four decades, as described in several reviews [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. It is worth noting that most C-glycosides reported in the literature are C-aldosides; the synthesis of the C-ketoside derivatives is rarely addressed, likely due to the challenging stereochemical control at the anomeric position.

The thiazolylketol acetates 4 (Scheme 1) were first developed by us as key intermediates for the stereoselective synthesis of the formyl C-glycosides 6 [16], which are very useful building blocks that allowed for the preparation of more complex products, i.e., C-glycosyl amino acids and C-oligosaccharides. Then, the thiazolylketol acetates proved to be also excellent glycosyl donors [17] when reacted (Scheme 2) in the presence of a Lewis acid with (a) O-nucleophiles such as sugar alcohols to afford keto-disaccharides and -oligosaccharides 10 or calixarene alcohols to give ketosyl calixarenes, (b) N-nucleophiles (trimethylsilyl azide) to provide ulosonyl amines (fused glycosyl glycines) 11, and (c) P-nucleophiles (triethylphosphite) to afford ketosyl (12) or ulosonyl (13) phosphonates. However, the glycosylation of even simple C-nucleophiles was never reported by us or other researchers. We describe here the chemical and stereochemical outcome of the C-glycosidation of thiazolylketol acetates using a good, commercially available nucleophile such as the trimethylsilyl cyanide. Moreover, our preliminary results demonstrate the feasibility of the thiazole-to-formyl unmasking reaction sequence and subsequent reduction to hydroxymethyl function without affecting the carbon–nitrogen triple bond.

2. Results and Discussion

The known [16] galactosyl and 2-azido-galactosyl thiazole derivatives 14 and 15 were treated with a slight excess of trimethylsilyl cyanide (TMSCN) in the presence of trimethylsilyl triflate (TMSOTf, 1 equiv.) and 4 Å powdered molecular sieves to give, after 30 min at room temperature, the corresponding C-glycosides 16 and 17 in 90% and 60% isolated yield (Scheme 3). As already observed [17] for the O-, N-, and P-glycosidation of the D-galacto- and D-manno-configured thiazolylketol acetates, these glycosides were obtained as single α-D (axial) anomers. In the absence of the other anomer, the comparison of their C-1 chemical shifts could not be made; therefore, the anomeric configuration was proven by the large vicinal coupling constant between the carbon atom of the cyanide group and the H-2 proton of the pyranose ring. In the H-coupled ¹³C-NMR spectra (125 MHz, CDCl₃) of 16 and 17 (Figures S13 and S14 in the Supplementary Materials), the CN carbon at 115.5 and 114.7 ppm, respectively, appeared as doublets with J³ = 7.9 (16) and 8.4 Hz (17). This behaviour was expected [18,19,20] for pyranosides in ⁴C₁ conformation where the CN carbon and the H-2 proton are in a trans-diaxial relationship (180° dihedral angle). In the case of the β-D-anomer, the equatorial CN carbon (CN−C-1−C-2−H-2 60° dihedral angle) should appear as a singlet (J³ < 1 Hz).

The C-galactoside 16 was then submitted to the usual [16,17] three-step thiazole-to-formyl unmasking protocol (Scheme 4) consisting in the N-methylation with methyl triflate, reduction of the N-methylthiazolium salt 17 by sodium borohydride, and copper-assisted hydrolysis of the thiazolidine intermediate 18 to afford the formyl C-galactoside 19. It is worth noting that the unmasking procedure can be carried out in less than two hours, including the workup treatment for each step. The crude aldehyde derivative 19 was directly reduced with NaBH₄ to give the C-heptuloside 20 in 37% overall yield after purification by column chromatography. This compound constitutes a versatile building block since it can act as a glycosyl acceptor due to the presence of the primary alcohol function, while the cyano group may be reduced to a primary amine or transformed into a carboxylic function.

