- Email: [email protected]
13C NMR spectroscopy of Pyrrolizidine alkaloids
Phytochemistry, Vol. 21, No. 2, pp. 439-443, 1982. Printed in Great Britain.
0031-9422/82/020439-05$03.00/0 Pergamon Press Ltd.
13C NMR SPECTROSCOPY RUSSELL
J. MOLYNEUX,
OF PYRROLIZIDINE
JAMES N. ROITMAN,
MABRY
ALKALOIDS
BENSON and ROBERT E. LUNDIN
Western Regional Research Center, Science and Education Administration, CA 94710, U.S.A.
U.S. Department
of Agriculture, Berkeley,
(Revised received 22 June 1981) Key Word Index-‘%2 NMR; pyrrolizidine alkaloids; retronecine;
pyrrolizidine
N-oxides.
Abstract-The 13C NMR spectra of nine pyrrolizidine alkaloids of the macrocyclic diester type, seven of the corresponding N-oxides and of the parent base retronecine have been recorded and the signals assigned. The 13C NMR signals were found to be sensitive to structural variation in both the diester moiety and the heterocyclic ring system, providing useful information for structural elucidation, particularly when the ‘H NMR spectra may be difficult to interpret.
INTRODUCTION
and riddelliine (4), the most typical and frequently encountered pyrrolizidine alkaloids of Senecio species, together with retronecine (lo), the necine base common to the above and many other alkaloids of this type. In addition, assignments are listed for the dihydropyrrolizidines platphylline (5) and hygrophylline (6) as well as the seco-type, senkirkine (7). These IZmembered macrocyclic ring alkaloids have been numbered in accordance with the system proposed by Culvenor et al. [13], extended by the addition of appropriate numbering for carbon atoms substituent
Pyrrolizidine alkaloids, particularly those of the macrocyclic diester type, which occur in the genus Senecio, and to a lesser extent in a number of other genera, present a hepatoxicity hazard to range animals and potentially to humans [l]. A continuing survey being conducted by this laboratory of a large number of Senecio species growing in the western United States, both uninvestigated and previously investigated, has yielded known alkaloids together with generally small amounts of unknown alkaloids of apparently similar structural type. Since ‘H NMR spectra of the latter provided only limited information in regard to slight structural variation in the necic acid portion of the molecule, due to overlapping or obscured signals, it became essential to obtain 13C NMR spectral data for known alkaloids in order to provide additional information for structural elucidation. Most of the 13C NMR spectra previously reported have been limited to individual compounds, lacking rigorous derivation of assignments [2-51, or to partial assignments of only the carbonyl and cr,p-unsaturated carboxyl groups [6--81. More recent publications on the 13C NMR of four macrocyclic pyrrolizidine alkaloids by Drewes et al. [9] and of several macrocyclic and non-macrocyclic pyrrolizidine alkaloids by Mody et al. [lo] report assignments which differ in significant respects from our results. Moreover, certain obviously incorrect assignments for retrorsine [ 1I] have been perpetuated by citation in a reference volume of 13C NMR shift assignments of alkaloids [12]. We now report the 13C NMR spectra and systematic derivation of signal assignments for a number of pyrrolizidine alkaloids of closely related structural types, together with certain of their N-oxides, in order to provide the correlations necessary for interpretation of the spectra of novel alkaloids.
