The structure of melinonine‐E, a quaternary indole alkaloid from Strychnos melinoniana baillon. Part 190: Studies on organic natural products

Abstract

SummaryThe structure of melinonine-E (2) with relative configuration was established on the basis of spectral data for 2 and its acetate 3.The isolation of the quaternary alkaloid melinonine-E from an extract of the bark of Strychnos melinoniuna Baillon (Loganiaceae) was first reported in 1957 [2]. On the basis of microanalyses of the crystalline picrate, perchlorate and iodide salts, the molecular formula for the cation was determined to be either C,,H,,N,O Further, the compound was shown to be devoid of vinyl groups and methyl groups of any kind. That the oxygen was present as an alcohol was substantiated by the formation of an 0-acetyl derivative on treatment with Ac,O/pyridine. UV and IR spectra suggested the presence of a beta-carbolinium chromophore in the molecule. On the basis of these data and biogenetic reasoning, structure 1 was proposed for melinonine-E, but limited quantities of the compound precluded degradation studies. The results of a re-investigation of this compound are presented in the following.Electron impact mass spectrometry (EI-MS) revealed for the cation a peak at m/z 293 which loses one H-atom to yield the base peak, mjz 292. This molecular formula is in agreement with "C-NMR spectral data which indicate the presence of 19 C-atoms and 19 protons bonded to C-atoms, and is still in fair agreement with the results of microanalysis. Significant fragments at [M-18] + and [M-32]' suggest the presence of a primary alcohol. This contention was confirmed by comparison of the 'H-NMR spectrum of 2 with that of its acetate 3, in which two one-proton dd's at 3.75 and 3.83 ppm (H,C(21)) in 2 are shifted downfield to 4.31 and 4.38 ppm in 3. That these signals appear as the AB-portion of an ABX-subspectrum indicates further that the hydroxymethyl group is attached to a C-atom bearing a single proton, a branch point in the skeleton.

A Reinvestigation of the Oxidative Rearrangement of Yohimbane‐Type Alkaloids. Part B. Formation of Oxindol (= 1,3‐Dihydro‐2H‐indol‐2‐one) Derivatives

“…but different from the earlier, clearly unrealistic drawing 3a [4]. ' ) Assignments of the aromatic protons in accordance with Hessr and coworkers [14].…”

“…4 ) In contrast to 2, thea-epimer 3 showed no BohIrnann band in the IR spectrum [12]. 5) Extensive force-field [14]). …”

“…The following observations strongly support the hypothesis that conformer 3 is the major conformer in solution: I ) The 'H-NMR coupling pattern within the CH, (5)/CH2 (6) fragment is more consistent with a staggered than with an eclipsed arrangement. 2) In a difference NOE experiment, both H-C (20) and H,-C ( 14) showed enhanced intensity, when the (axial) Hp-C (5) was irradiated. 3 ) Jn the "C-NMR spectrum, C (20) showed up at 36.2 ppm, i.e., shielded by 4 ppm compared to all other yohimbine derivatives mentioned in this paper, where C (20) invariably absorbed at 40.4 & 0.4 ppm.…”

“…Assignments in the aromatic section according to [IS]. Appears as a 1 : 1 :I t ('J = 23.8 Hz) of low intensity due to coupling with D-C (14). Appears as a 1:2:3:2:1 quint.…”

Application of Organic Azides for the Synthesis of Nitrogen-Containing Molecules

“…39 The first synthesis of (±)-melinonine-E (228) was accomplished by Bonjoch and co-workers. 40 We envisaged that the 2-azabicyclo 3 were needed to complete the reaction (Scheme 16).…”