Synthesis of the C-glycoside of α-D-mannose-(1 → 6)-d-myo-inositol†

Abstract

The dimannosylatedinositol pseudotrisaccharide phospholipid of the lipoarabinomannan (LAM) component of the mycobacterial cell wall has attracted interest as a therapeutic target because of its uniqueness to mycobacteria, its assembly at an early stage in LAM biosynthesis and the immunological activity of oligosaccharides containing this subunit. Accordingly, analogues of this pseudotrisaccharide, α-d-mannose-(1 → 2)-α-d-mannose-(1 → 6)-d-myo-inositol are of interest as mechanistic probes and drug leads. C-glycosides are of special interest because of their hydrolytic stability and conformational differences compared to O-glycosides. Herein, as a prelude to C-glycoside analogues of this pseudotrisaccharide, we describe the synthesis of the C-glycoside of α-D-mannose-(1 → 6)-d-myo-inositol. The synthetic strategy centers on the elaboration of a C1-linked glycal-inositol, the glycone segment of which is assembled via an oxocarbenium ion cyclization on a thioacetal-enol ether precursor that originates from “glycone” and “aglycone” components.

Introduction

The resurgence of mycobacterial infections (M. tuberculosis and AIDS-associated M. avium) has been associated with increasing populations of immunocompromised and homeless individuals.^1,2 Of particular concern is the emergence of M. tuberculosis strains that are resistant to many of the drugs that are used to treat tuberculosis.³ Consequently the development of new antimycobacterial agents is an active area of research.^4,5 The mycobacterial cell wall contains numerous components that are believed to be necessary for survival in the host and is the site of action of established antimycobacterial agents.⁶ The LAM subunit is composed of an outermost arabinomannan segment linked to a phosphotidylinositol mannoside (PIM) region. In addition to maintaining the structural integrity of the cell wall LAM has been implicated in the suppression of immune responses thereby contributing to pathogenesis and many of the clinical manifestations of tuberculosis.⁷ Accordingly, inhibition of the biosynthesis of LAM is a potential therapeutic strategy and the pseudotrisaccharide PIM₂ 2 an early stage intermediate is an attractive target because it is unique to mycobacteria (Fig. 1).⁸ An α-mannose transferase that catalyzes the synthesis of 2 from GDP-mannose and the pseudodisaccharide 1 has recently been isolated, and this presents and opportunity for rational drug design.⁹ C-glycoside analogues such as 3 that may act as hydrolytically stable, bi-substrate mimetics of the transition state leading to 2, are of interest as pharmaceutical leads and for enzyme studies.^10–12 Small PIM fragments have also shown interesting immunomodulatory activities and their C-glycosides may also find applications as probes of the underlying mechanisms.^13–16 However, while the synthesis of O-glycoside analogues is well documented, studies on C-glycosides thereof, are not.^17–20 Herein, en route to structures like 3, we describe the synthesis of 4, the C-glycoside of the core pseudodisaccharide, α-d-mannose-(1 → 6)-d-myo-inositol.

Fig. 1.

Biosynthesis and C-glycoside analogues of PIM₂.

Results and discussion

Our first approach to 4 followed the Beau α-C-manno-glycosidation protocol of coupling the mannosyl sulfone 5 and 6-C-formyl-myo-inositols like 6 (Scheme 1).²¹ Unfortunately this approach was not successful, possibly because of the highly hindered nature of the aldehyde partner. We therefore adopted a C-glycosidation strategy that is based on de novo synthesis of the glycone segment, and which we have used for C-disaccharides.^22–25 Thus, we envisaged that 4 could be fabricated by elaboration of a C-linked glycal-inositol 8, which is available via an oxocarbenium ion cyclization on thioacetalenol ether 7.

Scheme 1.

Strategies for α-C-manno-pseudodisaccharides.

The synthesis of the requisite C-linked glycal-inositol 17 started from the known 1-phenylthio-1,2-O-isopropylidene alcohol 14²³ and the 6-C-branched myo-inositol acid 13 (Scheme 2). The latter was prepared from the known d-myo-inositol derivative 10²⁶ using the Keck C-radical allylation methodology.²⁷ Thus, alcohol 10 was converted to the thiocarbonate 11 and treatment of 11 with allyltributyltin in the presence of AIBN gave the C-allyl derivative 12. Removal of ester and ketal protecting groups in 12 provided the pentaol derivative, which was protected as the penta-O-benzyl ether. Standard oxidative processing of the alkene in the latter gave acid 13. With acid 13 and alcohol 14 in hand DCC mediated esterification was next attempted. However, 15 was obtained in low yield. The Yamaguchi protocol was more successful, providing 15 in 86% yield.²⁸ Reaction of 15 with the Tebbe reagent yielded 16 in 77% yield. The oxocarbenium ion cyclization on 16 was promoted by methyl triflate to give 17 in 78% yield.

Scheme 2.

Synthesis of C-linked glycal-inositol.

Elaboration of C-glycal 17 to the desired α-C-mannoside motif started with a hydroboration–borane oxidation sequence on 17 (Scheme 3). This led to a 3 : 1 mixture of 18 : 19, which was more easily separated as the respective benzoates 20 and 21. ¹H NMR analysis suggested that 20 was the α-C-altro-isomer in predominantly the ^6′C_3′ conformation (J_4′,5′ = J_5′,6′ = 6.9 Hz), and 21 was the β-C-allo-favoring the ^3′C_6′ (J_4′,5′ = 3.9 Hz; J_5′,6′ = 6.9 Hz).^29,30 We envisaged that the desired α-C-manno motif could be obtained through configurational inversion at the C4′ position in 20 and towards this end 20 was next transformed to alcohol 23. Thus, acetonide cleavage in 20 provided diol 22, which was converted via the derived orthoacetate to a 2 : 1 mixture of 23 and its regioisomer 24.³¹ As for the precursor 20, the desired isomer 23 was also found to favor the ^6′C_3′ conformation (J_2′,3′ = J_3′,4′ = 3.8 Hz; J_4′,5′ = J_5′,6′ = 7.5 Hz). Reaction of 23 with trifluoromethanesulfonic anhydride and dry dichloromethane at −20 to 10 °C afforded the triflate derivative. The crude product was treated with potassium nitrite in DMF at 50 °C to give the desired alcohol 25 in 80% yield over two steps.³² The manno configuration and the ^3′C_6′ conformation of 25 was supported by J values (J_2′,3′ = J_3′,4′ = 9.6 Hz; J_4′,5′ = J_5′,6′ = 0 Hz) and H-2′/H-4′ and H-4′/H-7′ nOe’s. Straightforward removal of alcohol protecting groups in 25 provided the target α-C-manno-pyranosyl-myo-inositol 4. By starting with a myo-inositol precursor with orthogonal protecting groups on the C1 and C2 alcohols, this synthesis can be easily adapted to produce C-disaccharide derivatives of 25, that are primed for incorporation into pseudo-trisaccharide mimetics of 3.

Scheme 3.

Transformation of C-linked glycal to α-C-mannos.

