Untitled

Anti-inflammatory Activity of Pyrrolizidine Alkaloids from the Leaves of <i>Madhuca pasquieri</i> (Dubard)

“…The cis-trans conversion in pyrrolizidines, and the deciding effect of steric factors on their conformational equilibrium have been discussed. 13,14) In addition, NMR assignment studies showed that shift values for base ring protons are very distinctive and allow for the immediate recognition of important features, particularly any unsaturation and the oxygenation pattern. The usual oxygenation pattern is either mono-oxygenation at C-9 or the other PA with multi-oxygenation at C-2, C-7, and C-9.…”

Pyrrolizidine alkaloids

“…An NMR study conducted by Fleet's group conrmed the structures of previously isolated australine-type alkaloids in order to be sure that all compounds were, in fact, distinct natural products. 6 As part of this study, the group conducted a careful search for further pyrrolizidines in Castanospermum australe seeds, and discovered three new alkaloids (1)(2)(3). They also reported extensive biological testing of all isomers against a panel of glycosidase enzymes.…”

Cis-trans-conversion of the bicycle in pyrrolizidines: The effect of the preferred conformations on the properties of the bases and the thermodynamics of conformational transformation of the ring fusion type. (Review)

“…In addition, in the highly strained alcohols 26 and 27 with preferred trans-fusion of the rings there are favorable steric conditions for the formation of a strong intramolecular hydrogen bond. This weakens the interaction between the hydroxyl group and the polar stationary liquid phase and also promotes decrease in the retention time compared with the other isomers [15,31].…”

“…The pyrrolizidines were subdivided provisionally into unstrained and weakly and highly strained bases, according on the strength of the nonbonding interactions in the cis-fused forms B [14,15]. It should be understood that we are not discussing compounds in which the strain originates from interaction between sterically close substituents at the C (1) and C (7) atoms.…”

“…These nonbonding interactions are removed during the transition of the molecules to the trans-fused form 6A or 7A. However, there are strong angular strains in forms A [2,14,15,17]. In spite of this many pyrrolizidine derivatives of types 6 and 7 exist preferentially in the trans-fused conformations A [14,[17][18][19][20][21][22][23], demonstrating the considerable supremacy of the cis-fused forms B with respect to strain that determines the preference for the A forms.…”