1,3-PARALLEL O O-INTERACTIONS IN ACYCLIC CARBOHYDRATES - THE CRYSTAL-STRUCTURE AND MOLECULAR-STRUCTURES OF MESO-D-GLYCERO-L-ALTRO-HEPTITOL AND THE MONOHYDRATE, MESO-D-GLYCERO-L-IDO-HEPTITOL, AND MESO-D-GLYCERO-L-ALTRO-HEPTITOL AND D-GLYCERO-L-GALACTO-HEPTITOL HEPTAACETATES/

The molecular structures of the title compounds were determined by X-r ay crystallography, using direct methods, and refined to final convent ional residual parameters of R = 0.042, 0.035, 0.060, 0.062, and 0.047 , respectively, meso-D-glycero-L-altro-Heptitol crystallised usually a s a monohydrate from moist ethanol with single molecules occurring in the crystal as a racemate of chiral conformers adopting a planar zigza g conformation. Thus, a 1,3-parallel O-3//O-5 interaction is tolerated with a distance of 265.0 pm. By thorough exclusion of water, an unhyd rated morph was obtained from ethanol and investigated at 173 K. The c onformations of single molecules parallel those observed in the hydrat e. The O-3-O-5 distance is 267.6 pm. Molecules of the derived heptaace tate avoid this interaction by bending to a sickle, but tolerate, inst ead, an O-3//C-6 relationship with a distance of 295.0 pm. Also, in th is case, chiral conformers are observed, which are matched by their en antiomers in the unit cell. meso-D-glycero-L-ido-Heptitol is found in a doubly bent chiral conformation which is free of 1,3-parallel intera ctions and resembles that found in iditol hexaacetate, but not in idit ol itself. Again, a racemate of conformers is observed in the unit cel l. The molecules of the heptaacetate of chiral D-glycero-L-galacto-hep titol are found in a sickle conformation, free of 1,3-parallel interac tions, a situation already observed for the parent alditol but which c ould not have been predicted from recent investigations of the solid s tate structures of other alditol acetates. The occurrence of 1,3-paral lel O//O interactions in alditols and other acyclic carbohydrates is d iscussed comprehensively.