Conclusions

In summary the C-glycoside of α-d-mannose-(1 → 6)-d-myo-inositol 4 was prepared via a de novo synthesis of the “glycone” segment, over ten steps from thioacetal 13 and the C-branched inositol 14. This synthesis illustrates the feasibility of this strategy for synthetically challenging α-C-mannosides. The method is currently being applied to higher order saccharide mimetics of PIM and to stereochemically diverse C-glycoinositols. These results will be reported in due course.

Experimental

Solvents were purified by standard procedures or used from commercial sources as appropriate. Petroleum ether refers to the fraction of petroleum ether boiling between 40 and 60 °C. Ether refers to diethyl ether. Unless otherwise stated thin layer chromatography (TLC) was done on 0.25 mm thick precoated silica gel 60 (HF-254, Whatman) aluminium sheets and flash column chromatography (FCC) was performed using Kieselgel 60 (32–63 mesh, Scientific Adsorbents). Elution for FCC usually employed a stepwise solvent polarity gradient, correlated with TLC mobility. Chromatograms were observed under UV (short and long wavelength) light, and/or were visualized by heating plates that were dipped in a solution of ammonium(vi) molybdate tetrahydrate (12.5 g) and cerium(iv) sulfate tetrahydrate (5.0 g) in 10% aqueous sulphuric acid (500 mL), or a solution of 20% sulfuric acid in ethanol. Optical rotations ([α]d were recorded using a Rudolph Autopol III or Jasco P-1020 polarimeters with 10 or 5 cm cells (path lengths of 1 or 0.5 dm) respectively, and are given in units of 10⁻¹ deg cm² g at 589 nm (sodium d-line). Infra-red spectra were obtained using a Thermo Scientific Nicolet IS5 spectrometer as thin film liquid samples between sodium chloride plates. Only selected absorbances (ν_max) are reported. NMR spectra were recorded using either Varian Unity Plus 500, Bruker Ultra Shield and Bruker Ultra Shield Plus instruments. Unless otherwise noted, ¹H and ¹³C spectra were recorded at 500 and 125 MHz respectively in CDCl₃ or C₆D₆ solutions with residual CHCl₃ or C₆H₆ as internal standard (δ_H 7.27, 7.16 and δ_C 77.2, 128.4 ppm). Chemical shifts are quoted in ppm relative to tetramethylsilane (δ_H 0.00) and coupling constants (J) are given in hertz. First order approximations are employed throughout. High resolution mass spectrometry was performed on an Ultima Micromass Q-TOF or Waters Micromass LCT Premier mass spectrometers.

6-O-Phenylcarbonothioyl-2,3-O-(d-1′,7′,7′ -trimethyl-[2.2.1]-bicyclohept-2′-ylidene)-1,4,5-tris-O-pivaloyl-d-myo-inositol (11)

Phenyl chlorothionoformate (0.10 mL, 0.77 mmol) was added dropwise to a suspension of the alcohol 10 (196 mg, 0.35 mmol) and DMAP (4 mg, 0.04 mmol), in dry toluene 10 mL at rt. Pyridine (0.14 mL, 1.75 mmol) was then added and the resulting suspension was stirred for 2 h at rt. The reaction mixture was then washed with 0.1 M HCl and saturated aqueous NaHCO₃, and the organic extract dried (Na₂SO₄) and concentrated in vacuo. FCC of the residue gave 11 (0.20 g, 83%). R_f = 0.50 (10% ethyl acetate–petroleum ether); ¹H NMR (CDCl₃) δ ¹H NMR (CDCl₃) δ 0.88 (s, 3H), 0.98 (s, 3H), 1.09 (s, 3H), 1.25 (m, 28H), 1.45 (m, 2H), 1.74 (m, 2H), 1.91 (m, 2H), 4.10 (t, 1H, J = 6.0 Hz, H-2/3), 4.65 (dd, 1H, J = 4.3, 6.4 Hz, H-2/3), 5.25 (m, 3H, H-1, 4, 5), 6.31 (dd, 1H, J = 7.7, 10.7 Hz, H-6), 7.00 (d, 2H, J = 8.0 Hz), 7.30 (t, 1H, J = 8.0 Hz), 7.42 (t, 2H, J = 8.0 Hz); ¹³C NMR (CDCl₃) δ 9.93, 20.2 (two signals), 26.9, 27.0, 27.0, 27.1, 29.3, 38.8, 44.0, 45.1, 48.0, 69.4, 71.8, 72.1, 72.2, 72.9, 79.0, 118.7, 121.7, 126.6, 129.5, 153.4, 176.5, 177.0, 177.6, 194.5. HRMS (EI) m/z calcd for C₃₈H₅₄O₁₀NaS (M + Na)⁺ 725.3334, found 725.3334.

6-Deoxy-6-C-(3′-propenyl)-2,3-O-(d-1′,7′,7′-trimethyl-[2.2.1]-bicyclohept-2′-ylidene)-1,4,5-tris-O-pivaloyl-d-myo-inositol (12)

A solution of 11 (167 mg, 0.24 mmol), allyltributyltin (0.22 mL, 0.72 mmol), AIBN (7.88 mg, 0.05 mmol) and toluene (3.5 mL) was heated at reflux for 18 h. The mixture was then evaporated under reduced pressure. FCC of the residue afforded 12 (0.85 g, 61%). R_f = 0.69 (10% ethyl acetate–petroleum ether); IR ν 1734 cm⁻¹; ¹H NMR (CDCl₃) δ 0.90 (s, 3H), 1.00 (s, 3H), 1.05 (s, 3H), 1.22 (s, 18H), 1.22 (m, 1H, buried under singlet), 1.28 (s, 9H), 1.44 (m, 2H), 1.70 (m, 2H), 1.90 (m, 2H), 2.25 (m, 2H, H-6′), 2.57 (m, 1H, H-6), 3.97 (t, 1H, J = 6.2 Hz, H-3), 4.45 (dd, 1H, J = 4.4, 5.3 Hz, H-2), 4.95 (m, 3H, H-1, 5, =CH), 5.08 (m, 2H, H-4, =CH), 5.65 (m, 1H, =CH); ¹³C NMR (CDCl₃) δ 9.6, 20.2, 20.3, 26.9, 27.0, 27.1, 27.2, 29.2, 30.6, 38.1, 38.7, 38.8, 38.9, 45.0, 45.1, 48.0, 51.7, 68.7, 69.7, 72.8, 73.7, 74.5, 118.1, 119.0, 132.6, 177.0, 177.0, 177.5. HRMS (EI) m/z calcd for C₃₄H₅₄O₈Na (M + Na)⁺ 613.3710, found 613.3712.

