A new oxindole alkaloid, 21-oxogelsevirine (1), has been isolated from the MeOH extract of the stem of Gelsemium rankinii and its structure elucidated. The ¹H- and ¹³C-nmr spectra were assigned by comparison with the other three known analogs, gelsemine (4), 21-oxogelsemine (2), and gelsevirine (3). Gelsemium rankinii Small, native to the southeastern United States, is one of the three Gelsemium species belonging to the family Loganiaceae. Although the other two species, G. elegans and G. sempervirens, have been extensively studied, G. rankinii has hitherto not been explored. As part of our interest in the anticancer activity of extracts of plants of this genus, we have studied the alkaloids of G. rankinii. We report here the isolation and structure elucidation of a new oxindole alkaloid, namely 21-oxogelsevirine (1). Compound 1 displayed a molecular ion peak at m/z 366, 30 mu more than that of 21-oxogelsemine (2), and a base peak at m/z 122 characteristic of the fragmentation of 21-oxogelsemine (2), a minor alkaloid isolated from G. sempervirens (1). The observation in the ¹H-nmr spectrum of a three proton singlet at 3.96 ppm revealed the isolate to possess a methoxy group. The general similarity of all its spectra with those of compound 2 except for the chemical shifts of the aromatic protons, which were nearly identical with those of gelsevirine (3), pointed to compound 1 being 21-oxogelsevirine (Table 1). Instead of gelsemine (4), the major alkaloid of G. sempervirens, gelsevirine (3) was the predominant alkaloid of G. rankinii. Previously, we have been able to assign unambiguously the proton and carbon spectra of gelsemine (4) (2). Using the same method, we found it also necessary to revise the previously assigned proton and carbon spectra of gelsevirine (3). Thus, the previous assignments for H-16, H-15, H-14a, H-14e, and H-6 for gelsevirine should be H-15, H-14a, H-16, H-6, and H-14e, respectively, from the evidence of a homonuclear 2-D COSY experiment. From the heteronuclear 2-D correlation spectrum, the assignments for C-16, C-15, C-6, and N-CH₃ should be revised to C-15, C-16, N-CH₃, and C-6, respectively. Given the limited sample of 21-oxogelsemine isolated from the stem of G. sempervirens, it was not possible to employ a heteronuclear 2-D experiment for establishment of those nmr assignments. A simple and sensitive one-dimensional nmr technique for the correlation of proton and carbon chemical shifts (CSCM 1D) (4), however, made it possible to assign unambiguously the ¹³C spectrum of 21-oxogelsemine (2) (Figure 1). For example, spectrum (C) was obtained by transfer from the downfield ¹³C satellite of H-9 (920 Hz+80 Hz). Although the chemical shifts of C-9 and C-11 were close, their ¹H chemical shift difference (47 Hz) allowed selective irradiation to afford transfer only of C-9. In spectrum (D), the close ¹H chemical shifts of H-3 and H-5 led to the observation of both carbons. Each spectrum (B)-(H) is the result of only 400 scans. 21-Oxo substitution of gelsemine produced a significant downfield shift in C-18, C-21, C-20, C-6, and C-16, and an upfield shift for C-19, C-5, N-CH₃, and C-15 (Table 2). The close chemical shifts of C-6 and C-7 were distinguishable from an APT experiment, while C-17 and C-20 were confirmed by CSCM 1D for C-17. One of the carbonyl groups, C-21, was confirmed by the method of pulsed polarization transfer via long-range ¹H-¹³C couplings (5), i.e., irradiating the proton chemical shift of N-CH₃ at 2.79 ppm permitted observation of the signal of C-21 at 176.84 ppm. Subsequently, the ¹³C spectrum of the new isolate (1) was assigned by comparison with the three analogs, gelsemine, 21-oxogelsemine, and gelsevirine (Table 2). The ¹H-nmr spectra of 21-oxogelsemine and 21-oxogelsevirine were also assigned by a 2-D COSY experiment. The absence of H-21, the downfield shifts of H-18, H-5, H-14, H-15, and N-CH₃ and upfield shifts of H-19 and H-16, supported the presence of the 21-oxo group. The biological activity of these isolates will be reported subsequently.