Structures of furanosides: geometrical analysis of low-temperature X-ray and neutron crystal structures of five crystalline methyl pentofuranosides

Artem Evdokimov, A. Joseph Gilboa, Thomas F. Koetzle, Wim T. Klooster, Arthur J. Schultz, Sax A. Mason, Alberto Albinati & Felix Frolow, Acta Cryst. (2001). B57, 213-220

The paper reports an interesting study of five methyl pentofuranosides by low-temperature crystallography, whereby the crucial role of hydrogen bonding in this series can be discussed in detail for the first time, together with the wide-ranging conformations observed for the five-membered rings using accurate neutron diffraction results. The important context of the furanoside structures is reviewed with significant references to the literature, and the relevance of highly accurate crystal structures to modern modelling approaches in these compounds is discussed. Hitherto, data for only two methyl furanoside structures had been reported in the literature, so this paper represents a most important addition to the databases.

Previously in 1983, Jeffrey & Mitra had developed a method for classifying the hydrogen-bonding patterns observed in the then known crystal structures of 58 pyranoses and pyranosides and one furanoside into four types, and with the exception of methyl β-D-ribofuranoside, these new compounds all fall into classes I or III. Herein, however, the authors opted to use the Marzec & Day (1993) system of describing the exact geometry of the five-membered rings, using five bond lengths and four conformational variables, based largely on the Cremer & Pople (1975) descriptors, namely the pseudo-rotation angle (P, °), the maximum deviation along the normal to the Cremer-Pople plane (q, Å), the distortion of the ring in the Cremer-Pople plane (s, Å) and the direction of s (Γ, °). The authors compare and analyse these nine derived parameters of the methyl furanosides with one another and with the related methyl hexopyranosides, which naturally show less ring strain and any subsequent effects upon the ring geometry. The bond lengths for both series are remarkably similar, while bond angles are consistently smaller for the furanosides (4-8°). The extensive hydrogen-bonding network is entirely intermolecular in these crystal structures. The ring conformations show a wide range of pseudo-rotational phase angles, with a more closely averaged cluster of q, s, and Γ values. The authors discuss the differences and similarities in these conformations with those from several hundreds of known ribo- and deoxyribofuranoside rings in DNA, RNA and nucleosides/tides, bearing in mind the influence of the bulky anomeric substituents in nucleosides and nucleotides. Computational approaches to these problems through DFT methods for the isolated molecules are reported, and with the notable exception of α-D-lyxofuranoside, the gas-phase conformations are remarkably similar to those observed in the crystal structures. However, it is interesting to note that the energy-minimized structures for the isolated α-D-lyxofuranoside contain an intramolecular hydrogen bond, which appears to compensate for the strain induced by eclipsed ring substituents in these conformations.

Reviewed by Judith A.K. Howard, Chemistry Dept, U. of Durham, UK

References
Cremer, D. & Pople, J.A. (1975). J. Am. Chem. Soc. 97, 1354-1358.
Jeffrey, G.A. & Mitra, J. (1983). Acta Cryst. B39, 469-480.
Marzec, C. J. & Day, L. A. (1993). J. Biomol. Struct. Dyn. 10, 1091-1102.