4′,6′-O-Benzylidene- and 4′,6′-O-p-Methoxybenzylidene-α,β-cellobiose and Their Hexa-O-β-benzoyl Derivatives

Abstract

The preparation of 4′,6′-O-benzylidenecellobiose has been improved and its p-methoxybenzylidene counterpart was prepared in a similar way. As with other alkylidene derivatives of sugars, the corresponding per-O-benzoates are versatile intermediates in carbohydrate synthesis, allowing, after hydrolytic cleavage or opening of the phenylidene ring, regioselective manipulation of hydroxyl groups in cellobiose, or access to various derivatives of lactose.

In connection with synthetic work in this laboratory, a need arose for an intermediate that would allow access to derivatives of lactose modified at position 4′. A feasible route to such derivatives can start from cellobiose and involve S_N2 displacement of a suitable leaving group at C-4′. Derivatives of cellobiose that are amenable to chemical manipulation solely at that position can be prepared by reductive opening of the phenylidene ring in the title, fully protected compounds 3 and 5 (Scheme [1]) by various available methods[1] [2] [3] [4] [5] [6] to give the corresponding 6′-benzyl- or 6′-methoxybenzyl ether. Subsequent two-step inversion of configuration at C-4′ with a suitable reagent would provide either HO-4′-free or C-4′-substituted derivatives of lactose. Here we describe the syntheses and full characterization of the title compounds, which can be generally useful in carbohydrate synthesis.

The only benzylidination of cellobiose reported to date is that by Mani,[7] who used the DMF–Me₂SO₄ adduct[8] [9] as catalyst for the acetalation and isolated the product as the per-O-acetate. However, in our hands, several attempts to reproduce the reported 55% yield failed; the best yield we could consistently obtain by applying Mani’s protocol was ca. 30%. We found that a large amount of the starting material remained insoluble or unchanged (TLC) under those[7] conditions.

Because of the poor solubility of cellobiose in solvents commonly used for alkylidenation, preparation of 4′,6′-O-benzylidene- (2) and 4′,6′-O-p-methoxybenzylidene (4) cellobiose is difficult. However, the solubility problems were overcome when dimethyl sulfoxide (DMSO) was used as solvent. The use of this high-boiling solvent allowed the conversion to be conducted under homogeneous conditions, and a constant, gentle flow of inert gas aided removal of MeOH formed as a byproduct, thereby increasing the driving force for the reaction. The benefit of the removal of byproducts of acetalation during similar reactions has been previously recognized.[10] [11] The reaction conditions given below were established from a series of optimization experiments, when the amount of acetalation reagents and/or the acid catalyst, as well as reaction time and temperature were varied within practicability. When more forcing conditions were applied, to drive the conversion of the starting material to completion, much larger amounts of less polar products were formed. Because of higher reactivity of p-methoxybenzaldehyde dimethyl acetal, it was possible to conduct acetalation with this reagent under milder conditions than those applied with its non-methoxylated counterpart. Under the optimized conditions, when not all starting material was consumed but acceptable amount of byproducts were formed, compounds 2 and 4 were isolated consistently in ca. 40% yields, following isolation by chromatography. Both products were obtained in analytically pure state by crystallization, and were fully characterized for the first time. Crystallization of 2 or 4, which in both cases results in considerable losses, is not necessary for further conversion.

To obtain the corresponding per-O-benzoyl derivatives 3 and 5, we applied the method developed by Luo et al.,[12] which we previously found useful for making a series of β-benzoates of mono- and disaccharides.[13] These conversions proceeded uneventfully, but because of poor solubility of the fully protected compounds 3 and 5, difficulties were initially experienced during resolution of crude reaction mixtures by chromatography. The chromatographic tailing observed was minimized by the use of an unconventional, three-component elution solvent (see experimental section). As with other alkylidene derivatives of sugars, per-O-benzoates 3 and 5 are versatile intermediates in carbohydrate synthesis allowing, after hydrolytic cleavage or opening of the phenylidene ring with different reagents,[1] [3] [4] ^, [14] [15] [16] [17] [18] [19] [20] regioselective manipulation of hydroxyl groups in cellobiose, or access to various derivatives of lactose.

