13C nuclear magnetic resonance spectra of the Senecio alkaloids, retrorsine, swazine, isoline, and hygrophylline

High Resolution 1H-NMR and C-NMR Spectra of Saturated Pyrrolizidine Monoester Alkaloids

“…B. 13C-NMR Studies.-Although more than two hundred pyrrolizidine alkaloids have been isolated and characterized so far, use of 13C-nmr spectra has received only sporadic attention (12)(13)(14)(15). Table 4, which lists 13C-nmr spectra of compounds 1-12, indicates that chemical shift data can be used to discriminate between a) esters of trachelanthimidine and isoretronecanol and b) viridiflorates and trachelanthates.…”

Preparation of (+)-nemorensic acid and approaches to nemorensine using the partial reduction of electron deficient furans

“…1 The next step, stereoselective hydrogenation of retronecine, proved more difficult to accomplish. This reaction has been reported in the literature, 16 when Drewes and co-workers described successful hydrogenation using PtO 2 on carbon under one atmosphere of hydrogen. However, several attempts to repeat this work all met with failure in our hands.…”

Macrocyclic Pyrrolizidine Alkaloids from Senecio anonymus. Separation of a Complex Alkaloid Extract Using Droplet Counter-Current Chromatography

“…13C-nmr spectra were obtained for all the alkaloids except 4 and 8 (see Table 2), and unambiguous assignments for all H-containing carbons were based on 'H-'3C heteronuclear shift correlated nmr (HETCOR; Figure 2). For example, the clear distinction between the chemical shifts of C-2 and C-20 in retrorsine [31, which were previously interchanged (24) is illustrated in Figure 3-Table 2 indicates chemical shifts that were misassigned or interchanged in earlier reports.…”