Hydroxymethyl-substituted crown acetals with 35-C-14 and 40-C-16 skeletal backbones: synthesis and molecular geometries
摘要:
An oxidation/reduction sequence readily converts beta- and gamma -cyclodextrin into hydroxymethyl-substituted crown acetals with 35-C-14 and 40-C-16 skeletal cores. X-Ray analysis of their well crystallizing peracetates reveals the 40-membered ring of the gamma -CD derived octaacetal to mould into an undulated four-loop structure with alternating gauche and anti-conformations of the eight meso-butanetetrol units, the overall shape resembling a four-leaf clover. In the beta -CD derived, 35-membered crown heptaacetal, six of the seven glycolaldehyde/butanetetrol segments are lined up in alternating gauche/anti arrangements with the seventh, uneven unit inserted in gauche orientation. In solution, however, the macrocycles are highly flexible as evidenced by their H-1 and C-13 NMR spectra, which at 300 K show only one set of signals for the respective -CHR-CHR-O-CHR-O- units (R=CH2OH or CH2OAc). (C) 2001 Elsevier Science Ltd. All rights reserved.
Synthesis and Molecular Geometry of an Achiral 30-Crown-12 Polyacetal fromα-Cyclodextrin
摘要:
Periodate oxidation of alpha-cyclodestrin followed by borohydride reduction readily provided an octadecahydroxymethyl-substituted 30-crown-12 polyacetal 1, its 30-membered macrocycle being composed of six meso-butanetetrol/glycolaldehyde acetal units, which is, consequently, optically inactive, Its solid-state molecular geometry emerged from the X-ray structural analysis of the well-crystallizing octadeca-acetate 2, which revealed the undulated macrocycle to be molded into three loops with a unique order of succession of the -CHR-CHR-O-CHR-O- units: alternating gauche- and anti-conformations of the meso-butanetetrol portions and consecutive disposition of the glycolaldehyde-acetoxymethyl groups above and below the mean-plane of the macrocycle. In solution, however, as evidenced by H-1- and C-13-NMR spectra, the macrocycle is highly flexible at ambient and higher temperatures, its mobility becoming distinctly restricted only below - 20 degrees C.