Optical rotations were measured at ambient temperature in CHCl₃ with a Jasco automatic polarimeter, Model P-2000. All reactions were monitored by thin-layer chromatography (TLC) on silica gel-coated glass slides (Analtech, Inc). Column chromatography was performed by elution from columns of silica gel with an Isolera One Chromatograph (Biotage). Solvent mixtures less polar than those used for TLC were used at the onset of separations. NMR spectra were measured at 600 or 400 MHz (¹H) and 150 or 100 MHz (¹³C) with Bruker Avance 400 or 600 spectrometers in CDCl₃ or DMSO-d ₆ as solvents. ¹H and ¹³C NMR chemical shifts are referenced to signals of TMS (0 ppm), CDCl₃ (77.0 ppm), DMSO-d ₆ (2.5 ppm), or DMSO-d ₆ (39.51 ppm). Assignments of NMR signals were made by homonuclear and heteronuclear 2-dimensional correlation spectroscopy, run with the software supplied with the spectrometers. Solutions in organic solvents were dried with anhydrous MgSO₄, and concentrated under the conditions specified.

4′,6′-O-Benzylidene-α,β-cellobiose (2)

A suspension of cellobiose (1.71 g, 5 mmol; dried in a vacuum oven at 40 °C overnight) and camphorsulfonic acid (CSA; 0.25 mmol, 60 mg) in DMSO (40 mL) was stirred at 50 °C for 1 h, when a clear solution was formed. Benzaldehyde dimethyl acetal (2.0 equiv, 10 mmol, 1.5 mL) was added and stirring was continued at 50 °C for 3 d with a very slow stream of anhydrous N₂ passing over the solvent and escaping through a needle in the septum. TLC (CH₂Cl₂–MeOH, 6:1) showed the presence of the desired product (R_f ~0.2), multiple, less polar side products, and a small amount of cellobiose (base line). The reaction was quenched with Et₃N (0.71 mmol, 0.1 mL), and DMSO was removed first at 70 °C/2 Torr and then by multiple co-evaporations with H₂O, until a white foam was formed. Chromatography of the residue (MeOH–CH₂Cl_2, 13→25%) gave pure product 2 (0.95 g, 44%). Multiple crystallizations from DMF–MeCN gave material (mp 224–232 °C) that still contained about 3% of the β-anomer (NMR in DMSO-d ₆, in which anomerization does not occur[21]).

¹H NMR (600 MHz, DMSO-d ₆): δ (α anomer) = 7.45–7.37 (m, 5 H, H_arom), 6.33 (d, J = 4.6 Hz, 1 H, OH-1), 5.59 (s, 1 H, CHPh), 5.51 (d, J = 5.0 Hz, 1 H, OH-2′), 5.36 (d, J = 4.7 Hz, 1 H, OH-3′), 4.91 (t, J = 4.1 Hz, 1 H, H-1), 4.64 (d, J = 6.8 Hz, 1 H, OH-2), 4.50 (m, overlapped, 1 H, OH-6), 4.48 (d, overlapped, J = 7.7 Hz, 1 H, H-1′), 4.18 (dd, J _5′,6′a = 4.5 Hz, J _6′b,6′a = 10.5 Hz, 1 H, H-6′a), 4.11 (d, J = 1.9 Hz, 1 H, OH-3), 3.74−3.62 (m, 4 H, H-6a, H-5, H-6b, H-6′b), 3.57 (dt, J _3,4;3,2 = 9.2 Hz, 1 H, H-3), 3.45−3.37 (m, overlapped, 3 H, H-3′, H-4′, H-5′), 3.39 (t, overlapped, J = 9.4 Hz, 1 H, H-4), 3.20–3.17 (m, 1 H, H-2), 3.15–3.12 (m, 1 H, H-2′).

¹³C NMR (150 MHz, DMSO-d ₆): δ = 103.2 (C-1′), 100.6 (CHPh), 92.0 (C-1), 80.3 (C-4′), 79.8 (C-4), 74.3 (C-2′), 72.8 (C-3′), 72.2 (C-2), 71.1 (C-3), 69.9 (C-5), 67.6 (C-6′), 65.9 (C-5′), 60.1 (C-6).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₉H₂₇O₁₁: 431.1553; found: 431.1559.

Anal. Calcd for C₁₉H₂₆O₁₁: C, 53.02; H, 6.09. Found: C, 52.91; H, 6.25.