6-Deoxy-6-C-(2′-ethanoic acid)-1,2,3,4,5-O-penta-O-benzyl-d-myo-inositol (13)

Sodium hydroxide (5.5 g, 0.14 mol) was added to the solution of 12 (2.0 g, 3.4 mmol) in dry methanol (30 mL). The reaction mixture was heated at reflux for 3 h. Most of the volatiles were then removed under reduced pressure. The residue was diluted with water, neutralized with 1 M HCl and extracted with ethyl acetate. The organic phase was dried (Na₂SO₄) and concentrated in vacuo. An approximately 2 M solution of HCl in methanol (200 mL) was added to the residue, the mixture stirred for 16 h, then neutralized with methanolic NaOH and evaporated in vacuo. The product was taken up in a mixture of 20% MeOH in CHCl₃ and filtered through a short column of silica gel. The filtrate was evaporated in vacuo. To a solution of the residue in dry DMF (15 mL) at 0 °C under an argon atmosphere, was added NaH (2.1 g, 60% in mineral oil, 52 mmol) and Bu₄NI (0.54 g, 1.5 mmol). After 10 min BnBr (5.7 mL, 48 mmol) was introduced and stirring continued for an additional 0.5 h at rt. The reaction mixture was then diluted with water, and extracted with ether. The organic layer was washed with water, dried (Na₂SO₄) and concentrated in vacuo. The residue was purified by FCC to give the penta-O-benzyl derivative (1.5 g, 68%): clear oil; R_f = 0.75 (10% ethyl acetate–petroleum ether); IR ν 3433 (br), 1729, 1644 cm⁻¹; ¹H NMR (CDCl₃) δ 2.60 (m, 3H, H-6, 6′), 3.15 (dd, 1H, J = 1.7, 11.1 Hz, H-1), 3.25–3.40 (m, 2H, H-3/4, 5), 4.10–4.25 (m, 2H, H-2, 3/4), 4.46 (ABq, 2H, Δδ = 0.02 ppm, J = 11.4 Hz, PhCH₂), 4.68 (m, 3H, PhCH), 4.85 (m, 3H, PhCH), 5.05 (m, 4H, =CH₂, PhCH), 5.74 (m, 1H, =CH), 7.18–7.48 (m, 25H, ArH); ¹³C NMR (CDCl₃) δ 30.3, 41.2, 71.4, 72.7, 73.1, 73.7, 74.5, 75.6, 77.2, 79.5, 81.5, 83.6, 117.6, 127.0–128.5 (several lines), 135.0, 138.1, 138.5, 138.9, 139.2. HRMS (EI) m/z calcd for C₄₄H₄₇O₅ (M + H)⁺ 655.3418, found 655.3421.

A solution of the material from the previous step (1.53 g, 2.34 mmol) in 5 : 1 CH₂Cl₂–MeOH (72 mL) was cooled to −78 °C. A stream of O₃ in O₂ was bubbled through the solution until the starting material was not detectable by TLC. The mixture was flushed with argon and then triphenylphosphine (1.22 g, 4.68 mmol) was added. The mixture was warmed to rt, stirred for 2 h, and concentrated in vacuo. The residue was purified by FCC to provide the derived aldehyde (1.47 g, 96%): clear oil; R_f = 0.79 (30% ethyl acetate–petroleum ether); ¹H NMR (CDCl₃) δ 2.40–2.56 (m, 2H), 2.80–3.00 (m, 1H), 3.07 (dd, 1H, J = 2.0, 11.2 Hz), 3.20 (dd, 1H, J = 9.0, 10.7 Hz), 3.37 (dd, 1H, J = 2.2, 9.85 Hz), 4.07 (bt, 1H, J = 2.0 Hz), 4.08–4.19 (m, 2H), 4.30 (d, 1H, J = 11.5 Hz), 4.50 (dd, 2H, J = 2.0, 10.7 Hz), 4.75 (ABq, Δδ = 0.08, J = 11.8 Hz), 4.82 (d, 2H, J = 10.9 Hz), 8.87 (d, 1H, J = 11.9 Hz), 8.88 (d, 1H, J = 10.7 Hz), 5.01 (d, 1H, J = 10.8 Hz), 7.20–7.50 (m, 25H), 9.55 (t, 1H, J = 2.6 Hz); ¹³C NMR (CDCl₃) δ 38.8, 44.2, 71.6, 72.8, 73.0, 74.0, 74.9, 75.6, 78.4, 81.2, 81.4, 83.3, 127.4, 127.5, 127.7, 127.7, 127.9, 128.0, 128.1, 128.2, 128.2, 128.4, 128.4, 128.5, 128.5, 137.2, 138.0, 138.2, 138.8, 138.9, 202.0.

A solution of aldehyde from the previous step (1.47 g, 2.23 mmol), in THF (22.3 mL) was cooled to 0 °C. Solutions of 2,3-dimethyl-2-butene (1.1 mL, 2 M in THF), aqueous 1 M sodium biphosphate (2.20 mL, 2.20 mmol), and aqueous 1 M sodium chlorate (2.20 mL, 2.20 mmol) were then sequentially added. The reaction mixture was warmed to rt, stirred for 2 h, then extracted with ethyl acetate. The organic phase was washed with brine, dried (Na₂SO₄), filtered and concentrated in vacuo. The residue was purified by FCC to give 13 (1.37 g, 93%); R_f = 0.48 (30% ethyl acetate–petroleum ether); ¹H NMR (CDCl₃) δ 2.68 (d, 2H, J = 4.9 Hz), 2.70–2.78 (m, 1H), 3.30–3.34 (dd, 1H, J = 1.9, 11.4 Hz), 3.38 (dd, 1H, J = 2.3, 9.8 Hz), 3.40 (dd, 1H, J = 8.9, 10.9 Hz), 3.70–4.02 (m, 1H), 4.03–4.20 (m, 2H), 4.37 (d, 1H, J = 11.4 Hz), 4.52 (d, 1H, J = 11.4 Hz), 4.60 (d, 1H, J = 11.0 Hz), 4.61 (ABq, Δδ = 0.08, J = 8.3 Hz), 4.75–4.90 (m, 3H), 5.00 (t, 2H, J = 10.9 Hz), 7.20–7.49 (m, 25H); ¹³C NMR (CDCl₃) δ 23.9, 29.1, 32.0, 39.2, 67.7, 71.6, 72.8, 72.9, 73.8, 75.0, 75.6, 77.8, 80.2, 81.3, 83.4, 108.0, 127.3, 127.5, 127.6, 127.7, 127.7, 127.9, 127.9, 127.9, 128.0, 128.2, 128.4, 128.4, 128.4, 128.4, 128.5, 137.6, 138.4, 138.4, 138.8, 139.0. HRMS (ES) m/z calcd for C₄₃H₄₄O₇Na (M + Na)⁺ 695.2985, found 695.2963.