3. Materials and Methods

All moisture-sensitive reactions were performed under a nitrogen atmosphere using oven-dried glassware. Commercially available powdered 4 Å molecular sieves (5 µm average particle size) were used without further activation. The reactions were monitored by TLC on silica gel 60 F₂₅₄ with detection by charring with sulphuric acid. Flash column chromatography was performed on silica gel 60 (40–63 μm). Optical rotations were measured at 20 ± 2 °C in the stated solvent; [α]_D values are given in deg mL g⁻¹ dm⁻¹. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded in CDCl₃ at room temperature unless otherwise specified. In the ¹H NMR spectra reported below, the n and m values quoted in geminal or vicinal proton–proton coupling constants J_n,m refer to the number of the corresponding sugar protons. All the assignments were confirmed by 2D spectra (COSY and HSCQ). The ¹H-coupled ¹³C NMR spectra were recorded at 298 K on a Bruker AVANCE III 500 NMR spectrometer, operating at 125.7 MHz, equipped with a 5 mm H/X BBO helium CryoProbe, sequenced with gated decoupling, and by using a 30-degree flip angle (0.57 s acquisition time). FT-IR spectra were recorded using a Perkin Elmer Spectrum 3 instrument equipped with an ATR accessory. Melting points were determined with a capillary apparatus (Büchi 510). High-resolution mass spectrometry (Waters Micromass Q-TOF) analyses were carried out at the “Laboratoire de Mesures Physiques” (University of Montpellier).

3.1. (1R)-2,3,4,6-Tetra-O-benzyl-1-C-cyano-1-deoxy-1-C-(2-thiazolyl)-D-galactopyranose (16)

A mixture of 14 (520 mg, 0.82 mmol), activated 4 Å powdered molecular sieves (0.50 g), and anhydrous CH₂Cl₂ (7.5 mL) was stirred at room temperature for 10 min, then trimethylsilyl cyanide (120 µL, 0.98 mmol) and trimethylsilyl triflate (150 µL, 0.82 mmol) were added. The mixture was stirred at room temperature for 30 min, then diluted with triethylamine (0.30 mL) and CH₂Cl₂ (50 mL), filtered through a pad of celite, washed with water (2 × 20 mL), dried (Na₂SO₄), and concentrated. The residue was eluted from a column of silica gel with 6:1 cyclohexane-AcOEt to give 16 (449 mg, 90%) as a white solid. Mp 114–115 °C (cyclohexane); [α]_D = +32.4 (c = 1.0, CHCl₃). ¹H NMR: δ7.85 and 7.45 (2 d, 2H, J = 3.2 Hz, Th), 7.38−7.28 (m, 15H, Ar), 7.26−7.22 (m, 3H, Ar), 7.16−7.12 (m, 2H, Ar), 5.00 and 4.67 (2 d, 2H, J = 11.6 Hz, PhCH₂), 4.81 and 4.77 (2 d, 2H, J = 11.6 Hz, PhCH₂), 4.65 and 4.38 (2 d, 2H, J = 11.2 Hz, PhCH₂), 4.51 and 4.46 (2 d, 2H, J = 12.0 Hz, PhCH₂), 4.44 (d, 1H, J_2,3 = 9.6 Hz, H-2), 4.27 (ddd, 1H, J_4,5 = 0.8, J_5,6a = 7.6, J_5,6b = 5.6 Hz, H-5), 4.13 (dd, 1H, J_3,4 = 2.8 Hz, H-4), 4.02 (dd, 1H, H-3), 3.70 (dd, 1H, J_6a,6b = 9.2 Hz, H-6a), 3.65 (dd, 1H, H-6b). ¹³C NMR: δ 164.7 (C), 142.8 (CH), 138.4 (C), 137.9 (C), 137.6 (C), 137.3 (C), 128.4−127.5 (CH), 121.5 (CH), 115.5 (C), 81.6 (CH), 79.5 (C), 79.3 (CH), 76.0 (CH), 75.4 (CH₂), 74.7 (CH₂), 73.5 (CH₂), 73.5 (CH), 73.2 (CH₂), 67.7 (CH₂). FT-IR (cm⁻¹): 3031, 2919, 2870, 1498, 1454, 1344, 1087, 1056, 981, 942, 746, 733, 695. HRMS (ESI/Q-TOF): m/z calcd. for C₃₈H₃₇N₂O₅S [M+H]⁺ 633.2418, found 633.2419.