1,2,3,6,2′,3′-Hexa-O-benzoyl-4′,6′-O-benzylidene-β-cellobiose (3)

Et₃N (12.5 equiv, 15.63 mmol 2.2 mL) and Bz₂O (10 equiv, 12.5 mmol, 2.83 g) were added to a solution in DMF (11 mL) of pure (TLC) 4′,6′-O-benzylidene-α-β-cellobiose (538 mg, 1.25 mmol; dried in a vacuum oven at 40 °C overnight and then co-evaporated with toluene). The mixture was stirred at 45 °C for 2 d, when TLC (toluene–acetone, 9:1) showed the presence of only two fully benzoylated products with very similar mobility (R_f ~0.6). A sat. aq solution of NaHCO₃ (20 mL) and CH₂Cl₂ (20 mL) was added and the reaction was stirred at r.t. for 1 h. The mixture was concentrated and the residue was partitioned between CH₂Cl₂ (200 mL) and a mixture of sat. aq solutions of NaHCO₃ and NaCl (1:1, 200 mL). The aqueous layer was backwashed with CH₂Cl₂ (4 × 200 mL), the combined organic layers were concentrated, and purification by chromatography (toluene–CH₂Cl₂–EtOAc, 14:5:1) gave first pure, amorphous β compound 3.

Yield: 1.031 g (78%); [α]_D +39.3 (c 1.0, CHCl₃).

¹H NMR (400 MHz, CDCl₃): δ = 8.07–7.24 (m, 35 H, H_arom), 6.13 (d, J _1,2 = 8.0 Hz, 1 H, H-1), 5.88 (t, J = 9.2 Hz, 1 H, H-3), 5.72 (dd, 1 H, H-2), 5.63 (t, J = 9.7 Hz, 1 H, H-3′), 5.47 (dd, J _1′,2′ = 7.8 Hz, 1 H, H-2′), 5.23 (s, 1 H, CHPh), 4.87 (d, 1 H, H-1′), 4.50 (dd, J _5,6a = 1.9 Hz, J _6a,6b = 12.4 Hz, 1 H, H-6a), 4.43 (dd, J _5,6b= 3.9 Hz, 1 H, H-6b), 4.27 (t, 1 H, H-4), 4.02 (ddd, 1 H, H-5), 3.66 (t, 1 H, H-4′), 3.63 (m, 1 H, H-6′a), 3.33 (m, 1 H, H-5′), 2.87 (t, J = 10.4 Hz, 1 H, H-6′b).

¹³C NMR (100 MHz, CDCl₃): δ = 101.9 (C-1′), 101.2 (CHPh), 92.3 (C-1), 78.2 (C-4′), 77.7 (C-4), 73.7. (C-5), 73.2 (C-3), 72.4 (C-2′), 71.9 (C-3′), 70.7 (C-2), 67.6 (C-6′), 66.4 (C-5′), 61.9 (C-6).

HRMS (ESI): m/z [M + H]⁺ calcd for C₆₁H₅₁O₁₇: 1055.3126; found: 1055.3120.

Anal. Calcd for C₆₁H₅₀O₁₇·H₂O: C, 68.27; H, 4.88. Found: C, 68.47; H, 4.71.

Eluted next was a small amount of 1,2,3,6,2′,3′-hexa-O-benzoyl-4′,6′-O-benzylidene-α-cellobiose, slightly contaminated with the β anomer 3, which was purified by preparative TLC. The compound could not be crystallized from common organic solvents and was obtained as white amorphous solid.

[α]_D +117.5 (c 1.0, CHCl₃).

¹H NMR (600 MHz, CDCl₃): δ = 8.13–7.22 (m, 35 H, H_arom), 6.69 (d, J _1,2 = 3.7 Hz, 1 H, H-1), 6.14 (dd, J _2,3 = 10.4 Hz, J _3,4 = 8.2 Hz, 1 H, H-3), 5.63 (t, 1 H, H-3′), 5.54 (dd, 1 H, H-2), 5.48 (dd, J _2′,3′ = 9.5 Hz, J _1′,2′ = 7.8 Hz, 1 H, H-2′), 5.24 (s, 1 H, CHPh), 4.92 (d, 1 H, H-1′), 4.51−4.41 (m, 2 H, H-6a, H-6b), 4.29−4.20 (m, 2 H, H-4, H-5), 3.67 (t, 1 H, H-4′), 3.63 (dd, J _6′a,6′b = 10.8 Hz, J _5′,6′a 4.9 Hz, 1 H, H-6′a), 3.33 (td, J _4′,5′ = 9.7 Hz, J _5′,6′a = 5.0 Hz, 1 H, H-5′), 2.91 (t, 1 H, H-6′b).

