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Molecular Modeling of Saccharides, Part XXIX.

Hydroxymethyl-substituted crown acetals with 35-C-14 and 40-C-16 skeletal backbones: Synthesis and molecular geometries

S. Immel, F. W. Lichtenthaler, H. J. Lindner, and T. Nakagawa

Tetrahedron: Asymmetry 2001, 12, 2767-2774.

An oxidation/reduction sequence readily converts β- and γ-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 γ-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 β-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 1H and 13C 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).

Additional Graphics: Crown Acetals


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