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The stereostructures of linear chloral oligomers were determined by X-ray single crystal analysis. The chloral oligomers from dimer to pentamer were isolated by distillation and by GPC fractionation from the oligomer mixture prepared with t-BuOLi initiation and acetate endcapping. The dimer fraction was a diastereomenc mixture of the racemo (1) and meso (2) isomers, while the trimer, tetramer and pentamer fractions each contained a single diastereomer (3,4 and 5). The single crystals of 2,3,4 and 5 grown from methanol solutions belong to the space groups PI; P2,/n, P2,/0, and Ra2,, respectively. X-ray analyses showed that the acetal backbones of 2, 3 and 4 consist exclusively of meso dyads. The backbone arrangements of 2,3 and 4, are essentially identical and approximate the repeat gauche(+)-skew(-) sequences from the t-BuO group to the acetyl terminal group (for S-configurations). The conformation of (&S,S,S)-4 is very similar to a unit sequence of a 4A-helical structure of isotactic polychloral with left-handed helicity. The axis of the helix is parallel to the c-axis of the crystal, and the repeat distance is 4,81 A. The conformations of the acetal backbones of 4 and 5 in solution were also investigated by NMR spectroscopy, on the basis of dihedral angle dependence of 3Jc-0-c-H coupling constants.suggested that polychloral has a tactic structure, probably isotactic X-ray studies on a) Part 39, cf. 158).

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H and I3C NMR spectra of the mixture of chloral oligomers prepared by initiation with lithium tert-butoxide were examined in detail. ' H and I3C NMR signals due to meso and racemo diastereomers of the dimer fraction were unequivocally assigned through the I3C-'H COSY spectrum and the I3C NMR spectra with low-power selective 'H-decoupling. The twodimensional NOESY spectrum showed correlation peaks between the signals due to the methine (acetal) protons of the neighbouring chloral monomeric units of the most abundant isomers in the trimer and tetramer fractions. The ' H NOE enhancements and 3JcH coupling constants were measured for the dimers; the results confirmed the meso/racemo assignments based on earlier conformational analysis. ' )