On the Helicity of Oligomeric Formaldehyde

Abstract

Even this simple helical molecule, the benzyl‐protected pentamer 1, has a few surprises in store. Two independent conformers of 1 with the same absolute configuration are found in the unit cell which demonstrate an unprecedented form of isomerism. The helicity of the oligomeric formaldehyde is definitely a consequence of stereoelectronic effects.

1,17-Diphenyl-2,4,6,8,10,12,14,16-octaoxaheptadecane

“…The synthesis of polyoxymethylene helices that are terminated at both ends by phenyl groups has been reported by Noe et al (1994). We have previously reported the crystal structure of 1, 13-diphenyl-2,4,6,8,10,12-hexaoxatridecane (Noe et al, 1994) and report here the structure of the related title compound, (I).…”

Enantiomerically Pure Poly(oxymethylene) Helices: Correlating Helicity with Centrochirality

“…Conclusions. -We synthesized a set of enantiomerically pure poly(oxymethylene) helices with predictable handedness, which are capped by moieties derived from (1S)-2,2-dimethyl-1-phenylpropan-1-ol (14), up to a pentamer 20. By measuring the temperature-dependent optical rotation and temperature-dependent 1 H-NMR shifts, Fig.…”

“…It is well accepted that paraformaldehyde forms a helix as it was shown by theoretical calculations [5], by IR spectroscopy [6], and by X-ray analysis [7 -13] (see below). During our investigation of stereoelectronic effects, we became interested in the structure of oligomers of formaldehyde, i.e., of 1,13-diphenyl-2,4,6,8,10,12-hexaoxatridecane [14] and its corresponding heptamer (¼ 1,17-diphenyl-2,4,6,8,10,12,14,16octaoxaheptadecane) [15], respectively. These oligomers crystallize as helices and may serve as most basic reference for the investigation of stereoelectronic effects, the anomeric effect (interaction of the n 0 lone pair of the O-atom and the s* orbital of the CÀO bond), in particular, since there are no other interacting effects [16].…”

“…To synthesize for the first time an enantiomerically pure oligo(formaldehyde) (¼ oligo(oxymethylene)) helix with predicted handedness, we decided to use a chiral alcohol, (1S)-2,2-dimethyl-1-phenylpropan-1-ol (14), to generate both end groups (at the a and w terminus) of the formaldehyde oligomer. We report the synthesis and obtained with a diastereoselectivity of 4.5 : 1 by reacting lactol 2 with an excess of (AE)-4 in the presence of catalytic amounts of triphenylphosphonium bromide.…”

“…But, as in the case of (AE)-4, we obtained the dioxolane derivative 10, which is probably built by the same elimination, Prins reaction, and cyclization sequence as that leading to dioxolane 6. Finally, we decided to use (1S)-2,2-dimethyl-1-phenylpropan-1-ol (14) as chiralityinducing unit (Scheme 5). To separate racemic 14, we used lactol 12, which is accessible from lactone 11 by reduction with DIBAL.…”