The 1H NMR spectra of the Strychnos alkaloids Retuline and Isoretuline, and their N-deacetyl compounds

The 300 MHz 1H NMR spectra of the title compounds are discussed. The vicinal coupling constants 3J H2,H16 and 3J H16,H15 are about 10Hz and 4 Hz in the iso series, and about 7 Hz and 1 Hz for retuline series. 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 N1-COCH3 group in isoretuline.