¹³C NMR (150 MHz, CDCl₃): δ = 102.0 (C-1′), 101.2 (CHPh), 89.7 (C-1), 78.1 (C-4′), 76.9 (C-4), 72.5 (C-2′), 72.1 (C-3′), 71.0 (C-5), 70.7 (C-3), 70.2 (C-2), 67.7 (C-6′), 66.5 (C-5′), 61.6 (C-6).

HRMS (ESI): m/z [M+NH₄]⁺ calcd for C₆₁H₅₄O₁₇N: 1072.3386; found: 1072.3392.

Anal. Calcd for C₆₁H₅₀O₁₇: C, 69.44; H, 4.78. Found: C, 69.32; H, 4.81.

4′,6′-O-p-Methoxybenzylidene-α,β-cellobiose (4)

A suspension of cellobiose (1.71 g, 5 mmol; dried in a vacuum oven at 40 °C overnight) and CSA (0.25 mmol, 60 mg) in DMSO (40 mL) was stirred at 50 °C for 1 h, when a clear solution formed. The mixture was cooled to r.t. and treated with anisaldehyde dimethyl acetal (2.5 equiv, 12.5 mmol, 2.13 mL) for 1 d at r.t. with N₂ protection as described above. TLC (CH₂Cl₂–MeOH, 6:1) showed the presence of the desired product (R_f ~0.2), multiple less polar side products, and a little cellobiose (base line). After quenching the reaction with Et₃N (0.71 mmol, 0.1 mL), the mixture was worked up as described above for the preparation of 2, and chromatography (MeOH–CH₂Cl₂, 13→25%) gave pure 4 (0.87 g, 38%). Multiple crystallizations (DMF–MeCN) gave material (mp 211–218 °C) that still contained about 15% of the β anomer (anomeric ratio was established as mentioned above for 2).

¹H NMR (600 MHz, DMSO-d ₆): δ (α anomer) = 7.36–6.91 (m, 4 H, H_arom), 6.33 (d, J = 4.5 Hz, 1 H, OH-1), 5.52 (s, 1 H, CHPhOMe), 5.50 (d, J = 4.9 Hz, 1 H, OH-2′), 5.34 (d, J = 5.0 Hz, 1 H, OH-3′), 4.90 (t, J = 4.1 Hz, 1 H, H-1), 4.63 (d, J = 6.8 Hz, 1 H, OH-2), 4.50 (m, overlapped, 1 H, OH-6), 4.47 (d, overlapped, J = 7.9 Hz, 1 H, H-1′), 4.16 (dd, J _5′,6′a = 4.2 Hz, J _6′b,6′a = 10.6 Hz, 1 H, H-6′a), 4.11 (d, J = 1.7 Hz, 1 H, OH-3), 3.75 (s, 3 H, OCH₃), 3.71−3.61 (m, 4 H, H-6a, H-5, H-6b, H-6′b), 3.57 (br t, 1 H, H-3), 3.45−3.36 (m, overlapped, 3 H, H-3′, H-4′, H-5′), 3.39 (t, overlapped, J = 9.6 Hz, 1 H, H-4), 3.20–3.16 (m, 1 H, H-2), 3.14–3.11 (m, 1 H, H-2′).

¹³C NMR (150 MHz, DMSO-d ₆): δ = 103.2 (C-1′), 100.6 (CHPhOMe), 92.0 (C-1), 80.3 (C-4′), 79.8 (C-4), 74.4 (C-2′), 72.7 (C-3′), 72.2 (C-2), 71.1 (C-3), 69.9 (C-5), 67.6 (C-6′), 65.9 (C-5′), 60.1 (C-6), 55.1 (OCH₃).

HRMS (ESI): m/z [M + Na]⁺ calcd for C₂₀H₂₈O₁₂Na: 483.1478; found: 483.1488.

