Yucca Phenolics

Recently it has been recognized that yucca contains other physiologically-active constituents, particularly polyphenols. Two stilbenes, including trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene and trans-3,4',5-tetraxydroxystilbene (resveratrol) were identified in yucca bark. Also, some unique compounds with spiro confirmation were isolated and characterized. These included the spirobiflavonoid, larixinol, biosynthesized by combining two C15 units of flavonoid origin, previously identified in Larix gmelini and a number of novel spirostructures, which were given trivial names of yuccaols A-E [18,19] (Fig. 2). These compounds are composed of a C15 unit probably originating from the flavonoid skeleton and a C14 stilbenic compound linked via ?-lactone ring. Resveratrol makes up the stilbenic portion of yuccaols A and B and trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene is the stilbene in yuccaols C, D and E. By the analogy to the biosynthesis of larixinol it was presumed that most probably these compounds are synthesized by the attachment of the stilbenic derivative to the carbocationic intermediate occurring during the oxidation of flavanone to flavanol and subsequent rearrangement of this intermediate. Resveratrol was identified previously in grapes and is believed to be a phytoalexin produced by the plant to fight fungal colonization [20]. In yucca, this compound as well as its methoxyderivative and yuccaols can be found exclusively in yucca bark (Table 1), which is a dead tissue; it is not clear how these compounds are accumulated in this plant organ. Since yucca bark is a component of commercially available yucca powder, these compounds are present exclusively in this product; they are not present in yucca extract obtained by mechanical extraction. The chemistry and bioactivity of yucca saponins and phenolics have recently been reviewed by Piacente et al. [21].