Regioselective Acylation of Natural Glycosides

Saponins (e.g., asiaticoside, 17) are widely distributed in plants as well as in some lower marine animals and possess a broad range of biological activities [62a], Examples of acylation of these complex molecules have been reported by different authors. For instance, the regioselective esterification of 17 by CalB was quite remarkable, and only the 4 -0-acetate (17a) was isolated in 59% yields [66]. 

The regioselective esterification of other groups of saponins, e.g., a series of diosgenins [70], the haemolytic compound digitonin [71], and the so-called cardiac glycosides [72], have also been investigated by different groups. 

Flavonoid glycosides and their esters are an important group of natural compounds isolated from plants. They are widely used in pharmaceutical, cosmetic, and food preparations, and it is therefore not surprising that the search for new derivatives with improved antioxidant and antimicrobial activity, or-more sim-ply-with more suitable physico-chemical properties for specific applications (i.e., increased stability and/or solubility), has identified enzyme-catalyzed regioselective esterification as a particularly promising approach to the target molecules [21a], 

Because of industrial interest in these derivatives, several papers have appeared in the scientific literature, mainly focused on the optimization of the reaction conditions for the esterification of rutin (22) and naringin (23) [76]. When the best operating conditions were established, the conversion yield of the substrate could exceed 95%. Several pilot plants are at an advanced stage, and it seems that these acylated flavonoids will soon be marketed [21a]. 

Following the first examples of the acylation of adenosine and uridine [2], the regioselective lipase-catalyzed esterification of these important molecules has been extensively investigated by Gotor s group [21g]. Attention has been mainly focused on deoxynucleosides - compounds of special interest in medicinal chemistry because of their antiviral and antitumoral properties. Some significant examples are discussed below. 

---