CONFORMATIONAL ANALYSIS OF ALDITOLS: 1,2- AND 1,3-PARALLEL INTERACTIONS IN TRI-O-ISOPRO- PYLIDENE DERIVATIVES. J. Kopf* and P. Köell, Institut füer Anorganische und Angewandte Chemie der Universitäet Hamburg, Martin-Luther-King-Pl. 6, D-20146 Hamburg, Deutschland; Fachbereich Chemie der Universitäet Oldenburg, D-26111 Oldenburg, Deutschland

In continuation of our research program [1] we have determined the crystal and molecular structures of six tri-O-isopropylidene derivatives of openchain alditols. Contrary to the "so-called" Hassel-Ottar effect, our investigations show that the avoidance of 1,3-parallel interactions between O and C atoms (designated as O//O and C//O, respectively) is NOT the dominating factor in determining the conformations of higher alditols in the solid state [2]. Acetylated alditols in particular are able to adopt "unexpected" conformations [3]. A systematic search in the CAMBRIDGE FILE for the occurrence of such O//O and C//O interactions in the conformations of acyclic carbohydrate derivatives and the six new structure determinations confirm these results. Several examples will be discussed to illustrate that it is difficult to predict "preferred" conformations of alditols.

[1] Kopf, J. and Köell, P., Abstract MS-06.01.04, IUCr XVI, Beijing, China, 1993.

[2] Köell, P., Bischoff, M., Bretzke, C. and Kopf, J., Carbohydr. Res., 262, 1-8 (1994).

[3] Kopf, J., Morf, M., Hagen, B., Bischoff, M. and Köell, P., Carbohydr. Res., 262, 9-25 (1994).