Molecular modeling of saccharides, part 27 Synthesis and molecular geometry of an achiral 30-crown-12 polyacetal from alpha-cyclodextrin

Periodate oxidation of alpha-cyclodestrin followed by borohydride reduction readily provided an octadecahydroxymethyl-substituted 30-crown-12 polyacet al 1, its 30-membered macrocycle being composed of six meso-butanetetrol/gl ycolaldehyde acetal units, which is, consequently, optically inactive, Its solid-state molecular geometry emerged from the X-ray structural analysis o f the well-crystallizing octadeca-acetate 2, which revealed the undulated m acrocycle to be molded into three loops with a unique order of succession o f the -CHR-CHR-O-CHR-O- units: alternating gauche- and anti-conformations o f the meso-butanetetrol portions and consecutive disposition of the glycola ldehyde-acetoxymethyl groups above and below the mean-plane of the macrocyc le. In solution, however, as evidenced by H-1- and C-13-NMR spectra, the ma crocycle is highly flexible at ambient and higher temperatures, its mobilit y becoming distinctly restricted only below - 20 degrees C.