Grantianine, C18H23NO7: a pyrrolizidine alkaloid

Abstract

Abstract. M r = 365.1, monoclinic, P21, a = 12.93 (1), b=6.32(1), c=10.78(1)A, fl=102.73(9) ° , V= 859.3 A 3, Z = 2, D x = 1.411, D,,, = 1.40 (2) Mg m -3, ~,(Mo Ka) = 0.71069 A, g = 0.07 mm -1, F(000) = 388, room temperature. Final R =0.059 for 1321 observed reflections. Grantianine, a retronecine-derived pyrrolizidine alkaloid, has the carbonyl bonds of the ester functions on opposite sides of the 11-membered macrocycle with an angle of 139.6 (5) ° between the bonds. The y-butyrolactone ring is trans-fused to the macrocycle across bond C(13)-C(14). The exocyclic methyl group at C(24) is in a quasi-axial orientation.

Pyrrolizidine alkaloids

“…The selective attack at the more highly substituted carbonyl group is usually attributed to the approach trajectory of the nucleophile but may also be due to the electronic effect of the a-alkoxy group. The major isomer (82) was coupled with the protected derivative (83) of (+)retronecine (47) to yield alcohol (84) after selective removal of one of the silyl protecting groups. Lactonization was achieved by displacement of the mesylate after removal of the second silyl protecting group.…”

“…In the 1 1-membered analogue (1 36) which contains retronecine, the ester carbonyl groups are antiparallel as for trichodesmine. 84 The conformations of three macrocyclic diesters of synthanecine A have been established by Robins and Sim.H5 The alkaloid analogue (107a) has an unsymmetrical conformation with both ester carbonyl groups directed away from the 2-H. The dilactones (104) and ( 105) have conformations that approximate closely to C , symmetry with ester carbonyl groups that are antiparallel.…”

Pyrrolizidine alkaloids

“…The structure (69) for the new alkaloid desoxyaxillaridine was established by comparison of the mass and l H n.m.r. spectral data with those for the alkaloids ( 67) and (68).…”