Biological Properties of Hydroxytyrosol and Its Derivatives

Polyphenols are a wide family of compounds found in fruits and vegetables, wine, tea, cocoa, and extra-virgin olive oil, which exhibit strong antioxidant activity by scavenging different families of Reactive Oxygen Species (ROS). One of the most effective members of the polyphenol family in terms of free radical scavenging is hydroxytyrosol, 2-(3,4dihydroxyphenyl)ethanol (Fernández-Bolaños et al., 2008), a simple phenol found predominantly in olive tree (Olea europaea).

Hydroxytyrosol (HT) can be found in leaves and fruits of olive, extra virgin olive oil and it is specially abundant in olive oil mill wastewaters from where it can be recovered (Fernández-Bolaños et al., 2008;Sabatini, 2010). Hydroxytyrosol is a metabolite of oleuropein ( Fig. 1), another major phenolic component of olive products; they both give to extra-virgin olive oil its bitter and pungent taste (Omar, 2010a). Hydroxytyrosol shows a broad spectrum of biological properties due to its strong antioxidant and radical-scavenging properties. More active than antioxidant vitamins and synthetic antioxidants, hydroxytyrosol exerts its antioxidant activity by transforming itself into a catechol quinone (Rietjens, 2007).

Biological activity of hydroxytyrosol
Historically, olive tree leaves were used for traditional therapy by ancient civilizations. Extracts from olive tree leaves were found to have a positive effect on hypertension by the The high antioxidant efficiency of HT, attributed to the presence of the o-dihydroxyphenyl moiety, is due to its high capacity for free radical scavenging during the oxidation process and to its reducing power on Fe 3+ (Torres de Pinedo et al., 2007). The antioxidant properties of the o-diphenols are associated with their ability to form intramolecular hydrogen bonds between the hydroxyl group and the phenoxy radical (Visioli et al., 1998); therefore, the catechol avoids the chain propagation by donating a hydrogen radical to alkylperoxyl radicals (ROO · ) formed in the initiation step of lipid oxidation (Scheme 1). Oxidation of low-density lipoproteins (LDLs) is a lipid peroxidation chain reaction, which is initiated by free radicals. It has been shown that hydroxytyrosol can inhibit LDL oxidation efficiently due to its capacity to scavenge peroxyl radicals (Arouma et al., 1998;Turner et al., 2005). Hydroxytyrosol reduces oxidation of the low-density lipoproteins carrying cholesterol (LDL-C), which is a critical step in the development of atherosclerosis and other cardiovascular diseases (Gonzalez-Santiago et al., 2010;Vázquez-Velasco et al., 2011); hydroxytyrosol has also a potential protective effect against oxidative stress induced by tertbutyl hydroperoxide (Goya et al., 2007).
It has been reported that hydroxytyrosol enhances the lipid profile and antioxidant status preventing the development of atherosclerosis. This compound may also reduce the expression of vascular cell adhesion molecules (Carluccio et al., 2007) and inhibit platelet aggregation in rats (González- Correa et al., 2008a) and hypercholesterolaemia in humans (Ruano et al., 2007).

Anticancer activity
Numerous studies about the relationship between olive oil consumption and cancer prevention have been carried out (Pérez-Jiménez et al., 2005). Antioxidant compounds supplied in the diet can reduce the risk of cancer due to the fact that they can minimize DNA damage, lipid peroxidation and the amount of ROS generated (Omar, 2010a;Hillestrom, 2006;Manna, 2005).
Pre-treatment of HepG2 cells with hydroxytyrosol prevented cell damage, what could be due to the fact that hydroxytyrosol may prepare the antioxidant defense system of the cell to face oxidative stress conditions (Goya et al., 2007(Goya et al., , 2010.

Osteoporosis
Hydroxytyrosol may have critical effects on the formation and maintenance of bone, and could be used as an effective remedy in the treatment of osteoporosis symptoms, as it can stimulate the deposition of calcium and inhibit the formation of multinucleated osteoclasts in a dose-dependent manner. HT also suppressed the bone loss of spongy bone in femurs of ovariectomized mice (Hagiwara et al., 2011).

Antimicrobial activity
Antimicrobial activity of oleuropein, tyrosol and hydroxytyrosol has been studied in vitro against bacteria, viruses and protozoa (Bisignano et al., 1999).
The in vitro antimycoplasmal activity of HT has been investigated, concluding that this compound might be considered as an antimicrobial agent for treating human infections caused by bacterial strains or casual agents of intestinal or respiratory tract (Furneri et al., 2004).
It has been shown that polyphenols from olive oil are powerful anti-Helicobacter Pylori compounds in vitro , a bacteria linked to a majority of peptic ulcers and to some types of gastric cancer.