CH, CH,R
H
H \
18 IS 14 C=C-CH,-CH-
IS,/ Me
131
\
121
C-OH
1
Me’ %H;8 CO II
‘“C0:
3 4
I, R= H, senectonine 2 R=OH,retrorsine R H\
Me
/
1 coo
R=H,seneciphylline R=OH,riddelItine
I
-OH
I
5 R=H, plotyphylllne 6 R=OH, hygrophylline 13’
Me
7 senkirkine
12’
Me
RESULTS AND DISCUSSION
The 13C NMR spectral data are reported in Table 1 for senecionine (l), retrorsine (2), seneciphylline (3) 439
CH,OCO
Me
EC-CH--dH-C
Me
C=C-CH,!-!-OH
8 R=H, monocrotaline 9 R= COMe, spectabiline
IO retronecine
13CNMR of pyrrolizidine Monocrotaline
(8) and spectabiline
(9)
These compounds differ from those previously discussed in possessing an 11-membered rather than a 12-membered macrocyclic ring. Nevertheless, the resonances for the carbon atoms in the retronecine moiety show very little variation from those observed previously, the most significant being that due to C-2 which exhibits an upfield shift of 2.3 ppm relative to the signal for the same atom in senecionine (1). The ester carbonyl groups at C-11 and C-16, however, show significant shifts of 4.1 ppm upfield and 6.0 ppm downfield, respectively, for monocrotaline as compared to senecionine, probably as a result of the more limited conformational possibilities imposed on the smaller macrocyclic ring. Acetylation of the C-13 hydroxyl group to give spectabiline (9) produces little effect on the C-11 resonance but shifts that of the C-16 upfield by 4.7 ppm, possibly due to removal of hydrogen-bonding between the -OH and -C=O groups. The assignments for aii other carbon atoms in these two alkaloids can be made quite unequivocally, including those of the acetate group in spectabiline (Table 1). Pyrrolizidine
alkaloid
N-oxides
Despite the fact that macrocyclic diester pyrrolizidine alkaloid N-oxides frequently comprise the major portion oi these alkaioids in nature [20], their 13CNMR spectra have not been recorded, with the exception of those of retronecine and monocrotaline N-oxides recently reported by Barreiro et al. [21]. Table 2 records the “C NMR data for the N-oxides of senecionine (l), retrorsine (2), seneciplylline (3), riddelline (4), monocrotaline (8), spectabiline (9) and retronecine (lo), measured in D,O. These results accord with those reported [21] for retronecine N-oxide but comparison of the spectra for spectabiline and monocrotaline N-oxides indicates that the assignments for C-12 and C-13 in the latter should be interchanged. As expected C-3, C-5, and C-8 show large deshielding effects in the spectra of the N-oxides, compared to the tertiary bases, whereas the other carbon atoms show relatively minor shifts. In the macrocyclic diester N-oxides, C-8 exhibits the largest downfield shift, ranging from 19.0 to 20.6 ppm. For retronecine N-oxide, the smaller shift of 18.4 ppm emphasizes the effect of esterification upon this position. The C-3 position shows little variation in shift, ranging from 17.4 to 18.4 ppm but the shift of C-5 produced on N-oxidation is significantly different for the 1l-membered macrocyclic alkaloids monocrotaline and spectabiline (15.2-15.8 ppm) relative to the 1Zmembered macrocyclic alkaloids (17.4-17.9 ppm). This effect is undoubtedly due to the different conformations imposed upon the C-11 and C-12 necic acid moieties by diesterification with the necine base.
alkaloids
443