Anal. Calcd for C₂₀H₂₈O₁₂: C, 52.17; H, 6.13. Found: C, 52.30; H, 6.29.

1,2,3,6,2′,3′-Hexa-O-benzoyl-4′,6′-O-p-methoxybenzylidene-β-cellobiose (5)

Et₃N (12.5 equiv, 20.7 mmol, 2.89 mL) and Bz₂O (10 equiv, 16.58 mmol, 3.75 g) was added to a solution in DMF (15 mL) of pure (TLC) 4′,6′-O-p-methoxybenzylidene-α,β-cellobiose (763 mg, 1.66 mmol; dried in a vacuum oven at 40 °C overnight and then co-evaporated with toluene). The mixture was stirred at 45 °C for 2 d, when TLC (toluene–acetone, 9:1) showed the presence of only two fully benzoylated products with very similar mobility (R_f ~0.53). Sat. aq NaHCO₃ (20 mL) and CH₂Cl₂ (20 mL) were added and the reaction was stirred at r.t. for 1 h. The mixture was concentrated and the residue was partitioned between CH₂Cl₂ (200 mL) and a mixture of sat. aq solutions of NaHCO₃ and NaCl (1:1, 200 mL). The aqueous layer was backwashed with CH₂Cl₂ (4 × 200 mL), the combined organic layer was concentrated and purified by chromatography (toluene–CH₂Cl₂–EtOAc, 14:5:1) to first give pure β product 5.

Yield: 1.46 g (81%); [α]_D +29.3 (c 1.0, CHCl₃).

¹H NMR (400 MHz, CDCl₃): δ = 6.12 (d, J = 8.0 Hz, 1 H, H-1), 5.87 (t, J = 9.1 Hz, 1 H, H-3), 5.71 (dd, J _2,3 = 9.4 Hz, 1 H, H-2), 5.61 (t, J = 9.6 Hz, 1 H, H-3′), 5.45 (dd, J _1′,2′ = 7.8 Hz, 1 H, H-2′), 5.18 (s, 1 H, CHPMP), 4.86 (d, 1 H, H-1′), 4.50 (dd, J _5,6a = 1.9 Hz, J _6a,6b = 12.3 Hz, 1 H, H-6a), 4.43 (dd, J _5,6b= 4.0 Hz, 1 H, H-6b), 4.26 (t, 1 H, H-4), 4.01 (ddd, 1 H, H-5), 3.72 (s, 3 H, OCH₃), 3.63 (m, 1 H, H-4′), 3.62 (m, 1 H, H-6′a), 3.31 (m, 1 H, H-5′), 2.87 (t, J = 10.4 Hz, 1 H, H-6′b).

¹³C NMR (100 MHz, CDCl₃): δ = 101.9 (C-1′), 101.2 (CHPhOMe), 92.3 (C-1), 78.2 (C-4′), 76.7 (C-4), 73.7 (C-5), 73.2 (C-3), 72.4 (C-2′), 71.9 (C-3′), 70.7 (C-2), 67.6 (C-6′), 66.5 (C-5′), 62.0 (C-6), 55.2 (OCH₃).

HRMS (ESI): m/z [M + H]⁺ calcd for C₆₂H₅₃O₁₈: 1085.3232; found: 1085.3228.

Anal. Calcd for C₆₂H₅₂O₁₈: C, 68.63; H, 4.83. Found: C, 68.66; H, 5.02.

Eluted next was 1,2,3,6,2′,3′-hexa-O-benzoyl-4′,6′-O-p-methoxybenzylidene-α-cellobiose, which could not be crystallized and was obtained as a white, amorphous solid.

Yield: 114 mg (6%); [α]_D +94.9 (c 1.0, CHCl₃).

