Methyl 4-O-benzyl-α-l-rhamno­pyrano­side

Robert Pendrill; Lars Eriksson; Göran Widmalm

doi:10.1107/S1600536814007922

. 2014 Apr 16;70(Pt 5):o561–o562. doi: 10.1107/S1600536814007922

Methyl 4-O-benzyl-α-l-rhamnopyranoside

Robert Pendrill ^a, Lars Eriksson ^b,^*, Göran Widmalm ^a

PMCID: PMC4011307 PMID: 24860368

Abstract

In the title compound, C₁₄H₂₀O₅, an intermediate in the synthesis of oligosaccharides, the glycosidic [H—C—O—C(H₃)] torsion angle ϕ_H is 52.3° and the exo-cyclic [H—C—O—C(H₂)] torsion angle θ_H is −11.7°. The hexapyranose ring has a chair conformation. In the crystal, molecules are linked by O—H⋯O hydrogen bonds, forming chains propagating along [010]. Enclosed within the chains are R ₃ ³(12) ring motifs involving three molecules. The chains are linked via C—H⋯π interactions, forming a three-dimensional network.

Related literature

For a description of l-rhamnose as part of polysaccharides, see: Ansaruzzaman et al. (1996 ▶); Marie et al. (1998 ▶); Säwén et al. (2012 ▶). For a description of syntheses in which the title compound has been used, see: Eklund et al. (2005 ▶); Handa et al. (1979 ▶). For the structure of rhamnosyl-containing trisaccharides, see: Eriksson & Widmalm (2012 ▶); Eriksson et al. (1999 ▶); Jonsson et al. (2006 ▶). For further related literature on l-rhamnose, see: Anderson & Ijeh (1994 ▶); Varki et al. (1999 ▶); Haines (1969 ▶); Herget et al. (2008 ▶); Olsson et al. (2005 ▶). For puckering analysis, see: Cremer & Pople (1975 ▶). graphic file with name e-70-0o561-scheme1.jpg

Experimental

Crystal data

C₁₄H₂₀O₅
M _r = 268.30
Orthorhombic,
a = 6.5377 (1) Å
b = 9.1848 (2) Å
c = 23.2699 (5) Å
V = 1397.30 (5) Å³
Z = 4
Mo Kα radiation
μ = 0.10 mm⁻¹
T = 293 K
0.25 × 0.12 × 0.05 mm

Data collection

Oxford Diffraction Xcalibur 3 with sapphire 3 CCD diffractometer
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2004 ▶) T _min = 0.921, T _max = 1.000
9540 measured reflections
1665 independent reflections
1407 reflections with I > 2σ(I)
R _int = 0.040

Refinement

R[F ² > 2σ(F ²)] = 0.035
wR(F ²) = 0.081
S = 1.00
1665 reflections
177 parameters
H-atom parameters constrained
Δρ_max = 0.14 e Å⁻³
Δρ_min = −0.13 e Å⁻³

Data collection: CrysAlis CCD (Oxford Diffraction, 2004 ▶); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2004 ▶); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg, 2001 ▶); software used to prepare material for publication: enCIFer (Allen et al., 2004 ▶).

Supplementary Material

Crystal structure: contains datablock(s) I, rp1. DOI: 10.1107/S1600536814007922/su2708sup1.cif

e-70-0o561-sup1.cif^{(26KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536814007922/su2708Isup2.hkl

e-70-0o561-Isup2.hkl^{(82.1KB, hkl)}

CCDC reference: 996312

Additional supporting information: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

Cg is the centroid of the C41–C46 benzyl ring.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O2—H2A⋯O3ⁱ	0.82	2.00	2.813 (2)	172
O3—H3A⋯O5ⁱⁱ	0.82	2.05	2.799 (2)	151
C7—H7C⋯Cg ⁱⁱⁱ	0.96	2.89	3.652 (3)	137

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) .

Acknowledgments

This work was supported by grants from the Swedish Research Council and the Knut and Alice Wallenberg foundation.

supplementary crystallographic information

1. Comment

Bacteria contain many different sugar residues (Herget et al., 2008) in contrast to man where only a dozen monosaccharides are utilized in the formation of polysaccharides, glycoproteins and glycolipids [see Varki et al. (1999)]. In lipopolysaccharides L-rhamnose (6-deoxy-L-mannose) is often present as a sugar component, ranging from one residue per repeating unit, for example, as the terminal residue in the biosynthesized and polymerized oligosaccharide; consequently, it forms the side-chain residue in the O-antigen (Olsson et al., 2005), which often has 10 – 25 repeating units. Alternatively, L-rhamnose can make up the O-antigen polysaccharide per se, as a homopolymer (Ansaruzzaman et al., 1996).

The title compound, Fig. 1, has been used in the synthesis of a rhamnosyl-containing trisaccharide (Eklund et al., 2005), the crystal structure of which was recently determined (Eriksson & Widmalm, 2012). The title monosaccharide is the methyl glycoside of α-L-rhamnopyranose and carries a benzyl protecting group at O4 in an ether linkage; the remaining two hydroxyl groups are unprotected and available for further synthetic modifications.

The glycosidic torsion angle defined by H1-C1-O1-C7, φ_H is 52.3° (Fig. 1). The exo-cyclic torsion angle defined by H4—C4—O4—C40, θ_H = -11.7°, shows an almost eclipsed conformation. The corresponding torsion angle in the crystal structure of 4-O-Benzyl-2,3-O-isopropylidene-α-L-rhamnopyranose was 36.8° (Eriksson et al., 1999). Moreover, in the title compound the C4—O4—C40—C41 torsion is antiperiplanar and the benzyl ring plane deviates significantly from that defined by plane O4/C40/C41, with a dihedral angle of 54.85 (18)°.

The hexapyranose ring O5/C1-C5 has a chair conformation, with puckering parameters (Cremer & Pople, 1975) Q = 0.570 (2) Å, θ = 177.4 (2)° and φ = 11 (4)°. These puckering parameters reveal a ¹C₄ conformation close to the south pole, in contrast to another protected methyl α-L-rhamnopyranoside derivative carrying an isopropylidene group at O2 and O3 (Jonsson et al., 2006).

In the crystal, molecules are linked via O—H···O hydrogen bonds, involving both hydroxyl groups, forming chains along the a axis (Table 1 and Fig. 2). They enclose 12-membered R³₃(12) ring motifs. There are also C—H ··· π interactions present, between the C7 methyl group and the centroid of the (C41–C46) benzyl ring (Table 1), that link the chains forming a three-dimensional network.

The conformation of the exo-cyclic torsion angle (H4—C4—O4—C40) was analyzed by NMR measurements (see details in the archived CIF) of the long-range heteronuclear coupling constant between nuclei H4 and C40 using a J-HMBC experiment, which resulted in ³J_CH = 6.25 Hz. Interpretation of this coupling constant using the Karplus-type relationship ³J_C,H = 7.6 cos²θ - 1.7 cosθ + 1.6 (Anderson & Ijeh, 1994) leads to |θ_H| = 26° when interpreted as a single conformation, i.e., quite similar to the structure determined in the solid state. The corresponding torsion angle in the crystal structure of 4-O-Benzyl-2,3-O-isopropylidene-α-L-rhamnopyranose was 36.8° (Eriksson et al., 1999).

2. Experimental

The synthesis of the title compound was performed according to a published procedure (Haines, 1969), where the rhamnosyl residue has the L absolute configuration. The title monosaccharide was crystallized at ambient temperature by slow evaporation from chloroform yielding colourless prismatic crystals. Spectroscopic data and details of the NMR measurements are given in the archived CIF.