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

Crystal structures of all five crystalline methyl D-pentofuranosides, methy l alpha -D-arabinofuranoside (1), methyl beta -D-arabinofuranoside (2), met hyl alpha -D-lyxofuranoside (3), methyl beta -D-ribofuranoside (4) and meth yl alpha -D-xylofuranoside (5) have been determined by means of cryogenic X -ray and neutron crystallography. The neutron diffraction experiments provi de accurate. unbiased H-atom positions which are especially important becau se of the critical role of hydrogen bonding in these systems. This paper su mmarizes the geometrical and conformational parameters of the structures of all five crystalline methyl pentofuranosides, several of them reported her e for the first time. The methyl pentofuranoside structures are compared wi th the structures of the five crystalline methyl hexopyranosides for which accurate X-ray and neutron structures have been determined. Unlike the meth yl hexopyranosides, which crystallize exclusively in the C-1 chair conforma tion, the five crystalline methyl pentofuranosides represent a very wide ra nge of ring conformations.