Analysis of beta -isocinchonicine - the enolo-ethereal derivative of cincho
nine - by NMR, AM1 calculations, and X-ray diffraction resulted in the assi
gnment of its structure as 1(S)-ethyl-3-(quinol-4-yl)-2-oxa-6(S)-9-azabicyc
lo[4,4,0]dec-3-ene contrary to the structure with seven-membered enolo-ethe
real ring reported in the literature. The AM 1 calculations also showed sev
eral energetically favorable conformations with the ethyl group equatorial
in relation to the piperidine fragment. Four of these forms (II,V,VIII,XI),
supported by NOEs and being the rotamers of the quinolyl and ethyl substit
uents, are most probable components of the conformational equilibrium mixtu
re of beta -isocinchonicine. Three of the rotamers have been observed by Xr
ay diffraction: two in the crystals of beta -isocinchonicine hydroiodide mo
nohydrate (7 . HI .H2O) are linked into an asymmetric dimer by a pair of N-
HN+ hydrogen bonds and another rotamer in its hydroiodide dihydrate (7 . HI
. 7H(2)O) where the cations are linked by N-HN+ hydrogen bond into helices
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