2-( 2,4:3,5-Di-O-benzylidene-D-g lycer O-D-allo-pentitol- 1-yi)p yridine, C24H 2aN O s

C24H23NOs, Mr = 405.4, orthorhombic, P212121, a = 8.762(1), b= 13.984(1), c= 16.569 (1) /~, V2030 (1) A 3, Z = 4, Dx = 1.327 Mg m -3, ,~(Mo Ka) = 0.71073/~, /z = 0.087 mm- 1, F(000) = 856, room temperature, wR=0.034, R=0.035 for 2049 observed reflections [I _> 3o-(/)] out of 2536 measured reflections and 341 variables. The sugar moiety has the D-allo configuration, which provides further proof that the ring closure of the title compound to an a-nucleoside follows an SN2-1ike mechanism. The title compound consists of two 1,3-dioxane rings in the chair form, trans fused to each other, with two phenyl rings and a (2-pyridyl)hydroxymethyl substituent in equatorial positions. The variation of the C---O lengths in the dioxane rings follows the rules of the anomeric effect. The 1,3-dioxane rings are more puckered than their carbon analogues in transdecalin. The pyridine ring is nearer to C(3) than to 0(2), with a synclinal conformation around C(1)C(2). An intermolecular hydrogen bond is observed, but no intramolecular hydrogen bonds were found. Molecular-mechanics calculations (MM3 force field) reproduce the bond lengths with an e.s.d, of 0.011 A and the valence angles with an e.s.d, of 1.0 °. MM3 supports conclusions drawn from the X-ray determination concerning the conformational behaviour of the free molecule. In the force field the absence of parameters dedicated to aromatic N atoms makes calculations more difficult.