Thioacetal ester (15)

A mixture of acid 13 (0.38 g, 0.56 mmol), 2,4,6-trichlorobenzoyl chloride (0.09 mL, 0.56 mmol) and triethylamine (0.16 mL, 1.13 mmol) in THF (30 mL) was stirred for 3.5 h at 0 °C. A mixture of alcohol 14 (0.29 g, 0.56 mmol) and DMAP (89 mg, 0.73 mmol) in toluene (15 mL) were added, and stirring continued for 1 h. The mixture was then diluted with ether, washed with saturated aqueous NaHCO₃ and brine, dried (Na₂SO₄), filtered, and evaporated under reduced pressure. The residue was purified by FCC to give ester 15 (0.57 g, 86%): colorless oil; R_f = 0.58 (15% ethyl acetate–petroleum ether); IR ν 1727 cm⁻¹; ¹H NMR (CDCl₃) δ 1.02 (s, 9H), 1.37 (s, 3H), 1.44 (s, 3H), 2.68–2.85 (m, 3H), 3.38 (dd, 1H, J = 2.3, 9.9 Hz), 3.45–3.55 (m, 2H), 3.70–3.83 (m, 2H), 4.03–4.15 (m, 2H), 4.33 (dd, 1H, J = 4.5, 6.7 Hz), 4.40 (d, 1H, J = 11.4 Hz), 4.48 (d, 1H, J = 11.4 Hz), 4.52 (d, 1H, J = 10.9 Hz), 4.64 (d, 2H, J = 4.2 Hz), 4.75–4.85 (m, 3H), 4.90 (d, 1H, J = 10.8 Hz), 5.01 (d, 1H, J = 10.9 Hz), 5.23 (q, 1H, J = 5.0 Hz), 5.50 (d, 1H, J = 6.7 Hz), 7.15–7.73 (m, 40H); ¹³C NMR (CDCl₃) δ 19.1, 26.0, 26.8, 27.3, 31.5, 39.4, 62.1, 62.1, 71.7, 72.6, 72.8, 73.2, 73.7, 75.0, 75.4, 77.6, 79.4, 80.1, 81.3, 83.5, 84.9, 111.5, 127.6, 127.7, 127.7, 127.7, 127.8, 127.9, 127.9, 128.1, 128.3, 128.3, 128.4, 128.4, 128.4, 129.0, 132.0, 135.6, 135.6, 139.1, 172.0. HRMS (EI) m/z calcd for C₇₂H₇₈O₁₀NaSiS (M + Na)⁺ 1185.4977, found 1185.4977.

Thioacetal enol ether (16)

To a mixture of ester 15 (1.35 g, 1.16 mmol), and pyridine (0.10 mL) in anhydrous 3 : 1 toluene–THF (30 mL), was added, under an argon atmosphere and at −78 °C, Tebbe reagent (5.71 mL, 0.5 M in THF). The reaction mixture was warmed to rt, stirred at this temperature for 1 h, then slowly poured into 1 N aqueous NaOH at 0 °C. The resulting suspension was extracted with ether and the organic phase was washed with brine, dried (Na₂SO₄), filtered and concentrated in vacuo. FCC of the residue on basic alumina provided enol ether 16 (0.70 g, 77% based on recovered starting material) as a light yellow oil: R_f (basic alumina) = 0.45 (10% ethyl acetate–petroleum ether); ¹H NMR (C₆D₆) δ 1.16 (s, 9H), 1.40 (s, 3H), 1.51 (s, 3H), 2.61–3.00 (m, 3H), 3.47 (dd, 1H, J = 2.1, 10.0 Hz), 3.60 (dd, 1H, J = 1.8, 11.3 Hz), 3.77 (t, 1H, J = 9.1 Hz), 4.06 (dd, 1H, J = 4.8, 10.8 Hz), 4.10–4.20 (m, 2H), 4.20–4.26 (m, 3H), 4.40 (t, 1H, J = 9.5 Hz), 4.45–4.55 (m, 6H), 4.56 (q, 1H, J = 4.6 Hz), 4.61–4.77 (m, 2H), 4.80–5.11 (m, 5H), 5.26 (d, 1H, J = 11.4 Hz), 5.53 (s, 1H), 5.90 (d, 1H, J = 6.3 Hz), 6.87–7.85 (m, 40H); ¹³C NMR (C₆D₆) δ 19.7, 23.1, 26.5, 27.4, 27.5, 28.0, 30.4, 32.2, 34.8, 41.9, 62.7, 71.9, 73.2, 74.7, 74.9, 74.9, 75.9, 76.8, 79.1, 80.6, 80.7, 82.5, 85.0, 86.0, 86.4, 112.5, 112.6, 127.9, 127.9, 128.0, 128.1, 128.1, 128.1, 128.3, 128.5, 128.7, 128.7, 128.8, 128.8, 128.9, 129.0, 129.0, 129.6, 130.6, 130.6, 132.0, 133.8, 135.4, 136.4, 139.6, 139.7, 140.3, 140.3, 140.8, 160.8. HRMS (EI) m/z calcd for C₇₃H₈₀O₉NaSiS (M + Na)⁺ 1183.4184, found 1183.5188.

6-Deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(2′,6′-anhydro-5′,7′-dideoxy-3,4-O-isopropylidene-1′-O-tert-butyldiphenylsilyl-l-ribo-hept-5-enitol-7′-C-yl)-d-myo-inositol (17)

A mixture of enol ether 16 (0.49 g, 0.42 mmol), 2,6-di-tert-butyl-4-methylpyridine (1.02 g, 5.0 mmol), and freshly activated, powdered 4 Å molecular sieves (1.47 g) in anhydrous CH₂Cl₂ (20 mL), was stirred for 15 min, at rt, under an atmosphere of argon, then cooled to 0 °C. Methyl triflate (0.47 mL, 4.2 mmol) was then introduced, and the mixture warmed to rt, and stirred for an additional 18 h, at which time, triethylamine (1 mL) was added. The mixture was diluted with ether, washed with saturated aqueous NaHCO₃ and brine, dried (Na₂SO₄), filtered and evaporated in vacuo. FCC of the residue provided 17 (0.35 g, 78%) as a light yellow oil. R_f (basic alumina) = 0.35 (10% ethyl acetate–petroleum ether); ¹H NMR (C₆D₆) δ 1.16 (s, 9H, t-Bu), 1.23 (s, 3H, CH₃), 1.32 (s, 3H, CH₃), 2.70 (dd, 1H, J = 5.0, 14.3 Hz, H-7′a), 2.91 (dd, 1H, J = 3.1, J = 14.3 Hz, H-7′b), 2.97 (m, 1H, H-6), 3.39 (dd, 1H, J = 1.7, 12.0 Hz, H-1), 3.42 (dd, 1H, J = 2.0, 10.4 Hz, H-3), 3.64 (m, 1H, bdd, J = 2.5, 10.0 Hz, H-2′), 3.69 (dd, 1H, J = 9.1, 10.0 Hz, H-5), 3.99 (dd, 1H, J = 4.2, 11.0 Hz, H-1′a), 4.04 (dd, 1H, J = 5.5, 10.0 Hz, H-3′), 4.08 (bd, 1H, J = 11.0 Hz), 4.21 (bt, 1H, J = 1.9 Hz, H-2), 4.24 (t, 1H, J = 5.5 Hz, H-4′), 4.41 (t, 1H, J = 9.5 Hz, H-4), 4.46 (ABq, 2H, Δδ = 0.04 ppm, J = 11.1 Hz, PhCH₂), 4.55 (ABq, 2H, Δδ = 0.04 ppm, J = 11.9 Hz, PhCH₂), 4.80 (apparent d, 1H, J = 10.9 Hz, PhCH), 4.92 (ABq, 2H, Δδ = 0.03 ppm, J = 12.1 Hz, PhCH₂), 5.05 (m, 3H, H-5′, PhCH₂), 5.30 (apparent d, 1H, J = 11.3 Hz, PhCH), 7.00–7.28 (m, 21H, ArH), 7.31 (d, 2H, J = 1.2 Hz, ArH), 7.38 (d, 2H, J = 7.5 Hz, ArH), 7.46 (d, 4H, J = 7.5 Hz, ArH), 7.52 (d, 2H, J = 7.3 Hz, ArH), 7.84 (dt, 4H, J = 1.4, 7.9 Hz, ArH); ¹³C NMR (C₆D₆) δ 21.0, 27.2, 28.6, 30.2, 32.8, 42.6, 64.7, 70.7, 71.2, 72.9, 74.2, 75.4, 76.0, 76.1, 77.2, 78.7, 80.4, 82.3, 83.5, 85.8, 98.9 (C-5′), 109.5, 128.9–130.0 (several lines buried under C₆D₆), 131.4, 135.0, 135.1, 137.4, 137.5, 140.6, 140.7, 141.2, 141.6, 160.0 (C-6′). HRMS (EI) m/z calcd for C₆₇H₇₅O₉Si (M + H)⁺ 1051.5174, found 1051.5159.