Other smaller shifts produced on N-oxidation are most significant for the C-11 and C-16 ester carbonyl groups and for those atoms of the ester portion of the molecule which extend over the necine moiety. The assignments presented for the N-oxides in Table 2, in addition to confirming those derived for the free bases, provides correlations whereby the tertiary bases and their N-oxides can be readily distinguished by the use of 13CNMR spectroscopy. EXPERKMENTAL The “C NMR spectra were recorded at 25.03 MHz. 16000 data points were used to cover a spectral width of 6250HZ. Samples were pulsed every 2 set with a flip angle of 45”. REFERENCES
1. Bull, L. B., Culvenor, C. C. J. and Dick, A. T. (1%8) The Pyrrolizidine Akaloids. North-Holland, Amsterdam. 2. Culvenor, C. C. J., Johns S. R. and Smith, L. W. (1975) Ausr. i. Chem. 28,23i9. 3. Zalkow, L. H., Gelbaum, L. and Keinan, E. (1978) Phytochemistry 17, 172. 4. Wiedenfeld, H. and Ridder, E. (1979) Phytochemistry 18, 1083. 5. Ridder, E., Wiedenfeld, H. and Frisse, M. (1980) Phytochemistry 19, 1275. 6. Hikichi, M. and Furuya, T. (1974) Tetrahedron Letters 3657. -1.. . . . .I . I -7 n\ m.._ruruya, 1. (‘r/r, lerrf47. nucictu, M., Asaaa, r. anaI m hedron Letters 1233. 8. Hikichi, M. and Furuya, T. (1978) Tetrahedron Letters 767. 9. Drewes, S. E., Antonowitz, I., Kaye, P. T. and Coleman, P. C. (1981) J. Chem. Sot., Perkin Trans. 1,287. 10. Mody, N. V., Sawhney, R. S. and Pelletier, S. W. (1979) J. Natural Products 42, 417. 11. Casal, H., Altamirano, J. and Moyna, P. (1977) Gazz. Chim. Ital. 107, 361. 12. Shamma, M. and Hindenlag, D. M. (1979) Carbon-13 NMR Shift Assignments of Amines and Alkaloids. Plenum Press, New York. 13. Culvenor, C. C. J., Crout, D. H. G., Klyne, W., Mose, W. P., Renwick, J. D. and Scopes, P. M. (1971) J. Chem. Sot. (C), 3653. 14. Wehrli, F. W. and Nishida, T. (1979) Fortschr. Chem. Org. Naturst. 36, 1. 15. Wehrli, F. W. and Wirthlin, T. (1976) Interpretation of Carbon-13 NMR Spectra. Heyden, London. 16. Culvenor, C. C. J. (1966) Tetrahedron Letters 1091. 17. Chalmers, A. A., Pachler, K. G. R. and Wessels, P. L. (1974) Mag. Reson. 15,415. 18. Birnbaum, G. I. (1974) J. Am. Chem. Sot. %,6165. 19. Birnbaum, K. B. (1972) Acta Crystallogr. Sect. B 28, 2825. 20. Molyneux, R. J., Johnson, A. E., Roitman, J. N. and Benson, M. E. (1979) J. Agric. Food Chem. 27,4%. 21. Barreiro, E. J., De Lima Periera, A., Nelson, L., Gomes, L. F. and Da Silva, J. R. (1980) J. Chem. Res. S, 330.
0031-9422/82/020439-05$03.00/0 Pergamon Press Ltd.
13C NMR SPECTROSCOPY RUSSELL
J. MOLYNEUX,
OF PYRROLIZIDINE
JAMES N. ROITMAN,
MABRY
ALKALOIDS
BENSON and ROBERT E. LUNDIN
Western Regional Research Center, Science and Education Administration, CA 94710, U.S.A.
U.S. Department
of Agriculture, Berkeley,
(Revised received 22 June 1981) Key Word Index-‘%2 NMR; pyrrolizidine alkaloids; retronecine;
pyrrolizidine
N-oxides.
Abstract-The 13C NMR spectra of nine pyrrolizidine alkaloids of the macrocyclic diester type, seven of the corresponding N-oxides and of the parent base retronecine have been recorded and the signals assigned. The 13C NMR signals were found to be sensitive to structural variation in both the diester moiety and the heterocyclic ring system, providing useful information for structural elucidation, particularly when the ‘H NMR spectra may be difficult to interpret.