Antiinflammatory activity
Inflammation and its consequences play a crucial role in the development of atherosclerosis and cardiovascular diseases. Polyphenols have been shown to decrease the production of inflammatory markers, such as leukotriene B4, in several systems (Biesalski, 2007).
The effect of hydroxytyrosol on platelet function has been tested. Hydroxytyrosol was proven to inhibit the chemically induced aggregation, the accumulation of the proaggregant agent thromboxane in human serum, the production of the pro-inflammatory molecules leukotrienes and the activity of arachidonate lipoxygenase (Visioli et al., 2002).

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integrase. Thus, these agents provide an advantage over other antiviral therapies in which both, viral entry and integration, are inhibited (Lee-Huang et al., 2007a, 2007b, 2009).
HT and its derivatives are also useful, when applied topically, as microbicide for preventing HIV-infection, as well as other sexually transmitted diseases caused by fungi, bacteria or viruses (Gómez-Acebo et al., 2011). Furthermore, it has been reported that hydroxytyrosol inactivated influenza A viruses, suggesting that the mechanism of the antiviral effect of HT might require the presence of a viral envelope (Yamada et al., 2009).

Hydroxytyrosol as an antinitrosating agent
The antinitrosating properties of hydroxytyrosol and other plant polyphenols of dietary relevance have been investigated (De Lucia et al., 2008). It has been shown that HT reacts with sodium nitrite at pH 3 to give 2-nitrohydroxytyrosol, supporting a protective role of HT as an efficient scavenger of nitrosating species (Fig. 2).

Lipophilic hydroxytyrosol esters
Many different hydroxytyrosol lipophilic analogues occur naturally in olive fruit and in virgin olive oil. The amount of these compounds is related to olive variety and ripeness, climate, location, type of crushing machine and oil extraction procedures. As an example, the concentration of hydroxytyrosyl acetate is similar to that of HT in some olive oil varieties such as Arbequina, twice as high in the Picual variety, and between one third and one fourth in the Manzanilla and Hojiblanca oils .
Due to the limited solubility of HT in lipid media, the search for new lipophilic hydroxytyrosol esters with enhanced properties is of great interest, both in food industry and in medicine. Studies on olive polyphenols have shown the importance of the lipophilicity of the antioxidants on the cell uptake and membrane crossing, and on the substrate to be protected (membrane constituents or LDL), (Grasso et al., 2007). These facts explain the efforts made in the development of new synthetic analogues with increased lipophilicity.

Synthetic approaches
Phenolic acids, such as caffeic acid, have been esterified with good chemoselectivity in the presence of strong protic acids (Fischer esterification), but the severe reaction conditions together with the large excess of alcohol required make this strategy of limited applicability (Burke et al., 1995). Under basic catalysis, phenols can be easily deprotonated, so the esterification of phenolic alcohols and phenolic acids via acyl nucleophilic substitution requires previous protection of the phenolic hydroxyl groups, due to the competition between aliphatic and phenolic hydroxyl groups (Appendino et al., 2002;Gambacorta et al., 2007).

Protection of phenolic hydroxyl groups
As an example, benzyl groups have been used to carry out the HT esterification under basic conditions, followed by catalytic hydrogenation to remove the protective groups (Gordon et al., 2001), as depicted in Scheme 2.

Scheme 2. Synthesis of hydroxytyrosyl acetate via benzylation of phenolic hydroxyls
A two-step procedure involving the reaction of methyl orthoformate-protected hydroxytyrosol with acetyl chloride, and hydrolytic deprotection in phosphate buffer under very mild conditions (pH=7.2) to get hydroxytyrosyl acetate (87% overall yield) (Scheme 3) was also described as a successful procedure for the preparation of HT-derived esters (Gambacorta et al., 2007). The key synthetic orthoester intermediate was also used for the synthesis of HT upon reduction with LiAlH 4 and acidic deprotection. In order to overcome the problems associated to the protection and deprotection steps of the phenolic hydroxyl groups, different methods for the preparation of hydroxytyrosyl esters by reaction of hydroxytyrosol with various acylating agents have been described, such as esterification with free acids (Appendino et al., 2002), transesterification with methyl or ethyl esters (Alcudia et al., 2004;Trujillo et al., 2006), acyl chlorides (Torregiani et al., 2005) and the use of enzymatic methodologies (Grasso et al., 2007;Mateos et al., 2008;Torres de Pinedo et al., 2005;Buisman, 1998).