6-Deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(2′,6′-anhydro-3′,4′-O-isopropylidene-1′-O-tert-butyldiphenylsilyl-7′-deoxy-d-glycero-d-talo-heptitol-7′-C-yl)-d-myo-inositol (18) and 6-deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(2′,6′-anhydro-3′,4′-O-isopropylidene-1′-O-tert-butyldiphenylsilyl-7′-deoxy-l-glycero-l-allo-heptitol-7′-C-yl)-d-myo-inositol (19)

BH₃·Me₂S (1.3 mL, 1 M solution, 1.3 mmol) was added at 0 °C to a solution of 17 (0.35 g, 0.33 mmol) in anhydrous THF (15 mL) under an atmosphere of argon. The mixture was warmed to rt and stirred for an additional 1 h, then recooled to 0 °C and treated with a mixture of 3 N NaOH (2.4 mL) and 30% aqueous H₂O₂ (2.4 mL) for 30 min. The mixture was then diluted with ether and washed with saturated aqueous NaHCO₃ and brine, dried (Na₂SO₄), filtered and evaporated under reduced pressure. FCC of the residue provided an unseparated mixture of 18 : 19 (0.30 g, ca. ratio 3 : 1, 83%). Repeated FCC on the mixture provided samples of separated 18 and 19.

For 18: R_f = 0.25 (20% ethyl acetate–petroleum ether); ${[\alpha ]}_{\mathrm{D}}^{23}-13.3$ (c 1.02, CHCl₃); IR ν 3411 cm⁻¹; ¹H NMR (CDCl₃) δ 1.05 (s, 9H, t-Bu), 1.34 (s, 3H, CH₃), 1.46 (s, 3H, CH₃), 1.75 (m, 1H, H-7′), 1.87 (m, 1H, H-7′), 2.65 (m, 1H, H-6), 3.15 (m, 3H, H-1, H-5, OH), 3.30 (dd, 1H, J = 2.1, 9.9 Hz, H-3), 3.44 (m, 1H, H-5′), 3.57 (m, 1H, H-6′), 3.71 (dd, 1H, J = 4.8, 10.5 Hz, H-1′a), 3.78 (dd, 1H, J = 6.7, 10.6 Hz, H-1′b), 3.89 (m, 2H, H-2′, H-4′), 4.05 (bs, 1H, H-2), 4.09 (t, 1H, J = 9.2 Hz, H-4), 4.26 (apparent d, 1H, J = 11.3 Hz, PhCH), 4.31 (dd, 1H, J = 3.6, 6.10 Hz, H-3′), 4.48 (apparent d, 1H, J = 11.2 Hz, PhCH), 4.64 (m, 3H, PhCH × 3), 4.72–4.89 (m, 3H, PhCH × 3), 4.94 (apparent d, 1H, J = 12.0 Hz, PhCH), 4.96 (apparent d, 1H, J = 12.0 Hz, PhCH), 7.11 (m, 3H, ArH), 7.20–7.50 (m, 28H, ArH), 7.65 (m, 4H, ArH); ¹³C NMR (CDCl₃) δ 19.2, 25.8, 26.9, 28.0, 33.2, 37.7, 64.1, 71.9, 72.6, 72.8, 73.5, 73.5, 73.6, 73.7, 74.8, 74.8, 75.4, 76.8, 80.3, 81.5, 82.4, 83.3, 108.8, 127.3–128.6 (several signals), 129.8 (two signals), 133.3, 135.6, 137.1, 138.5, 138.6, 138.9, 138.9. HRMS (EI) m/z calcd for C₆₇H₇₇O₁₀Si (M + H)⁺ 1069.5280, found 1069.5266.

For 19: R_f = 0.23 (20% ethyl acetate–petroleum ether); IR ν 3455 cm⁻¹; ¹H NMR (CDCl₃) δ 0.93 (s, 9H, t-Bu), 1.25 (s, 3H, CH₃), 1.27 (s, 3H, CH₃), 1.88 (m, 2H, H-7′), 2.58 (d, J = 8.3 Hz, OH), 2.50 (m, 1H, H-6), 3.09 (d, J = 10.9 Hz, H-1), 3.27 (m, 3H, H-2′, 3′, 3), 3.40 (t, J = 9.3 Hz, H-5), 3.60 (m, 2H, H-1′, 6′), 3.73 (bd, J = 11.2 Hz, H-1′), 3.87 (dd, J = 4.9, 9.3 Hz, H-3′), 4.00 (t, J = 9.6 Hz, H-4), 4.07 (bs, 1H, H-2), 4.26 (apparent d, J = 11.3 Hz, PhCH), 4.29 (t, J = 4.5 Hz, H-4′), 4.49 (m, 3H, PhCH × 3), 4.73 (m, 4H, PhCH × 4), 4.88 (apparent d, J = 10.8, PhCH), 4.94 (apparent d, J = 11.3, PhCH), 7.10–7.30 (m, 28H, ArH), 7.60 (m, 4H, ArH); ¹³C NMR (CDCl₃) δ 19.5, 26.6, 27.1, 28.4, 22.1, 38.4, 64.5, 71.8 (2C), 72.8, 72.9 (two signals), 73.7, 75.0, 75.3 (2C), 75.8, 78.8, 79.9, 81.8, 82.7, 83.7, 110.0, 127.4–128.6 (several signals), 129.7 (two signals), 133.7, 133.8, 135.8, 135.9, 138.1, 138.8, 138.9, 139.1, 139.3. HRMS (EI) m/z calcd for C₆₇H₇₆O₁₀NaSi (M + Na)⁺ 1091.5105, found 1091.5076.