¹H NMR (600 MHz, CDCl₃): δ = 8.12–6.77 (m, 34 H, H_arom), 6.69 (d, J = 4.0 Hz, 1 H, H-1), 6.13 (dd, J _3,4 = 8.4 Hz, J _3,2 = 9.8 Hz, 1 H, H-3), 5.61 (t, J = 9.7 Hz, 1 H, H-3′), 5.53 (dd, 1 H, H-2), 5.47 (dd, J _1′,2′ = 7.8 Hz, 1 H, H-2′), 5.19 (s, 1 H, CHPhOMe), 4.92 (d, 1 H, H-1′), 4.49–4.43 (m, 2 H, H-6a, H-6b), 4.27–4.21 (m, 2 H, H-4, H-5), 3.73 (s, 3 H, OCH₃), 3.65 (t, 1 H, H-4′), 3.61 (dd, J _6′a,6′b = 4.9 Hz, J _5′,6′a = 10.6 Hz, 1 H, H-6′a), 3.31 (ddd, J _5′,6′b = 9.8 Hz, 1 H, H-5′), 2.91 (t, 1 H, H-6′b).

¹³C NMR (150 MHz, CDCl₃): δ = 102.1 (C-1′), 101.2 (CHPhOMe), 89.8 (C-1), 78.1 (C-4′), 76.9 (C-4), 72.6 (C-2′), 72.1 (C-3′), 71.1 (C-5), 70.7 (C-3), 70.2 (C-2), 67.7 (C-6′), 66.5 (C-5′), 61.7 (C-6), 55.2 (OCH₃).

HRMS (ESI): m/z [M + H]⁺ calcd for C₆₂H₅₃O₁₈: 1085.3232; found: 1085.3278.

Anal. Calcd for C₆₂H₅₂O₁₈: C, 68.63; H, 4.83. Found: C, 68.69; H, 4.92.

Acknowledgment

This research was supported by the Intramural Research Program of the NIH, NIDDK.

• References

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• References

• 1 Fügedi P, Lipták A, Nanási P, Szejtli J. Carbohydr. Res. 1982; 104: 55
  CrossrefSearch in Google Scholar
• 2 Garegg PJ, Hultberg H, Wallin S. Carbohydr. Res. 1982; 108: 97
  CrossrefSearch in Google Scholar
• 3 Johansson R, Samuelsson B. J. Chem. Soc., Perkin Trans. 1 1984; 2371
  Crossref
• 4 Panchadhayee R, Misra AK. Synlett 2010; 1193
  Thieme Connect
• 5 Sakagami M, Hamana H. Tetrahedron Lett. 2000; 41: 5547
  CrossrefSearch in Google Scholar
• 6 Tanaka K, Fukase K. Synlett 2007; 164
• 7 Mani NS. Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem. 1989; 28: 602
  Search in Google Scholar
• 8 Kantlehner W, Gutbrod HD. Liebigs Ann. Chem. 1979; 1362
• 9 Kantlehner W, Gutbrod HD, Gross P. Liebigs Ann. Chem. 1979; 522
• 10 Evans ME. Carbohydr. Res. 1972; 21: 473
  CrossrefSearch in Google Scholar
• 11 Murguía C, Vaillard SE, Ricardo JG. Synthesis 2001; 1093
  Thieme Connect
• 12 Luo S.-Y, Kulkarni SS, Chou C.-H, Liao W.-M, Hung S.-C. J. Org. Chem. 2006; 71: 1226
  CrossrefSearch in Google Scholar
• 13 Sail D, Kováč P. Carbohydr. Res. 2012; 357: 47
  CrossrefSearch in Google Scholar
• 14 Chretien Synth. Commun. 1990; 20: 1589
  Crossref
• 15 Debenham SD, Toone EJ. Tetrahedron: Asymmetry 2000; 11: 385
  CrossrefSearch in Google Scholar
• 16 Crich D, Yao Q, Bowers AA. Carbohydr. Res. 2006; 341: 1748
  CrossrefSearch in Google Scholar
• 17 Zhang Y.-J, Dayoub W, Chen G.-R, Lemaire M. Eur. J. Org. Chem. 2012; 1960
• 18 Lipták A, Czegényi I, Harangi J, Nanási P. Carbohydr. Res. 1979; 73: 327
  CrossrefSearch in Google Scholar
• 19 Hanessian S, Plessas NR. J. Org. Chem. 1969; 34: 1045
  CrossrefSearch in Google Scholar
• 20 Hanessian S, Plessas NR. J. Org. Chem. 1969; 34: 1053
  CrossrefSearch in Google Scholar
• 21 Casu B, Reggiani M, Gallo GG, Vigevani A. Tetrahedron Lett. 1964; 2839

• Supplementary Material