Acid catalyzed transesterification
HT transesterification using methyl or ethyl esters and p-toluenesulfonic acid as catalyst has been described as a method without the need of protection of the aromatic hydroxyl groups due to its total chemoselectivity (Alcudia et al., 2004;Trujillo et al., 2006). This method involves heating a solution solution of hydroxytyrosol in the corresponding ethyl or methyl ester, containing a catalytic amount of p-toluenesulfonic acid (Scheme 4). This protocol has been optimized for HT acetate (86%), and also for longer aliphatic chains like hydroxytyrosyl butyrate, laureate, palmitate, stearate, oleate and linoleate, obtained in acceptable to good yields (62-76%) (Mateos et al., 2008).
Scheme 4. General procedure of acid-catalyzed transesterification

Acylation of polyphenolic alcohols with the couple CeCl 3 -RCOCl
Cerium (III) chloride has been reported to be an efficient promoter for the chemoselective esterification of unprotected polyphenolic alcohols with acyl halides as acyl donors, thereby making it possible to avoid the protection of phenolic hydroxyl groups and providing polyphenolic esters of interest (Torregiani, 2005). This reaction is one example of the socalled Lewis acid catalysis by lanthanide salts (Ishihara et al., 1995). The reaction presumably involves the formation of an electrophilic Lewis adduct between acyl chlorides and cerium (III) chloride, which is quenched by the more nucleophilic aliphatic hydroxyl group of the substrate, with formation of the ester, and regeneration of the lanthanide promoter. The yields obtained are acceptable for HT using nonanoyl and oleoyl chlorides (53 and 52%), respectively (Scheme 5).
Scheme 5. Acylation of hydroxytyrosol with acyl chlorides and Ce(III)

Esterification with free acids: Mitsunobu esterification
The Mitsunobu reaction has been also applied to the chemoselective esterification of phenolic acids with phenolic alcohols (Appendino et al., 2002) as demonstrated by the condensation of hydroxytyrosol with gallic acid, and of vanillyl alcohol with caffeic acid in a one step procedure with 48% and 50% yields, respectively. The esterification is carried out using DIAD (diisopropyl azodicarboxylate) and TPP (triphenylphosphine) in THF (Scheme 6). The removal of byproducts arising during the Mitsunobu reaction, a major problem of this type of reactions, could be solved by gel-permeation chromatography on Sephadex LH-20. Scheme 6. Mitsunobu esterification of hydroxytyrosol and vanillyl alcohol

Syntheses of hydroxytyrosol esters from tyrosol and homovanillyl alcohol
The syntheses previously described in the previous sections had all in common hydroxytyrosol as a precursor of its esters, but some efforts have also been done to get hydroxytyrosyl esters starting from different and cheaper reagents. In this context, the syntheses of hydroxytyrosol esters from tyrosol and homovanillyl alcohol have been proposed (Bernini et al., 2008b). This procedure involves the selective esterification of tyrosol and homovanillyl alcohol with acyl chlorides in dicholoromethane as solvent, to give tyrosyl and homovanillyl acetates in 90% and quantitative yields, respectively, by using only a little excess of acetyl chloride in dichloromethane without any catalysts. The authors suggested acid catalysed acylation due to traces of hydrochloric acid derived from the hydrolysis of the acetyl chloride under the experimental conditions. A similar selectivity was observed by using several saturated or unsaturated acyl chlorides with longer chains such as hexanoyl, palmitoyl, oleoyl and linoleoyl chlorides.
The subsequent oxidation with 2-iodoxybenzoic acid (IBX) or Dess-Martin periodinane reagent (DMP) and in situ reduction with sodium dithionite (Na 2 S 2 O 4 ) of tyrosyl and homovanillyl esters led to the corresponding hydroxytyrosol derivatives. In general, the oxidation of tyrosol derivatives proceeded with higher yields (92-77%) compared to those of homovanillyl derivatives (88-58%). The use of DMP gave similar results to those obtained with IBX. The procedure of oxidation/reduction with IBX/Na 2 S 2 O 4 to obtain the different esters is under protection of two patents (Bernini et al. 2007(Bernini et al. , 2008c).
Scheme 7. Synthesis of hydroxytyrosol esters from tyrosol and homovanillyl alcohol

