A detailed 13C-n.m.r. investigation has been carried out on the conformational changes in proline oligomers brought about by interaction with lithium and calcium perchlorates. Interaction of lithium and calcium salts with Piv-(Pro)n-OMe, n = 2, 4 and 5 results in trans-cis isomerization. In the case of pentaproline, metal salts also give rise to other trans-isomers caused by the rotation about the C
α
C(O) bond (Ψ, cis). Calcium salts seem to stabilize cis'-isomers and produce effects somewhat different from those of lithium salts.
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