INTRODUCTION
and riddelliine (4), the most typical and frequently encountered pyrrolizidine alkaloids of Senecio species, together with retronecine (lo), the necine base common to the above and many other alkaloids of this type. In addition, assignments are listed for the dihydropyrrolizidines platphylline (5) and hygrophylline (6) as well as the seco-type, senkirkine (7). These IZmembered macrocyclic ring alkaloids have been numbered in accordance with the system proposed by Culvenor et al. [13], extended by the addition of appropriate numbering for carbon atoms substituent
Pyrrolizidine alkaloids, particularly those of the macrocyclic diester type, which occur in the genus Senecio, and to a lesser extent in a number of other genera, present a hepatoxicity hazard to range animals and potentially to humans [l]. A continuing survey being conducted by this laboratory of a large number of Senecio species growing in the western United States, both uninvestigated and previously investigated, has yielded known alkaloids together with generally small amounts of unknown alkaloids of apparently similar structural type. Since ‘H NMR spectra of the latter provided only limited information in regard to slight structural variation in the necic acid portion of the molecule, due to overlapping or obscured signals, it became essential to obtain 13C NMR spectral data for known alkaloids in order to provide additional information for structural elucidation. Most of the 13C NMR spectra previously reported have been limited to individual compounds, lacking rigorous derivation of assignments [2-51, or to partial assignments of only the carbonyl and cr,p-unsaturated carboxyl groups [6--81. More recent publications on the 13C NMR of four macrocyclic pyrrolizidine alkaloids by Drewes et al. [9] and of several macrocyclic and non-macrocyclic pyrrolizidine alkaloids by Mody et al. [lo] report assignments which differ in significant respects from our results. Moreover, certain obviously incorrect assignments for retrorsine [ 1I] have been perpetuated by citation in a reference volume of 13C NMR shift assignments of alkaloids [12]. We now report the 13C NMR spectra and systematic derivation of signal assignments for a number of pyrrolizidine alkaloids of closely related structural types, together with certain of their N-oxides, in order to provide the correlations necessary for interpretation of the spectra of novel alkaloids.
CH, CH,R
H
H \
18 IS 14 C=C-CH,-CH-
IS,/ Me
131
\
121
C-OH
1
Me’ %H;8 CO II
‘“C0:
3 4
I, R= H, senectonine 2 R=OH,retrorsine R H\
Me
/
1 coo
R=H,seneciphylline R=OH,riddelItine
I
-OH
I
5 R=H, plotyphylllne 6 R=OH, hygrophylline 13’
Me
7 senkirkine
12’
Me
RESULTS AND DISCUSSION
The 13C NMR spectral data are reported in Table 1 for senecionine (l), retrorsine (2), seneciphylline (3) 439
CH,OCO
Me
EC-CH--dH-C
Me
C=C-CH,!-!-OH
8 R=H, monocrotaline 9 R= COMe, spectabiline
IO retronecine
13CNMR of pyrrolizidine Monocrotaline
(8) and spectabiline
(9)
These compounds differ from those previously discussed in possessing an 11-membered rather than a 12-membered macrocyclic ring. Nevertheless, the resonances for the carbon atoms in the retronecine moiety show very little variation from those observed previously, the most significant being that due to C-2 which exhibits an upfield shift of 2.3 ppm relative to the signal for the same atom in senecionine (1). The ester carbonyl groups at C-11 and C-16, however, show significant shifts of 4.1 ppm upfield and 6.0 ppm downfield, respectively, for monocrotaline as compared to senecionine, probably as a result of the more limited conformational possibilities imposed on the smaller macrocyclic ring. Acetylation of the C-13 hydroxyl group to give spectabiline (9) produces little effect on the C-11 resonance but shifts that of the C-16 upfield by 4.7 ppm, possibly due to removal of hydrogen-bonding between the -OH and -C=O groups. The assignments for aii other carbon atoms in these two alkaloids can be made quite unequivocally, including those of the acetate group in spectabiline (Table 1). Pyrrolizidine
alkaloid
N-oxides
Despite the fact that macrocyclic diester pyrrolizidine alkaloid N-oxides frequently comprise the major portion oi these alkaioids in nature [20], their 13CNMR spectra have not been recorded, with the exception of those of retronecine and monocrotaline N-oxides recently reported by Barreiro et al. [21]. Table 2 records the “C NMR data for the N-oxides of senecionine (l), retrorsine (2), seneciplylline (3), riddelline (4), monocrotaline (8), spectabiline (9) and retronecine (lo), measured in D,O. These results accord with those reported [21] for retronecine N-oxide but comparison of the spectra for spectabiline and monocrotaline N-oxides indicates that the assignments for C-12 and C-13 in the latter should be interchanged. As expected C-3, C-5, and C-8 show large deshielding effects in the spectra of the N-oxides, compared to the tertiary bases, whereas the other carbon atoms show relatively minor shifts. In the macrocyclic diester N-oxides, C-8 exhibits the largest downfield shift, ranging from 19.0 to 20.6 ppm. For retronecine N-oxide, the smaller shift of 18.4 ppm emphasizes the effect of esterification upon this position. The C-3 position shows little variation in shift, ranging from 17.4 to 18.4 ppm but the shift of C-5 produced on N-oxidation is significantly different for the 1l-membered macrocyclic alkaloids monocrotaline and spectabiline (15.2-15.8 ppm) relative to the 1Zmembered macrocyclic alkaloids (17.4-17.9 ppm). This effect is undoubtedly due to the different conformations imposed upon the C-11 and C-12 necic acid moieties by diesterification with the necine base.