6-Deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(2′,6′-anhydro-5-O-benzoyl-3′,4′-O-isopropylidene-1′-O-tert-butyldiphenylsilyl-7′-deoxy-d-glycero-d-talo-heptitol-7′-C-yl)-d-myo-inositol (20) and 6-deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(2′,6′-anhydro-5-O-benzoyl-3′,4′-O-isopropylidene-1′-O-tert-butyl-diphenylsilyl-7′-deoxy-l-glycero-l-allo-heptitol-7′-C-yl)-d-myo-inositol (21)

A mixture of 18 and 19 (244 mg, 0.229 mmol) was dissolved in CH₂Cl₂ (20 mL), and pyridine (0.055 mL, 0.675 mmol) and benzoyl chloride (0.034 mL, 0.296 mmol) were added to the reaction mixture. The reaction was monitored by TLC. The mixture was diluted with CH₂Cl₂, washed with saturated aqueous NaHCO₃ and brine, dried (Na₂SO₄), filtered and evaporated in vacuo. FCC of the residue provided recovered 19 (30 mg, 12%), 20 (94 mg, 40% brsm), 21 (77 mg, 33% brsm), and unseparated 20/21 (38 mg, 16% brsm).

For 20: R_f = 0.63 (20% ethyl acetate–petroleum ether); IR ν 1720 cm⁻¹; ¹H NMR (CDCl₃) δ 1.11 (s, 9H, t-Bu), 1.28 (s, 3H, CH₃), 1.49 (s, 3H, CH₃), 1.62 (ddd, J = 2.1, 7.4, 12.8 Hz, 1H, H-7′a), 2.18 (dt, J = 1.8, 12.8 Hz, 1H, H-7′b), 2.67 (apparent q, J = 11.1 Hz, 1H, H-6), 3.19 (m, 2H, H-1, H-5), 3.26 (dd, 1H, J = 2.2, 9.9 Hz, H-3), 3.82 (dd, 1H, J = 4.1, 10.9 Hz, H-1′a), 3.89 (dd, 1H, J = 4.7, J = 10.9 Hz, H-1′b), 4.00 (q, 1H, J = 4.5 Hz, H-2′), 4.03 (bt, 1H, J = 2.0 Hz, H-2), 4.15 (t, 1H, J = 9.3 Hz, H-4), 4.23 (t, 1H, J = 6.1 Hz, H-4′), 4.29 (m, 2H, H-6′, PhCH × 2), 4.35 (t, 1H, J = 5.1 Hz, H-3′), 4.46 (apparent d, 1H, J = 11.6 Hz, PhCH), 4.60 (m, 3H, PhCH × 3), 4.75 (apparent d, 1H, J = 10.7 Hz, PhCH), 4.81 (s, 2H, PhCH₂), 4.97 (apparent t, 2H, J = 9.6 Hz, PhCH × 2), 5.22 (t, 1H, J = 6.9 Hz, H-5′), 7.09–7.40 (m, 33H, ArH), 7.60 (t, 1H, J = 7.5 Hz, ArH), 7.75 (m, 4H, ArH), 7.95 (d, 2H, J = 8.0 Hz, ArH); ¹³C NMR (CDCl₃) δ 19.5, 26.4, 27.2, 27.9, 30.4, 37.8, 64.7, 71.5, 72.4, 72.5, 72.8, 72.9, 73.1, 73.5, 74.3, 75.6, 81.0, 81.1, 81.6, 83.4, 109.5, 127.6–130.4 (several signals), 133.2, 133.6, 133.8, 135.9, 136.0, 138.2, 138.7, 138.9, 139.2, 139.4, 165.8. HRMS (EI) m/z calcd for C₇₄H₈₁O₁₁Si (M + H)⁺ 1173.5542, found 1173.5525.

For 21: R_f = 0.55 (20% ethyl acetate–petroleum ether); ¹H NMR (CDCl₃) δ 0.96 (s, 9H, t-Bu), 1.20 (s, 3H, CH₃), 1.28 (s, 3H, CH₃), 1.82 (m, 1H, H-7′a), 1.94 (m, 1H, H-7′b), 2.58 (m, 1H, H-6), 3.18 (dd, 1H, J = 0.7, 11.5 Hz, H-1), 3.23 (dd, 1H, J = 2.1, 10.0 Hz, H-3), 3.44 (dd, 1H, J = 6.3, 7.8 Hz, H-2′), 3.52 (t, 1H, J = 10.0 Hz, H-5), 3.64 (dd, 1H, J = 5.9, 11.3 Hz, H-1′a), 3.78 (bd, 1H, J = 10.0 Hz, H-1′b), 3.91 (t, 1H, J = 11.0 Hz, H-4), 3.95 (dd, 1H, J= 4.8, 9.4 Hz, H-3′), 4.12 (bs, 1H, H-2), 4.17 (dt, 1H, J = 2.9, 9.8 Hz, H-6′), 4.30 (apparent d, 1H, J = 11.0, PhCH), 4.42 (ABq, 2H, J = 11.8 Hz, Δδ = 0.06 ppm, PhCH₂), 4.54 (apparent d, 1H, J = 10.9 Hz, PhCH), 4.58 (t, 1H, J = 4.3 Hz, H-4′), 4.63 (apparent d, 1H, J = 10.8 Hz, PhCH), 4.72 (apparent d, 1H, J J= 11.0 Hz, PhCH), (s, 2H, PhCH₂), 4.79 (m, 3H, PhCH × 3), 4.85 (partially buried dd, 1H, J = 3.9, 9.7 Hz, H-5′), 4.88 (apparent d, 1H, J = 10.8, PhCH), 7.05–7.40 (m, 33H, ArH), 7.45 (t, 1H, J = 7.5, ArH), 7.62 (m, 4H, ArH); 7.78 (d, 2H, J = 7.8 Hz, ArH); ¹³C NMR (CDCl₃) δ 19.5, 26.5, 27.2, 28.3, 29.8, 38.4, 64.5, 71.9, 72.8, 72.9 (two signals), 73.0, 73.2, 73.9, 74.7, 75.8, 79.0, 79.4, 81.1, 81.8, 81.9, 84.0, 110.4, 127.4–128.7 (several signals), 129.8, 130.1, 130.2, 133.2, 133.6, 133.8, 135.8 (two signals), 138.0, 139.0, 139.2, 139.3, 139.4, 166.0. HRMS (ES) m/z calcd for C₇₄H₈₀O₁₁NaSi (M + Na)⁺ 1195.5368, found 1195.5410.

