The alkaloid otosenine. Evidence of a weak –>N:...C<–O intra-annular bond

Abstract

CI9H2707N , orthorhombic, P2t2~2 a = 16.355 (7), b = 12.843 (2), c = 8.961 (1) A, Z = 4, D m = 1.34 g cm -3. The alkaloid otosenine is a pyrrolizidine derivative isolated from a senecio plant. Some features observed in the pyrrolizidine ring suggest the presence of a N... C (2.18 A) intra-annular 'one electron bond'. The macrocycle can be described in terms of pseudo twofold symmetry. Introduction.A crystal of dimensions 0.30 x 0.20 × 0.16 mm was selected for recording intensity data on a fully automated diffractometer. Graphite-monochromated Mo KCt radiation was used to collect the I002 diffractometer-significant (no time was wasted for accidentally absent reflexions) independent reflexions up to t9 = 25 ° . Of these, 848 were considered as observed according to the criterion I > 2o(1). The data were corrected for Lorentz and polarization effects, but no absorption correction was applied.The structure was solved by direct methods using the MULTAN 74 system (Main, Woolfson, Lessinger, Germain & Declercq, 1974). An E map, generated from the best of 200 phase sets, located 23 of the 27 non-hydrogen atoms in the asymmetric unit. The rest of the structure, including the H atoms, was found from difference maps. Refinement was by full-matrix least

Characterization of Two Structural Forms of Otonecine-Type Pyrrolizidine Alkaloids from Ligularia hodgsonii by NMR Spectroscopy

“…Therefore, it was suggested that there is a transannular interaction between the nitrogen atom and C-8 in the structure of 1 . Similar suggestions have also been made previously in view of various X-ray crystallographic studies, regarding the structure of otonecine base 1-3,5,12 and other otonecine-type PAs, − including 2 , which was originally isolated from Ligularia elegans Cass …”

“…11 Therefore, it was suggested that there is a transannular interaction between the nitrogen atom and C-8 in the structure of 1. Similar suggestions have also been made previously in view of various X-ray crystallographic studies, regarding the structure of otonecine base [1][2][3]5,12 and other otonecine-type PAs, [18][19][20][21] including 2, which was originally isolated from Ligularia elegans Cass. 10 However, it should be noted that in all these X-ray crystallographic studies the compounds were examined as the dried crystalline form, and consequently, there has been no previous report regarding the dependency of the type of media in determining whether otonecine-type PAs exist in non-ionized or ionized forms.…”

Crystal and molecular structures of 1-isoquinolyl(phenyl)methyl benzoate and 1-isoquinolylisopropyl benzoate

“…Among many factors influencing the base strengths of amines, an intramolecular interaction of the lone pair of electrons at the nitrogen with a carbonyl function is well known (Perez-Salazar et al, 1977;Kaftory and Dunitz, 1975;Btirgi et al, 1973;Birnbaum, 1972;Hall, 1968). Such a type of interaction could have been expected for the acylated isoquinolylmethanols.…”