Nuclear magnetic resonance study of the conformations of atropine and scopolamine cations in aqueous solution

Abstract

We have measured the ¹H and ¹³C chemical shifts and ¹H–¹H coupling constants from the n.m.r. spectra of atropine and scopolamine cations. For both species, nuclei in symmetrical positions in the tropane ring shown non-equivalence because they experience different shielding effects from the aromatic substituent. We have estimated the different ring current shift contributions at the tropane ring nuclei for a wide range of conformations (varying the torsion angles ϕ₂–ϕ₅) and searched for the conformations which give the best fit to the measured chemical shift differences for the symmetrical pairs of nuclei. We find that the aromatic ring in atropine and scopolamine occupies approximately the same region of conformational space in both the crystal and solution states. In this special case where we are calculating several shielding differences (which can be measured accurately) attributed to ring current shifts this method of conformational analysis would be expected to give reliable results.