6-Deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(2′,6′-anhydro-5-O-benzoyl-1′-O-tert butyldiphenylsilyl-7′-deoxy-l-glycero-l-allo-heptitol-7′-C-yl)-d-myo-inositol (22)

5% Acetyl chloride in methanol (0.1 mL) was added to a solution of 20 (206 mg, 0.176 mmol) in dry CH₂Cl₂ (15 mL). The reaction mixture was stirred at rt for 20 min. The mixture was neutralized by addition of sodium methoxide. Removal of the volatiles under reduced pressure and FCC of the residue provided 22 (142 mg, 71%): IR ν 3443 cm⁻¹; R_f = 0.54 (40% ethyl acetate–petroleum ether); ¹H NMR (CDCl₃) δ 1.10 (s, 9H, t-Bu), 1.54 (m, 1H, H-7′a), 2.56 (m, 2H, H-6, 7′b), 2.67 (d, 1H, J = 3.1 Hz, OH), 2.93 (d, 1H, J = 4.0 Hz, OH), 3.17 (m, 2H, H-1, 5), 3.33 (dd, 1H, J = 2.3, J = 9.9 Hz, H-3), 3.80 (dd, 1H, J = 6.8, 10.5 Hz, H-1′a), 3.83 (dd, 1H, J = 4.3, J = 10.5 Hz, H-1′b), 3.92 (m, 1H, H-2′), 4.02 (m, 1H, H-4′), 4.07 (m, 2H, H-2, 3′), 4.18 (t, 1H, J = 9.3 Hz, H-4), 4.32 (m, 2H, H-6′, PhCH), 4.51 (apparent d, 1H, J = 11.5 Hz, PhCH), 4.62 (m, 3H, PhCH × 3), 4.80 (m, 3H, PhCH × 3), 5.03 (apparent d, 1H, J = 11.5 Hz, PhCH), 5.04 (apparent d, 1H, J = 11.5 Hz, PhCH), 5.15 (dd, 1H, J = 3.7, J = 4.8 Hz, H-5′), 7.20–7.41 (m, 33H, ArH), 7.54 (t, 1H, J = 6.5 Hz, ArH), 7.68 (m, 4H, ArH), 7.95 (d, 2H, J = 7.0 Hz, ArH); ¹³C NMR (CDCl₃) δ 19.2, 27.0, 30.0, 37.2, 65.1, 69.0, 69.2, 69.3, 71.4, 72.6 (two signals), 73.3, 73.5, 74.0, 75.4, 75.5, 81.4, 81.5, 81.8, 83.6, 127.5, 127.5, 127.7, 127.9, 128.0, 128.0, 128.1, 128.3, 128.3, 128.3, 128.4, 128.4, 129.9, 130.0, 132.8, 133.0 135.6, 135.7, 135.7, 137.7, 138.5, 138.6, 138.7, 139.2, 166.0. HRMS (EI) m/z calcd for C₇₁H₇₇O₁₁Si (M + H)⁺ 1133.5229, found 1133.5215.

6-Deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(3′-O-acetyl-2′,6′-anhydro-5-O-benzoyl-1′-O-tert-butyldiphenylsilyl-7′-deoxy-l-glycero-lallo-heptitol-7′-C-yl)-d-myo-inositol (23) and 6-deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(4′-O-acetyl-2′,6′-anhydro-5-O-benzoyl-1′-O-tert-butyldiphenylsilyl-7′-deoxy-l-glycero-l-allo-heptitol-7′-C-yl)-d-myo-inositol (24)

A solution of 22 (0.041 g, 0.036 mmol) in dry benzene (6 mL) and triethyl orthoacetate (6 mL) was treated with CSA (1.1 mg) at rt for 1 h. Triethylamine (3 mL) was then added and the mixture diluted with water and extracted with ethyl acetate. The organic phase was washed with saturated aqueous NaHCO₃, water and brine, dried (Na₂SO₄) and concentrated under reduced pressure. The residue was dissolved in CH₂Cl₂ (10 mL) and treated with 80% aqueous acetic acid (20 mL). The mixture was stirred at rt for 35 min, then diluted with toluene and evaporated under reduced pressure. FCC of the residue afforded 23 (0.027 g, 64%) and 24 (0.014 g, 33%).

For 23: R_f = 0.17 (20% ethyl acetate–petroleum ether); IR ν 3443, 1730 cm⁻¹; ¹H NMR (CDCl₃) δ 1.10 (s, 9H, t-Bu), 1.66 (m, 1H, H-7′a), 2.00 (s, 3H, CH₃CO), 2.20 (dd, 1H, J = 6.4, 10.4 Hz, H-7′b), 2.63 (m, 1H, H-6), 3.18 (m, 2H, H-1, 5), 3.26 (dd, 1H, J = 2.2, 9.9 Hz, H-3), 3.76 (dd, 1H, J = 4.7, 10.9 Hz, H-1′b), 3.81 (dd, 1H, J = 6.0, 10.9 Hz, H-1′a), 3.99–4.17 (m, 4H, H-2, 4, 2′, 4′), 4.7 (m, 1H, PhCH, H-6′), 4.48 (apparent d, 1H, J = 11.4 Hz), 4.60 (m, 3H, PhCH), 4.7 (m, 3H, PhCH), 4.90 (m, 2H, PhCH), 5.13 (t, 1H, J = 7.5 Hz, H-5′), 5.43 (t, 1H, J = 3.8 Hz, H-3′), 7.18–7.46 (m, 33H, ArH), 7.51 (t, J = 7.5 Hz, 1H, ArH), 7.68 (m, 4H, ArH), 7.86 (d, 2H, J = J = 7.8 Hz, ArH); ¹³C NMR (500 MHz, CDCl₃) δ 19.2, 21.1, 26.8, 30.6, 37.3, 62.4, 68.9, 71.1, 71.4, 72.6, 72.7, 72.8, 73.2, 73.3, 74.5, 74.7, 75.5, 81.0, 81.2, 81.4, 83.3, 127.2, 127.5, 127.6, 127.7, 127.7, 127.8, 127.8, 128.0, 128.1, 128.2, 128.3, 128.4, 129.8, 129.9, 133.2, 135.6, 135.7, 138.0, 139.1, 166.5, 170.2. HRMS (ES) m/z calcd for C₇₃H₇₈O₁₂NaSi (M + Na)⁺ 1197.5160, found 1197.5205.

For 24: R_f = 0.20 (20% ethyl acetate–petroleum ether); IR ν 3452, 1726 cm⁻¹; ¹H NMR (CDCl₃) δ 0.97 (s, 9H, t-Bu), 1.45 (m, 1H, H-7′a), 1.86 (s, 3H, CH₃CO), 2.06 (bs, 1H, OH), 2.32 (t, 1H, J = 12.2 Hz, H-7′b), 2.50 (m, 1H, H-6), 3.07 (m, 2H, H-1, 5), 3.17 (dd, 1H, J = 2.2, 9.9 Hz, H-3), 3.72 (m, 1H, CH₂-1′), 3.88 (m, 1H, H-2′), 3.94 (bs, 1H, H-2), 4.02 (t, 1H, J = 9.3 Hz, H-4), 4.14 (apparent d, 1H, J = 11.4 Hz, 1H, PhCH), 4.22 (m, 1H, H-3′), 4.27 (m, 1H, H-6′), 4.35 (apparent d, 1H, J = 11.4 Hz, PhCH), 4.50 (m, 3H, PhCH), 4.68 (m, 3H, PhCH), 4.91 (m, 2H, PhCH), 5.13, (t, 1H, J = 5.10, H5′), 5.30 (dd, 1H, J = 3.6, 6.3 Hz, H-4′), 7.10–7.30 (m, 33H, ArH), 7.46 (t, 1H, J = 7.5 Hz, ArH), 7.62 (m, 4H, ArH), 7.80 (d, 2H, J = 7.8 Hz, ArH); ¹³C NMR (500 MHz, CDCl₃) δ 19.4, 21.1, 27.1, 29.8, 37.7, 63.9, 67.1, 71.1, 71.5, 72.2, 72.6, 72.7, 72.8, 73.5, 75.0, 75.6, 81.1, 81.2, 81.6, 83.6, 127.8–128.6 (several lines), 129.8, 129.9, 130.0, 130.1, 133.1, 133.2, 133.4, 135.8, 135.9, 138.0, 138.7, 138.8, 139.1, 139.3, 166.5, 170.2. HRMS (ES) m/z calcd for C₇₃H₇₈O₁₂NaSi (M + Na)⁺ 1197.5160, found 1197.5188.

