SINGLE-CRYSTAL X-RAY STRUCTURES OF THE 2 4-HEPTADECYL DERIVATIVES OF (1R,5S)-3,6,8-TRIOXABICYLO[3.2.1]OCTANE

The single crystal structures of the two diastereomeric 4-heptadecyl d erivatives of (1R,5S)-3,6,8-trioxabicyclo[3.2.1]octane have been deter mined by X-ray diffraction to be ,4R,SS)-heptadecyl-3,6,8-trioxabicycl o[.2.1]octane (II) and SS)-4-heptadecylo[3,6,8-trioxabicyclo[3.2.1]oct ane (II), respectively, which have an exo or axial 4-heptadecyl group, and an endo or equatorial 4-heptadecyl group, respectively. The struc tures of I and II had been suggested by their phase-sensitive 2D NOESY H-1-NMR spectra, but are now established unambiguously. These optical ly pure non-ionic lipid-like amphipathic molecules (I and II) represen t the first 3,6,8-trioxabicyclo[3.2.1]octanes for which single crystal structures have been solved. Crystals of both isomer I and isomer II were orthorhombic with space group P2(1)2(1)2(1), and had unit cell di mensions of a = 9.586, b = 43.14, c = 5.289 Angstrom, and a = 7.34, b = 51.8, c = 5.636 Angstrom, respectively. The structures of I and II w ere both solved by using direct methods to R = 0.045 and R = 0.086, re spectively. Both I and II pack in stacked bilayers with interdigitatin g and tilting hydrocarbon chains. The molecular and hydrocarbon cross sections are I: S = 50.70 Angstrom(2), Sigma = 19.00 Angstrom(2); and II: S = 41.37 Angstrom(2), Sigma = 18.26 Angstrom(2).