Received 15 July 2005
aChemical Crystallography, Chemical Research Laboratory, University of Oxford, Oxford OX1 3TA, England,bDivision of Organic Synthesis, National Chemical Laboratory, Pune 411 008, India,cBiological Chemistry, Division of Biomedical Sciences, Imperial College, London SW7 2AZ, England, and dDepartment of Organic Chemistry, Chemical Research Laboratory, Mansfield Road, Oxford OX1 3TA, England
Correspondence e-mail: email@example.com
Determination of the crystal structure of the title compound, C40H50O5Si2, firmly established its relative configuration and hence that of some related tetrahydrofuran carboxylates. The material crystallizes with Z' = 2. Except for the chiral centres, the two independent molecules are related by a pseudo-centre of symmetry.
The reaction of methanol with lactones containing 2-O-trifluoromethanesulfonates (trifluoromethanesulfonates) in the presence of either acid (Wheatley et al., 1993) or base (Choi et al., 1992) provides a general synthesis of methyl tetrahydrofuran-2-carboxylates. Such materials have been exploited in the preparation of sugar amino acids (SAAs) for use as peptidomimetics (Chakraborty et al., 2004; Grotenberg et al., 2004; Smith et al., 2003). Many THF SAA scaffolds are predisposed to form secondary structures in short oligomers (Claridge et al., 2005; Long et al., 1999, 2002; Hungerford et al., 2000). There are only limited reports of -peptides based on cyclic templates (Curran et al., 1996; Crisma et al., 2001). In a programme directed towards the synthesis of -THF SAAs, it was found that reaction of the -lactone trifluoromethanesulfonate (1) (Stewart et al., 2002) with methanol in the presence of sodium carbonate gave a mixture of the THF carboxylates (2) and (3). In order to ensure the correct assignment of the stereochemistry at C-2 in the epimers, (3) was converted to the crystalline disilyl ether (4), the structure of which is reported in this paper (Fig. 1).
The structure of (4) contains two molecules in the asymmetric unit (Z' = 2). Except for the 1,4-anhydroribonate units (which are chiral and therefore cannot be related by an improper operator), the molecules are related by a pseudo-centre of symmetry at (,,). The absolute configuration of the material was known unambiguously from the synthesis; the Flack (1983) parameter is in agreement with this assignment.
The structure consists of molecular layers (Fig. 2) lying parallel to the bc plane, and characterized by a hydrophilic and a hydrophobic surface. The hydrophobic surface of one layer faces the equivalent surface of the adjacent layer (Fig. 3).
| || Figure 1 |
The structure of one molecule of the title compound with displacement ellipsoids drawn at the 50% probability level. All H atoms, except for H31, H41, H61 and H71, have been omitted for clarity. The H atoms are drawn with an arbitary radius.
| || Figure 2 |
The crystal structure projected on to the bc plane.
| || Figure 3 |
The crystal structure projected along the b axis, showing two hydrophobic faces opposing each other. By symmetry, pairs of hydrophilic faces also oppose each other.
The H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H = 0.93-98 Å) and displacement parameters [Uiso(H) = 1.2-1.5Ueq(parent atom)], after which they were refined with riding constraints. The pseudo-centre of inversion did not lead to any refinement problems.
Data collection: COLLECT (Nonius, 2001); cell refinement: DENZO/SCALEPACK; data reduction: DENZO/SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS.
Financial support (to AAE) from the EPSRC and (to GJS) from DST, New Delhi, for a BOYSCAST Fellowship, is gratefully acknowledged.
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