6-Deoxy-1,2,3,4,5-penta-O-benzyl-6-C-(3′-O-acetyl-2′,6′-anhydro-5-O-benzoyl-1′-O-tert-butyldiphenylsilyl-7′-deoxy-d-glycero-d-manno-heptitol-7′-C-yl)-d-myo-inositol (25)

To a solution of 23 (0.025 g, 0.021 mmol), in CH₂Cl₂ (0.3 mL) was added pyridine (0.03 mL) at −20 °C. Trifluoromethane-sulfonic anhydride (0.007 mL, 0.021 mmol) in CH₂Cl₂ (0.5 mL) was then added dropwise, and the mixture warmed to 10 °C over 2 h. The solution was then diluted with CH₂Cl₂ and washed with 1 M HCl, saturated aqueous NaHCO₃, water and brine. The organic phase was dried (Na₂SO₄) and concentrated in vacuo at rt. KNO₂ (0.020 g, 0.24 mmol) was added to a solution of the crude product in dry DMF (1 mL). After stirring at 50 °C for 6 h, the mixture was diluted with CH₂Cl₂ and washed with brine. The organic phase was dried (MgSO₄) and concentrated in vacuo. Purification of the residue by FCC afforded 25 (0.020 g, 80%): R_f = 0.10 (20% ethyl acetate–petroleum ether); ${[\alpha ]}_{\mathrm{D}}^{23}-23$ (c 1.0, CHCl₃); IR ν 3468, 1717 cm⁻¹; ¹H NMR (CDCl₃) δ 1.11 (s, 9H, t-Bu), 1.41 (m, 1H, H-7′a), 2.00 (s, 3H, CH₃CO), 2.08 (m, 1H, H-7′b), 2.30 (d, 1H, J = 9.9 Hz, OH), 2.54 (m, 1H, H-6), 3.12 (m, 2H, H-1, 5), 3.31 (dd, 1H, J = 2.1, 9.9 Hz, H-3), 3.51 (dd, 1H, J = 3.5, 11.3 Hz, H-1′a, 2′), 3.68 (m, 2H, H-1′b, 2), 3.86 (m, 1H, H-4′), 4.15 (m, 2H, H-2, 4), 4.26 (apparent d, 1H, J = 11.6 Hz, PhCH), 4.65 (m, 5H, H-6′, PhCH × 4), 4.77 (apparent d, 1H, 10.7 Hz, PhCH), 4.82 (s, 2H, PhCH₂), 5.01 (apparent d, 1H, J = 10.7 Hz, PhCH), 5.03 (apparent d, 1H, J = 11.2 Hz, PhCH), 5.28 (bs, 1H, H-5′), 5.49 (t, 1H, J = 9.6 Hz, H-3′), 7.16–7.48 (m, 33H, ArH), 7.55 (t, 1H, J = 7.41, ArH), 7.69 (dd, 2H, J = 1.1, 7.7 Hz, ArH), 7.71 (dd, 2H, J = 1.2, 7.8 Hz, ArH), 8.09 (d, 2H, J = 7.3 Hz, ArH); ¹³C NMR (CDCl₃) δ 19.3 (C-Si), 21.0 (CH₃), 26.7 [(CH₃)₃C], 28.1 (C-7′), 37.2 (C-6), 62.7 (C-1′), 69.7 (C-4′), 70.2 (C-3′), 71.0 (PhC), 71.2 (C-2′), 72.4 (C-2), 72.6 (PhC), 73.4 (PhC), 74.5 (PhC), 75.2 (C-5′, 6′), 75.4 (PhC), 80.6 (C-1), 81.3 (C-3, 5), 83.7 (C-4), 127.2–128.5 (several signals), 129.4, 129.5, 130.2, 133.0, 133.3, 133.5, 135.7, 135.8, 137.5, 138.5, 138.6, 138.9, 139.2 (all Ar), 166.0 (CvO), 171.4 (CvO). HRMS (ES) m/z calcd for C₇₃H₇₈O₁₂NaSi (M + Na)⁺ 1197.5160, found 1197.5167.

6-Deoxy-6-C-(2′,6′-anhydro-7′-deoxy-d-glycero-d-manno-heptitol-7′-C-yl)-d-myo-inositol [C-glycoside of α-d-mannose-(1 → 6)-d-myo-inositol] (4)

A mixture of 25 (0.008 g, 0.01 mmol) and NaOMe (ca. 5 mg, 0.01 mmol) in methanol (5 mL) was stirred at rt for 30 min. The solvent was then removed under reduced pressure and THF (1 mL) and TBAF (0.012 mL of a 1 M solution in THF, 0.012 mmol) were added to the residue. The mixture was stirred for 5 h at rt, then diluted with saturated aqueous NaHCO₃ and extracted with ether. The organic phase was dried (Na₂SO₄) and concentrated in vacuo. FCC of the residue afforded a homogeneous material (0.0040 g) as a colorless oil: R_f = 0.21 (2% methanol–ethyl acetate). A mixture of this product, 20% Pd on carbon (0.0123 g), and methanol (1 mL) was stirred under an atmosphere of hydrogen (balloon), for 12 h. The mixture was then purged with argon and filtered through Celite. The filtrate was concentrated in vacuo, and the residue purified by FCC to give 4 (0.002 g, 87%); R_f = 0.12 (40% methanol–CH₂Cl₂); ${[\alpha ]}_{\mathrm{D}}^{19}$ 6 (c 0.2, H₂O); ¹H NMR (600 MHz, D₂O, external standard: Me₄NBr) δ 1.70 (m, 1H, H-7′a), 1.90 (m, 1H, H-6), 2.18, (bt, 1H, J = 13.8 Hz, H-7′b), 3.20 (t, 1H, J = 9.1 Hz, H-5), 3.46 (bd, 1H, J = 10.0 Hz, H-3), 3.55–3.78 (m, 5H), 3.83 (m, 2H), 3.92 (bd, 1H, J = 1.5 Hz), 3.99 (bd, 1H, J = 2.4 Hz), 4.25 (bd, 1H, J = 8.4 Hz, H-6′); ¹³C NMR (150 MHz, D₂O) δ 29.1 (C-7′), 41.7 (C-6), 63.8 (C-1′), 69.9, 73.1, 73.5, 73.6, 74.4, 75.2 (C-5), 75.7, 76.0, 76.5 (C-3), 79.3; HRMS (ES) m/z calcd for C₁₃H₂₄O₁₀Na (M + Na)⁺ 363.1267, found 363.1267.