3.2. (1S)-2-Azido-3,4,6-tri-O-benzyl-1-C-cyano-1-deoxy-1-C-(2-thiazolyl)-D-galactopyranose (17)

A mixture of 15 (300 mg, 0.50 mmol), activated 4 Å powdered molecular sieves (0.30 g), and anhydrous CH₂Cl₂ (6 mL) was stirred at room temperature for 10 min, then trimethylsilyl cyanide (75 µL, 0.61 mmol) and trimethylsilyl triflate (100 µL, 0.56 mmol) were added. The mixture was stirred at room temperature for 30 min, then diluted with triethylamine (0.20 mL) and CH₂Cl₂ (40 mL), filtered through a pad of celite, washed with water (2 × 10 mL), dried (Na₂SO₄), and concentrated. The residue was eluted from a column of silica gel with 5:1 cyclohexane-AcOEt to give 17 (170 mg, 60%) as a white solid. Mp 137–138 °C (AcOEt-cyclohexane); [α]_D = +36.2 (c = 0.6, CHCl₃). ¹H NMR: δ 7.91 and 7.53 (2 d, 2H, J = 3.2 Hz, Th), 7.45−7.27 (m, 15H, Ar), 4.93 and 4.62 (2 d, 2H, J = 11.6 Hz, PhCH₂), 4.83 and 4.79 (2 d, 2H, J = 11.4 Hz, PhCH₂), 4.52 (d, 1H, J_2,3 = 10.4 Hz, H-2), 4.51 and 4.46 (2 d, 2H, J = 12.0 Hz, PhCH₂), 4.23 (ddd, 1H, J_4,5 = 0.8, J_5,6a = 7.6, J_5,6b = 5.6 Hz, H-5), 4.12 (dd, 1H, J_3,4 = 2.8 Hz, H-4), 3.88 (dd, 1H, H-3), 3.69 (dd, 1H, J_6a,6b = 9.2 Hz, H-6a), 3.64 (dd, 1H, H-6b). ¹³C NMR: δ 163.4 (C), 143.2 (CH), 138.0 (C), 137.4 (C), 137.1 (C), 128.6−127.8 (CH), 121.9 (CH), 114.7 (C), 80.0 (CH), 78.2 (C), 76.2 (CH), 74.8 (CH₂), 73.6 (CH₂), 73.1 (CH₂), 72.0 (CH), 67.5 (CH₂), 63.8 (CH). FT-IR (cm⁻¹): 2876, 2116, 1495, 1454, 1354, 133, 1274, 1217, 1140, 1101, 1058, 1028, 1003, 986, 919, 899, 876, 849, 792, 771, 758, 725, 699. HRMS (ESI/Q-TOF): m/z calcd. for C₃₁H₃₀N₅O₄S [M+H]⁺ 568.2013, found 568.2013.

3.3. 2,6-Anhydro-3,4,5,7-tetra-O-benzyl-2-C-cyano-D-glycero-L-manno-heptitol (20)

