ISSN:
0025-116X
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Oligomerization of trichloroacetaldehyde (chloral) was carried out with the lithium salt of (1 S, 2 R, 4 S)-(-)-borneol as the initiator. The unimer and dimer fractions consisted of two and four diastereomers, respectively; the individual isomers were isolated from the fractions by crystallization from methanol or by high performance liquid chromatography. The X-ray single-crystal analysis showed that the major isomer in the unimer fraction is the (R)-form which has the acetal carbon in (R)-configuration. The diastereomeric ratios were determined by 1H NMR as R/S = 57/43 for the unimers and RS/SR/RR/SS = 34/25/23/18 for the dimers. The conformations of the meso-dimers [(R,R)- and (S,S)-isomers] in the crystalline form corresponded to a half turn of the 41-helix of isotactic polychloral, whereas the conformations of the racemo-dimers (the (R,S)- and (S,R)-isomers) were different from the helical structure in the crystal and in solution. The trimer fraction consisted of six of the eight possible diastereomers; among these isomers, the mm isomers (the (R,R,R)- and (S,S,S)-isomers) were highly predominant. The amount of the (R,R,R)-trimer formed was only slightly larger than that of the (S,S,S)-isomer. (a) Part 42, cf. G. D. Jaycox, O. Vogl, J. Am. Chem. Soc., submitted.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/macp.1991.021920104
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