An investigation to optimize the extraction yield and the radical scavenging activity from the agricultural by-product olive tree wood (Olea europaea L., cultivar Picual) using six different extraction protocols was carried out. Four olive wood samples from different geographical origin, and harvesting time have been used for comparison purposes. Among the fifty olive wood extracts obtained in this study, the most active ones were those prepared with ethyl acetate, either through direct extraction or by successive liquid-liquid partitioning procedures, the main components being the secoiridoids oleuropein and ligustroside. An acid hydrolysis pretreatment of olive wood samples before extractions did not improve the results. In the course of this study, two compounds were isolated from the ethanolic extracts of olive wood collected during the olives’ harvesting season and identified as (7′′R)-7′′-ethoxyoleuropein (1) and (7′′S)-7′′-ethoxyoleuropein (2). 1. Introduction Since agricultural and industrial residues are attractive sources of natural antioxidants, basically due to their null or low value [1–4], different residues and by-products from fruits [5, 6], vegetables [7, 8], or olive oil manufacturing [9] have been screened for the presence of antioxidants. Due to the large amounts of biomass from pruning generated every year (more than 7 million tonnes per year in Spain), olive tree wood constitutes an important agricultural by-product. During the search of natural antioxidants from Olea europaea L. residues and by-products, both solid and liquid residues from olive oil and table olives processing have been studied [2, 10–19]. Our preliminary studies on the radical scavenging activity of olive wood extracts, cultivar Picual, showed that this agricultural by-product could be a source of natural antioxidants [20]. The isolation and radical scavenging activity of the main constituents [21] as well as some minor components present in olive wood extracts have been reported by us [22]. The secoiridoids oleuropein and ligustroside are among the main components. Other compounds present in olive wood are the lignan (+)-cycloolivil, the phenolic alcohol hydroxytyrosol, and several secoiridoids related to oleuropein, such as (7′′S)-7′′-hydroxyoleuropein or oleuropein 3′-O-β-D-glucoside. Moreover, the human platelet antiaggregant properties of two olive wood components, oleuropein and (+)-cycloolivil, have been evaluated [23]. The cultivar Picual was selected for these studies since it is one of the most important Spanish olive varieties for oil extraction,
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