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The helical‐chiral character of the diradical intermediate 2, which cyclizes quicker than it equilibrates, explains the memory effect of chirality that occurs during the enantioselective photocyclization of alanine derivative 1 to give the proline derivative 3. Ts=H3CC6H4SO2.

Methods of molecular mechanics were applied to investigate the conformation of the (methyl 2-O-sulfate-4-methyl-α-L-idopyranose) uronic acid (DMIS), in order to correlate the peculiar vicinal proton coupling constants observed in polysaccharides containing the iduronate ring to the conformational characteristics of this sugar ring. We found three conformers with comparable energies, namely the two chair forms C and C and the skew-boat form S (L); the latter is separated from each chair form by a barrier of about 9 kcal/mol. Along the pseudorotational path three additional minima ( S , S , and S ) were found, yet at least 4 kcal/mol higher than S . The results obtained for the relative energies of the three conformers and the conformation of the side groups were affected by the inclusion of the electrostatic term and, in particular, by the charge assigned to the ionic groups of DMIS. However, the conformational properties of the idopyranosidic ring in DMIS (and in related compounds) should still be interpreted in terms of equilibrium among these three conformers only.

A rotational-isomeric-state treatment of ethylene−norbornene copolymers has been worked out for the purpose of establishing a correlation between conformation and 13C NMR chemical shifts of this new class of polymers and of clarifying their microstructure. A comparison of conformer populations computed for the stereoregular alternating (N−E) x chains and for copolymers including such defects as N−N, E−E, E−E−E, and N−N−N indicates the need for accounting for the tacticity of the chain, besides its composition, in the interpretation of the spectra. Some stereochemical shifts were suggested by the calculations. In particular, conformational analysis predicted and evidenced the origin of distinct meso / racemic signals for methines C2 and C1 and for the ethylene carbon of the regularly alternating copolymers. This prediction induced a more thorough analysis of the 13C NMR spectra of mainly alternating (N−E) x copolymers, allowing us to recognize and distinguish isotactic and syndiotactic N−E−N sequences. Then, a detailed and unambiguous assignment of the spectra of stereoregular and irregular alternating E−N copolymers was achieved. The strong effect of ring distortions in the presence of N−N dyads and triads is also discussed.

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SUMMARY: A series of ethylene-norbornene copolymers were synthesized in the presence of zirconocenes with different symmetries and ligand patterns and at different norbornene (N)/ethylene (E) mole ratios. Copolymers were characterized by 13 C NMR spectroscopy; Inadequate NMR sequences were used also. The comparison of 13 C NMR spectra of copolymers prepared with different norbornene content and the correlation between 13 C NMR chemical shifts and conformational structures of the chain on the basis of molecular mechanics calculations were performed. Preliminary assignments were revised and new comonomer sequences such as ENNE which contain meso and racemo NN dyads were assigned.

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