The 1H NMR spectra of the strychnos alkaloids retuline isoretuline, and their N‐deacetyl compounds.

The 300 MHz 1 H NMR spectra of the title compounds are discussed. The vicinal coupling constants 3 J H2, H16 and H16, H15 are about 10 Hz and 4 Hz in the iso series, and about 7 Hz and 1 Hz for 1 and 2. In isoretuline, the piperidine ring (ring D) has a chair conformation, but in retuline a boat conformation, with C21 in the tip position, prevails. This conclusion is based on the pattern of allylic and homoallylic couplings around the double bond, and rests also on comparison with the spectrum of strychnine. Evidence is presented for a hydrogen bond between the C17 hydroxymethyl and the Nl‐COCH 3 , group in isoretuline.