An NMR study of spatial arrangements in natural alkaloids—I: Indor spectra and conformations of pachycarpine (d‐sparteine), lupanine and 13‐hydroxylupanine

Abstract

Abstract-The INDOR homonuclear PMR spectra of three quinolizidine alkaloids-pachycarpine (d-sparteine), lupanine and 13-hydroxylupanine-have been studied. The molecular conformations including the boat-structure of the ring C have been established. QUINOLIZIDINE alkaloids are known to have widely different structures because their molecules, which sometimes also contain lactam and other functions, can differ in the spatial arrangement of the rings.The alkaloids which have already been studied long ago1 had many problems of their stereochemistry left rather vague. The experimental data obtained do not often fit any of the conformational structures possible for the base. The examples considered here are pachycarpine (d-sparteine) (l), lupanine (2) and 13-hydroxylupanine (3). The O (3)presence of two non-equivalent quinolizidine rings in 1 has been chemically verified2 to and this leads to the hypothetical AIB-trans and CID-cis configurations for the allchair c o n f~r m a t i o n .~,~ IR spectra of 1 and 2, however, recorded in the range from 2800 to 2500 cm-I showed that the overall integral intensity of the C-H stretching vibration (the latter characterises the bonds which are in the cc-position with regard to the N and enter into a specific interaction with the anticoplanar lone pairs of the nitrogenslO) was lower than that expected for an all-chair conformation. The PMR spectra (100 MHz) of lupanine, 17-and 15-oxysparteines, sparteine and its deuterated analogues revealed11*12 that the spectra of 1 contained the signals of equatorial cr-protons (H2e 2-63 ppm; H,,,2-76 ppm; H,,,2.51 ppm; H,,,2.32 ppm) in the 2.6 to 2.0 ppm region. The fact that all the signals which could have been associated with the axial a-protons were absent at lower fields made it evident that 1 as well as 2 contained six axial protons anticoplanar to the axial lone pairs of the two nitrogens.13 The boat conformation was also verified for the ring C. 837

Application of two‐dimensional NMR spectroscopy to the analysis of the proton NMR spectrum of sparteine and its lactams

“…8 Fragmentary data in CDC1, solution have also been published by Bohlmann et a1. ' and Skolik et ~1 .~ In the present experiments benzene was used as the solvent; this choice was dictated by the requirement of achieving resolution of deuterium signals of deuterated alkaloids obtained in biosynthetic experimenkz…”

Chemistry of 3-azabicyclo[3.3.1]nonanes

“…A similar value of Jhcoh (ca.12 Hz) for the amino alcohol 144 indicates no2 152 broad multiplet at 8 4.55 due to the carbinol proton has a half-band width of 20 Hz, indicating diaxial coupling of this proton with the C-6 and C-8 protons. In the spectrum of 144 the half-band width of 11 Hz for the multiplet at 8 4.20 due to the C-7 proton provides additional support for the intramolecular bonding.…”

“…The close similarity of the chemical shifts of the 7-methylene protons of bispidine and those of N-methylpiperidine proves the chair conformation since the latter shows a marked preference for chair conformation with the methyl group equatorial. 191 On the basis of the JHcoh value, the conformation of l,5-dinitro-3-methyl-3-ABN-7-endo-ol (144) Large values of ^hcoh118™ (ca.12 Hz) have been reported for compounds in which the OH hydrogen is constrained by H bonding to be anti to the carbinol hydrogen. A similar value of Jhcoh (ca.12 Hz) for the amino alcohol 144 indicates no2 152 broad multiplet at 8 4.55 due to the carbinol proton has a half-band width of 20 Hz, indicating diaxial coupling of this proton with the C-6 and C-8 protons.…”