¹³C NMR spectral studies of arecoline, hordenine, strychnine and brucine

Abstract

Abstract-The n.m.r. spectra of arecoline, hordenine, strychnine, brucine and their salts were determined and the chemical shifts reported previously for arecoline have been verified by single frequency proton decoupling techniques. Effects of protonation on the carbon resonances in aiding unambiguous assignments in strychnine and brucine are discussed. The quaternary N-methiodide of brucine was also studied to confirm the assignments made for carbon atoms adjacent to the basic N atom.THE APPLICATION of 13C n.m.r. spectroscopy to structural studies of natural products is well-known. Among these compounds alkaloids have received considerable attention;1-5 in particular, Wenkert and co-workers have not only assigned the resonances of many classes of alkaloids, but have provided significant guidelines for efficient use of 13Cn.m.r. in these kinds of structural studies. This subject has been reviewed extensive1y.l Work in progress in our laboratory is directed towards the application of 15N n.m.r. to structure elucidation of alkaloids and we wish to correlate and compare our results with corresponding 13C data. Perusal of the literature revealed that the 13C spectra of arecoline, hordenine, strychnine and brucine (1-4) have not been completely reported, but only a partial assignment of strychnine (3), based on T, values, exists6 Furthermore, we wish to confirm the reported2 assignments of C-2 and C-6 in arecoline (l), which were based on assumed substituent effects. Although brucine is a minor structural analog of strychnine, assignments of both serve as mutual confirmations. EXPERIMENTALProton noise-decoupled or off-resonance decoupled Fourier transform 13C spectra were obtained at 25.03 MHz on a JEOL PS/PFT-100 n.m.r. spectrometer equipped with the JEOL EC-100 data system. From 250 to 2500 transients were accumulated for proton noise-decoupled spectra over a 5 KHz range using 8 K words of memory (1.22 Hzladdress); 1000-15000 transients were accumulated for proton off-resonance decoupled spectra. A 30' pulse was employed at repetition rates ranging from 1.5-2.0 s depending upon the sample. All compounds were commercial samples and were used without further purification. CDCI, solutions in 10 mm 0.d. tubes were employed for the free alkaloids as well as for strychnine + trifluoroacetic acid (TFA, 1:1) and brucine t TFA (l:l), using TMS as internal reference. The RESULTS AND DISCUSSION Arecoline and arecoline hydrobromideAlthough the 13C spectrum of arecoline had been reported earlier,2 we re-examined it and its hydrobromide (la) in order to assign unambiguously the resonances of C-2 and C-6. In addition, we found analysis of the shifts of the olefinic carbons upon protonation of (1) to be helpful in assignment of the olefinic carbons in strychnine and brucine and their TFA derivatives.The proton noise-decoupled spectrum of 1 (CDCI,) showed eight resonances at 26-68(t), 45*70(q), 51 -4O(q), 51*40(t), 53*30(t), 128. and q denote the multiplicity of the signal as singlet, doublet, triplet and quartet, respectively, in t...

Probing Platinum Azido Complexes by 14N and 15N NMR Spectroscopy

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Synthesis of new 4-(1-ethylthio-2,2,2-trifluoroethyl)-6-methylpyridazin-3(2H)-ones starting from S-ethyl 4-oxo-2-(pentafluoroethyl)pentanethiolate and hydrazines

“…A concentric capillary tube filled with a mixture of acetamide and N-methyl acetamide in DMSO-d 6 (1.1 M solution of each) was used as internal standard so that sample HSQC cross-peaks are unambiguous assigned as arising from either NH or NH 2 groups. Acetamide and N-methyl acetamide signals were identified by their high intensity and their 15 N chemical shifts values (respectively 110 and 105 ppm, saturated ammonia as standard [10]). Compound 5B provided a single HSQC peak at d( 15 N) = 50 ppm, that has the same sign as the ones of acetamide.…”

Sulfonation of strychnine and some related compounds by sulfur dioxide – manganese dioxide (the Leuchs reaction)

“…The assignments of Table 3 follow directly from those reported for the free base (28,29), the hydrochloride (29), and the trifluoroacetate (28).…”