A mixture of 16 (50 mg, 0.08 mmol), activated 4 Å powdered molecular sieves (80 mg), and anhydrous CH₃CN (1.0 mL) was stirred at room temperature for 10 min, then methyl triflate (34 µL, 0.19 mmol) was added. The mixture was stirred at room temperature for 20 min and then concentrated to dryness without filtering off the molecular sieves. NaBH₄ (8 mg, 0.21 mmol) was added to a cooled (0 °C), stirred suspension of the crude N-methylthiazolium salt 17 in CH₃OH (1.0 mL). The mixture was stirred at room temperature for an additional 15 min, diluted with acetone (10 mL), filtered through a pad of celite, and concentrated. CuO (51 mg, 0.64 mmol) and CuCl₂^.2H₂O (13.6 mg, 0.08 mmol) were added to a solution of the crude diastereomeric mixture of N-methylthiazolidines 18 in 10:1 CH₃CN-H₂O (1.0 mL). The mixture was stirred at room temperature for 20 min, diluted with CH₃CN (10 mL), filtered through a pad of celite, and concentrated to give a brown syrup. A solution of the residue in Et₂O (10 mL) was filtered through a pad (3 × 0.5 cm, d × h) of Florisil (100–200 mesh), and the pad was washed with AcOEt (5 mL). The colourless solution was concentrated to afford the crude aldehyde 19. ¹H NMR (selected data): δ 9.13 (s, 1H, CHO), 7.55−7.20 (m, 20H, 4 Ph). NaBH₄ (6 mg, 0.16 mmol) was added to a cooled (0 °C), stirred solution of crude 19 in 1:1 CH₃OH-Et₂O (1.0 mL). The mixture was stirred at room temperature for an additional 10 min, diluted with acetone (2 mL), and concentrated. The residue was eluted from a column of silica gel with 4:1 cyclohexane-AcOEt to give 20 (17 mg, 37%) as a white solid. Mp 92–93 °C (cyclohexane); [α]_D = +53.7 (c = 0.6, CHCl₃). ¹H NMR: δ 7.40−7.27 (m, 20H, Ar), 5.01 and 4.77 (2 d, 2H, J = 11.6 Hz, PhCH₂), 4.96 and 4.59 (2 d, 2H, J = 11.6 Hz, PhCH₂), 4.79 and 4.77 (2 d, 2H, J = 11.6 Hz, PhCH₂), 4.52 and 4.48 (2 d, 2H, J = 12.0 Hz, PhCH₂), 4.16 (d, 1H, J_3,4 = 9.6 Hz, H-3), 4.09 (ddd, 1H, J_5,6 = 0.8, J_6,7a = J_6,7b = 6.4 Hz, H-6), 4.06 (dd, 1H, J_4,5 = 2.8 Hz, H-5), 3.94 (dd, 1H, H-4), 3.89 and 3.76 (2 d, 2H, J_1a,1b = 12.0 Hz, 2 H-1), 3.60 (d, 2H, 2 H-7). ¹³C NMR: δ 138.2 (C), 137.8 (C), 137.52 (C), 137.51 (C), 128.5−127.6 (CH), 116.3 (C), 81.8 (CH), 79.6 (C), 75.4 (CH₂), 75.2 (CH), 74.8 (CH₂), 73.47 (CH), 73.46 (CH₂), 73.1 (CH), 72.9 (CH₂), 67.8 (CH₂), 64.5 (CH₂). FT-IR (cm⁻¹): 3428, 3062, 3030, 2921, 2878, 2852, 1454, 1363, 1178, 1093, 1044, 1027, 990, 914, 749, 729, 7001. HRMS (ESI/Q-TOF): m/z calcd. for C₃₆H₃₈NO₆ [M+H]⁺ 580.2694, found 580.2695; m/z calcd. for C₃₆H₄₁N₂O₆ [M+NH₄]⁺ 597.2959, found 587.2960.

4. Conclusions

The preliminary results reported in the present work pave the way to the synthesis of other C-ketosides starting from different monosaccharides (e.g., D-glucose and D-mannose) and/or using different C-nucleophiles (e.g., silyl enol ethers, allyltrimethylsilane, furane). Central to our synthetic strategy is the use of thiazole as a masked form of the formyl group, because this stable heterocycle allows a variety of substrate elaborations but can be then transformed into the formyl under very mild conditions, leaving unaltered the stereocenters and the functional groups of the substrate.