Semi-synthesis of taraxerane triterpenoids from oleanolic acid

Biotransformation of oleanolic and maslinic methyl esters by Rhizomucor miehei CECT 2749

“…Then, the regeneration of the carbonyl group and the opening of the lactone ring could occur, forming an alkoxide ion at C-13. The subsequent attack of the alkoxide ion to C-12, by the b-face, with the output of the bromide ion would generate the 12b,13b-epoxy group (Martinez et al, 2015). The microbial transformation of compound 13 by R. miehei for 13 days yielded a mixture of metabolites very difficult to separate.…”

Structural determination of oleanane-28,13β-olide and taraxerane-28,14β-olide fluorolactonization products from the reaction of oleanolic acid with SelectfluorTM

“…Since acid catalysis is unlikely under the electrophilic fluorinating conditions employed with Selectfluor TM and given that we failed to isolate any intermediates containing a C-14 C-15 double bond, we propose that formation of (2) follows a different mechanism to that proposed for the bromolactonization of OA (Martinez et al, 2015). We propose that following electrophilic addition of fluorine to the C12 C13 double bond, the tertiary carbocation formed at C-13 can either be stabilized by the intramolecular nucleophilic attack of the C-28 carboxyl to form (1) or by the Wagner-Meerwein 1,2-shift of C-27 to C-13.…”

“…However, our findings parallel previous reports in the literature which report formation of other oleanane-type 28,13�-�-lactone derivatives under different conditions. For instance, using NMR analysis for characterization, the formation of 12-bromo-3�-hydroxytaraxeran-28,14�-olide as a minor product from the reaction of OA with bromine in CCl 4 has been reported (Martinez et al, 2015). The formation of 3-O-acetyl-taraxeran-28,14�-olide from the reaction of OA under oxidative conditions (formic acid/hydrogen peroxide at 373 K for several hours) has also been reported and characterized by NMR analysis (Heise et al, 2021).…”

“…While the precise mechanism underlying the formation of compound (2) following reaction of OA with Selectfluor TM remains unclear, Martinez and co-workers have previously elucidated a possible mechanism for the formation 12-bromo-3�-hydroxytaraxeran-28,14�-olide (Martinez et al, 2015). Following bromination of oleanolic acid with Br 2 /CCl 4 , the authors report isolation of 12�-bromo-3�-hydroxyolean-28,13�-olide (80%), together with 12�-bromo-3�-hydroxytaraxer-14-en-28-oic acid (3%), and 12�-bromo-3�-hydroxyolean-28,13�-olide (13%), as minor products when the reaction was maintained for 12 h. The authors propose that formation of 12�-bromo-3�-hydroxytaraxer-14-en-28-oic acid could be explained due to traces of HBr in the reaction…”

Coumarinolignoids and Taraxerane Triterpenoids from Sapium discolor and Their Inhibitory Potential on Microglial Nitric Oxide Production

“…Coumarinolignoids are a class of natural products in which a phenylpropanoid unit is linked to a coumarin moiety through a 1,4-dioxane bridge . These substances are relatively rare and exhibit a wide variety of biological properties, such as anti-inflammatory, antioxidant, and hepatoprotective effects. − Taraxeranes, a type of triterpenoids derived biosynthetically from oleananes, display cytotoxic and allelopathic activities. − Both coumarinolignoids and taraxerane triterpenoids occur in plants in the family Euphorbiaceae. − …”