The structure and absolute configuration of serpenticine, a new anhydronim base isolated from Rauwolfia vomitoria Afz., has been determined with the aid of W, IR and 13c-nmr spectral data of the base along with ORD, proton nmr and high resolution mass spectra of its tetrahydro product 8. The isolation and structure of two new alkaloids from the methanolic extract of Rauwolfia vomitoria, has been communicated earlier. Following further studies in the alkaloidal constituents, a new yellow anhydronium base has been isolated from the strongly basic fraction and provisionally named as serpenticine. The procedure of its isolation, details of which are given in the experimental, was based on the sparing solubility of its hydroiodide in water and alcohol. Serpenticine forms lemon yellow shining plates, melts at 275-276' and analysed for C22H22N2O4.5H2O. It contains one C-methyl and two methoxyl groups. The W spectrum of serpenticine is very similar to serpentine, showing maxima at 265 nm (log c 4.41), 310 nm (log E 4.2) and 365 nm (log E 3.6). The IR spectrum shows indolic NH (3400 cm-1) and twin peaks of equal intensity at 1695 cm-1 and 1618 cm-1 due to carbonyl and C=C in the grouping H3COOC-CH=CH-OR? Prominent bands at 1580, 1460 cm-1 and other bands at lower frequency region between 650-900 cm-1 provided evidence of aromaticity. The absorption at 1200 cm-1 represented C-O-C asymmetric stretching in the grouping C=C-O-C while that at 1260 cm-1 corresponded to O-CH3 group. The multiple bands from 1235-1300 cm-1 are due to the C-C(=O)-C stretching of the ester of α,β-unsaturated acid. The completely dehydrated base in vacuo did not show the presence of NH group. This along with colour, strongly basic character and W spectrum, strongly suggested an anhydronium base structure for serpenticine.On catalytic reduction serpenticine yields tetrahydroserpenticine C22H26N2O4, m.p. 264-265°C, [α]D +68° (chloroform). The presence of methoxycarbonyl group was established by alkaline hydrolysis of tetrahydroserpenticine to an amorphous amino acid - tetrahydroserpenticinic acid, from which the mother base could be regenerated by esterification with diazomethane. The W spectrum of tetrahydroserpenticine exhibits maxima at 230 and 298 nm, with inflection at 246-250 nm. The summation of the W spectra of 2,3-dimethyl-6-methoxyindole with 2,6-dimethyl-3-carboethoxy-5,6-dihydro-1,4-pyran is superimposable on the spectrum of tetrahydroserpenticine, and the latter is therefore to be considered as a derivative of 7-methoxytetrahydrocarboline. This part of the structure in the molecule is supported by the positive Adamkiewicz reaction, as well as close similarity in UV and IR spectra of tetrahydroserpenticine with those of tetraphylline and reserpinine. The high resolution mass spectrum of tetrahydroserpenticine shows a fragmentation pattern similar to that of ajmalicine and yohimbine with an increment of 30 for all the fragments containing the benzene ring. The molecular ion peak was at 382.1892 which agrees with the molecular formula C22H26N2O4. Aside of that, it showed intense M-1 peak at 381 resulting from the loss of a hydrogen atom from C-3, and peaks at m/e 367, 351 and 323 due to the loss of methyl group, O-CH3 and COOCH3 respectively. The peak at m/e 255 is due to fragment of composition C16H19N2O. It arises from the cleavage at D/E ring junction. The loss of ethylene from it gives another prominent peak at m/e 227 for fragment. The strong peaks at m/e 214 (C13H14N2O), 200 (C12H12N2O) and 199 (C12H11N2O) result from the homolysis of allylically labilized 3-14 bond of tetrahydroserpenticine to give the intermediate. Further cleavage of the 4-21 linkage then leads to the ionized dihydro-β-carboline (m/e 200), while the expulsion of an additional hydrogen atom from it offers a route to the β-carbolinium ion (m/e 199). On the other hand, cleavage at 20-21 linkage gives ion radical (m/e 214). A very intense peak at m/e 186 arises from the ion C12H12NO. Its genesis can be explained through the mechanism proposed earlier by Djerassi et al. which involves retro Diels-Alder fragmentation of ring C, followed by homolytic fission of the allylically activated 14-15 bond with generation of the conjugated ion (m/e 186).On the basis of cumulative evidence tetrahydroserpenticine appears to be a stereoisomer of reserpinine, isoreserpinine and tetraphylline. This was supported by the proton nmr spectrum in CDCl3. It showed a 1H broad singlet at δ 8.20 for indolic NH, a three proton doublet at δ 1.35 for C-19 methyl (J=7.1 Hz), a sharp 3H singlet at δ 3.89 for aromatic methoxyl group and another 3H singlet at δ 3.7 due to COOCH3. A one proton singlet at δ 7.5 is attributed to the ethylenic proton in the grouping ROOC-C=CH-OR. The signal for H-19 comes at δ 4.3. It is broken into a quartet by the methyl group (J=7.1 Hz) and then into an octet due to proton at C-20 (J=10.5 Hz). A doublet at δ 6.8 may be assigned to H-12 which has a higher electron density. The proton showed meta coupling with H-10 (J=2.5 Hz) and a small para coupling with H-9 (J=0.5 Hz) visible under high resolution only. The signal of H-10 is at δ 7.7 as a distorted quartet. It exhibited ortho coupling with H-9 (J=9 Hz) and meta coupling with H-12 (J=2.5 Hz). The H-9 signal is again a doublet at δ 8.00 showing ortho coupling with H-10 (J=9 Hz) and para coupling with H-12 (J=0.5 Hz) visible under high resolution only.Tetrahydroserpenticine showed a complex pattern of Bohlmann bands in the IR spectrum between 2700-2900 cm-1, which is typical of C-3 hydrogen and axial to ring D. The C/D ring junction is therefore trans with reference to the bond C-3 to H and the free electronic doublet of Nb. This conclusion is further supported by proton nmr spectrum of Nb-methiodide of tetrahydroserpenticine which showed N+-CH3 chemical shift at δ 3.36, which is typical of trans quinolizidine system. The configuration at C-20 was established through ORD studies. Tetrahydroserpenticine gave two Cotton effects, one at 270-278 mμ (shoulder, [φ]=+8900 to +8450) and the other at 238-253 mμ (peak at 253 mμ, [φ]=+15100; trough at 238 mμ [φ]=-3500; molecular amplitude 186). The latter arises from α,β-unsaturated ester grouping in ring E. The positive sign of this effect can be explained by any one of the three possible combinations (3α,20β-; 3β,20α- and 3β,20β-). In view of axial hydrogen at C-3, the acceptable combination is 3α,20β. This is further confirmed by small amplitude of Cotton effect at 235-255 nm. It depends on relative positions of the two chromophores (α,β-unsaturated ester and indole nucleus). In 3α,20β combination the two chromophores are more nearly coplanar, and the ORD amplitude is small. Considering α configuration at C-15 (Wenkert rule), the D/E ring junction is trans and tetrahydroserpenticine, therefore, belongs to normal series of heteroyohimbine alkaloids.The configuration at C-19 was established through proton nmr spectrum. Ajmalicine and tetraphylline, having α and axial configuration of C-19 methyl group, show methyl doublet at δ 1.16, further upfield than any other heteroyohimbine alkaloid. The methyl doublet of tetrahydroserpenticine comes at δ 1.35 which compares well with those of raumitorine and rauvanine, having β and equatorial configuration of C-19 methyl group. Tetrahydroserpenticine is, therefore, 19-epimer of tetraphylline. This is further confirmed by large coupling constant (J=10.5 Hz) between H-19 and H-20 due to trans diaxial interaction. The absolute configuration of tetrahydroserpenticine can therefore be represented as. Tetrahydroserpenticine is a reduction product of serpenticine, into which it can be converted back through dehydrogenation with lead tetraacetate. Consequently the following structure can be assigned to serpenticine, keeping in view the relationship between ajmalicine and serpentine.Conclusive support to the above structure is provided by 13C-nmr spectrum of serpenticine. It showed the following signals which were identified through comparison with the spectrum of serpentine. Of particular significance in the context of this data is the downfield shift for the singlet of C-11 due to oxygen substituent, and also comparative downfield shifts of C-19 and C-24 due to epimerization at C-19. Such shifts have been described by Wenkert et al. in case of ajmalicine and 19-epimer of its iso base. The chemical shifts are referred to tetramethylsilane (δ 0.00). * Refers to off resonance spectrum.