Three new secoiridoid derivatives from Olea europaea flowers: isolation, identification and biological evaluation

Olea europaea L. is historically one of the most important trees of the Mediterranean countries. The scientific literature contains plenary of phytochemical and biological works regarding its fruits, leaves and olive oil in contrast to olive flowers were only few data are available. The main goal of the current work was to elaborate a phytochemical study of olive flowers for the isolation and identification of their major secondary metabolites.

The analysis was contacted in hydroalkoholic extract and was achieved in two steps. Initially the extract was fractionated by step-gradient CPC and subsequently the chosen fractions were further purified using preparative HPLC. The result of our study was the isolation of nine natural compounds, two of which are flavonoids (rutin, quercetin-3-O-sophoroside), two triterpenic acids (oleanolic acid, maslinic acid) and five secoiridoid derivatives (oleuropein, 2ʹ-hydroxy-oleuropein, 2ʹ-ethoxy-oleuropein, elenolic acid rutinoside and oleofloside). It is important to say that the compounds 2ʹ-ethoxy-oleuropein (1), elenolic acid rutinoside (2) and oleofloside (3) are described for the first time and thus are considered new natural products.

The structural elucidation of isolated compounds was confirmed by NMR and HRMS/MS spectroscopy. In parallel, the hydroalcoholic extract and the isolated secoiridoid derivatives were evaluated for their ability to inhibit tyrosinase, elastase and collagenase activity. The results showed that both extract and the isolated compounds exhibit remarkable biological activity in inhibiting the collagenase enzyme with 2΄-ethyloxy-oleuropein and 2΄-hydroxy-oleuropein being the most active. In contrast, there was no activity observed, as far as the inhibition of the enzymes tyrosinase and elastase.

Fig. 1 The structures of the new secoiridoid derivatives isolated from olive flowers.