Lipase-catalyzed transesterification
The use of enzymes, like lipases, as catalysts in non-aqueous solvents to prepare lipophilic derivatives directly from HT has been widely described in the last few years (Grasso et al., 2007;Torres de Pinedo et al., 2005;Mateos et al., 2008;Buisman et al., 1998). This procedure avoids the use of toxic reagents and allows mild reaction conditions.
The esterification of phenols with carboxylic fatty acids and lipases as biocatalysts was firstly investigated by Buisman et al., (1998), using hydroxytyrosol, octanoic acid in hexane, and immobilized lipases from Candida antartica (CAL-B). Furthermore, a strong dependence of the yield on the solvent used was observed; so, in diethyl ether a conversion of 85% was obtained within 15 hours (35 ºC), while conversions of roughly 20% were found in the case of solvents like chloroform, dichloromethane or THF. Yields of 70-80% were observed using n-pentane and n-hexane, in spite of the poor solubility of HT in such solvents.
Different enzymes have been tested on hydroxytyrosol (Grasso et al., 2007) including lipases from A. niger, C. cylindracea, M. javanicus, P. cepacia, M. miehei, C. viscosum, P. fluorescens, R. arrhizus, R. niveus, C. antarctica, porcine pancreas and wheat germ, using vinyl acetate as reagent and tert-butyl methyl ether as solvent. The best results were obtained with C. antarctica in terms of short reaction time, chemioselective conversion and good yield. C. antarctica lipase (CAL) was selected for acylation of hydroxytyrosol and homovanillic alcohol with vinyl esters of different acyl chains on a preparative scale, as shown in Table 1. The use of C. antarctica with increasing alkyl chain length required longer reaction times. The homovanillyl alcohol and its esters were found to exhibit scarce effectiveness both as radical scavengers and antioxidant agents.
Transesterification of HT with ethyl saturated, mono-and poly-unsaturated fatty acid esters, catalized by Novozym 435 (immobilized C. antarctica lipase B), in vacuum under solventless conditions, has been successfully developed (Torres de Pinedo et al. 2005). This procedure gave hydroxytyrosyl esters in 59-98% yield for the saturated fatty acid esters, and 32-97% yield for the mono-and poly-unsaturated fatty acid esters.

Antioxidant activity
The antioxidant activity of hydroxytyrosyl esters has been measured with different methods, including DPPH (1,1-diphenyl-2-picrylhydrazyl radical), ABTS (2,2'-azino-bis(3etilbenzotiazolin-6-sulfonic acid), FRAP (ferric reducing antioxidant power) and Rancimat (Mateos et al., 2008;Gordon et al., 2001;Bouallagui et al., 2011b). The Rancimat test is a method commonly used to evaluate the antioxidant power in lipophilic food matrices, such as oils and fats, while the ABTS and FRAP assays are used for the evaluation of antioxidant activity in hydrophilic medium; the ABTS assay evaluating the radical-scavenging capacity, and the FRAP method determining the reducing activity.
The Rancimat test revealed a lower activity for ester derivatives compared to HT, in agreement with the so-called polar paradox, according to which hydrophilic antioxidants are more effective in less polar media, such as bulk oils, whereas lipophilic antioxidants are more effective in relatively more polar media, such as in oil-in-water emulsions or liposomes (Frankel et al., 1994;Shahidi & Zhong, 2011 (Grasso et al., 2007) The order of the scavenging activities toward the ABTS radical was hydroxytyrosyl esters ≥ -tocopherol > hydroxytyrosol > tyrosyl >tyrosyl esters  BHT. In a similar trend, comparison of FRAP values obtained for the free hydroxytyrosol and tyrosol with the corresponding esters revealed that while hydroxytyrosyl esters showed a significantly higher reducing activity than their precursor, all the tyrosyl esters showed a lower antioxidant activity than that of tyrosol. The same conclusion was obtained from DPPH assay of the radical scavenging activity (Grasso et al., 2007).
In connection with the size of the acyl chain, the reported literature seems to conclude that the antioxidant capacity of hydroxytyrosyl esters is better for medium-sized (C4C9) alkyl chains in comparison with HT, whereas further elongation of the acyl chain does not improve the antioxidant activity. This confirms that antioxidant capacity does not depend only on lipophilicity. A possible explanation could be related to the fact that the conformational freedom of the ester chain increases with the acyl chain length, and this could result in folded structures in which catechol hydroxyls are shielded (Tofani et al., 2010;Pereira-Caro et al., 2009;Medina et al., 2009).
This antioxidant activity has also been proved in biological assays, in order to check the ability of hydroxytyrosyl esters to protect proteins and lipids against oxidation caused by peroxyl radicals, using a brain homogenate as an ex vivo model (Trujillo et al., 2006) and cumene hydroperoxide to induce oxidation. The results obtained showed a protective effect in these systems, which was more effective in preventing the generation of carbonyl groups in proteins than the generation of malondialdehyde in lipid; hydroxytyrosyl linoleate showed the greatest activity. This fact proves that the introduction of a lipophilic chain in the hydroxytyrosol molecule increases both protein and lipid protection.
Dichlorodihydrofluorescein (DCF) fluorometric assay on whole cells, carried out to check the antioxidant activity of a large serie of hydroxytyrosyl esters (Tofani et al., 2010) on rat muscle cells, showed that hydroxytyrosol esters had a better antioxidant activity compared to HT due to the better penetration into the cells of the lipophilic derivatives.
Hydroxytyrosol fatty acid esters have shown a nonlinear tendency in antioxidant capacity in fish oil-in-water emulsions (Lucas et al., 2010), where a maximum of antioxidant efficiency appeared for hydroxytyrosol octanoate in a study of hydrosytyrosyl esters with alkyl chains varying from C2 to C18. These results seem to be in disagreement with the antioxidant polar paradox.