alkaloids
443
Other smaller shifts produced on N-oxidation are most significant for the C-11 and C-16 ester carbonyl groups and for those atoms of the ester portion of the molecule which extend over the necine moiety. The assignments presented for the N-oxides in Table 2, in addition to confirming those derived for the free bases, provides correlations whereby the tertiary bases and their N-oxides can be readily distinguished by the use of 13CNMR spectroscopy. EXPERKMENTAL The “C NMR spectra were recorded at 25.03 MHz. 16000 data points were used to cover a spectral width of 6250HZ. Samples were pulsed every 2 set with a flip angle of 45”. REFERENCES
1. Bull, L. B., Culvenor, C. C. J. and Dick, A. T. (1%8) The Pyrrolizidine Akaloids. North-Holland, Amsterdam. 2. Culvenor, C. C. J., Johns S. R. and Smith, L. W. (1975) Ausr. i. Chem. 28,23i9. 3. Zalkow, L. H., Gelbaum, L. and Keinan, E. (1978) Phytochemistry 17, 172. 4. Wiedenfeld, H. and Ridder, E. (1979) Phytochemistry 18, 1083. 5. Ridder, E., Wiedenfeld, H. and Frisse, M. (1980) Phytochemistry 19, 1275. 6. Hikichi, M. and Furuya, T. (1974) Tetrahedron Letters 3657. -1.. . . . .I . I -7 n\ m.._ruruya, 1. (‘r/r, lerrf47. nucictu, M., Asaaa, r. anaI m hedron Letters 1233. 8. Hikichi, M. and Furuya, T. (1978) Tetrahedron Letters 767. 9. Drewes, S. E., Antonowitz, I., Kaye, P. T. and Coleman, P. C. (1981) J. Chem. Sot., Perkin Trans. 1,287. 10. Mody, N. V., Sawhney, R. S. and Pelletier, S. W. (1979) J. Natural Products 42, 417. 11. Casal, H., Altamirano, J. and Moyna, P. (1977) Gazz. Chim. Ital. 107, 361. 12. Shamma, M. and Hindenlag, D. M. (1979) Carbon-13 NMR Shift Assignments of Amines and Alkaloids. Plenum Press, New York. 13. Culvenor, C. C. J., Crout, D. H. G., Klyne, W., Mose, W. P., Renwick, J. D. and Scopes, P. M. (1971) J. Chem. Sot. (C), 3653. 14. Wehrli, F. W. and Nishida, T. (1979) Fortschr. Chem. Org. Naturst. 36, 1. 15. Wehrli, F. W. and Wirthlin, T. (1976) Interpretation of Carbon-13 NMR Spectra. Heyden, London. 16. Culvenor, C. C. J. (1966) Tetrahedron Letters 1091. 17. Chalmers, A. A., Pachler, K. G. R. and Wessels, P. L. (1974) Mag. Reson. 15,415. 18. Birnbaum, G. I. (1974) J. Am. Chem. Sot. %,6165. 19. Birnbaum, K. B. (1972) Acta Crystallogr. Sect. B 28, 2825. 20. Molyneux, R. J., Johnson, A. E., Roitman, J. N. and Benson, M. E. (1979) J. Agric. Food Chem. 27,4%. 21. Barreiro, E. J., De Lima Periera, A., Nelson, L., Gomes, L. F. and Da Silva, J. R. (1980) J. Chem. Res. S, 330.