Cardiovasvular diseases
Platelet aggregation is considered one of the main events in arterial thrombosis; therefore aggregation prevention is a major goal of cardiovascular research. It has been proved that hydroxytyrosol acetate inhibits platelet aggregation induced by ADP, collagen or arachidonic acid and stimulates nitric oxide production, more efficiently than hydroxytyrosol, and as effectively as acetylsalicylic acid; the latter is the most widely used drug in the world to prevent ischaemic cardiovascular diseases because of its antiplatelet aggregating action. This conclusion has been achieved in vivo in a study of oral administration of this ester to rats (González- Correa et al., 2008b), and in vitro in both human whole blood and platelet-rich plasma (González-Correa et al., 2009).

DNA damage oxidative protection
The atypical Comet test on whole blood cells has been applied to several hydroxytyrosyl esters to check their capacity to counteract the oxidative stress caused by H 2 O 2 and the basal DNA damage.
The results obtained show that antidamaging properties on DNA of HT acetate and propanoate are comparable to those of HT, whereas the protective effect progressively decreases in the order butanoate < decanoate  estearate (Fig. 3). This behavior was not observed for the lipophilic analogues of homovanillyl alcohol which appear to be scarcely protective, indicating that o-diphenols are more effective antioxidants than simple phenols (Grasso et al., 2007).

Prevention of oxidative stress
The ability of hydroxytyrosol and its esters to prevent iron-induced oxidative stress has been studied on human cervical cells (HeLa cells) by the TBARS protocol (Bouallagui et al., 2011b). Pre-incubation of HeLa cells in the presence of 100 μM phenolic compounds led to a significant improvement of the oxidative status. In fact, thiobarbituric acid-reactive substance (TBARS) production was decreased by 30%, 36% and 38% with hydroxytyrosol, hydroxytyrosyl acetate and hydroxytyrosyl oleate, respectively.

Transport, absorption and metabolism
The study of the metabolism of hydroxytyrosol, tyrosol, and hydroxytyrosyl acetate has been carried out using human hepatoma cells (HepG2) as a model system of the human liver (Mateos et al., 2005). The results showed extensive uptake and metabolism of hydroxytyrosol and scarce metabolism of tyrosol, while hydroxytyrosyl acetate showed an interesting behavior, with formation of deacetylated hydroxytyrosol after only 2 h. Because hydroxytyrosyl acetate was stable in the culture medium, the hydroxytyrosol detected in the extracellular medium should be attributed to the action of the hepatic cells.

Neuroprotective effect of hydroxytyrosyl and hydroxytyrosol acetate
Neuroprotection exerted by HT derivatives has been investigated in rat brain slices subjected to hypoxia-reoxygenation, both in vitro and after oral administration (González- Correa et al., 2008). This study was carried out to confirm to the previously demonstrated neuroprotective effects of virgin olive oil in rats (González- Correa et al., 2007). Although the studies gave positive results in the neuroprotective activity of both HT and hydroxytyrosyl acetate, mechanisms that underlie this effect are still unknown.

Synthetic approaches
Hydroxytyrosyl alkyl ethers have been obtained ) in a three-step procedure starting from hydroxytyrosol isolated from olive oil waste waters (Scheme 8). This procedure requires first the selective protection of the aromatic hydroxyl groups via benzylation with benzyl bromine in the presence of K 2 CO 3 , and then the addition of an alkyl iodide under basic conditions, and the subsequent deprotection by catalytic hydrogenation (Pd/C) to obtain the corresponding ethers.
The yield for the alkylation step varies depending on the length of the alkyl chain; as depicted in Scheme 8, the yields decrease as the length of the alkyl chain increases, due to the reduced solubility of the corresponding long chain alkyl iodides in the solvent (DMSO).

Scheme 8. Synthesis of hydroxytyrosyl alkyl ethers by alkylation with alkyl iodides
The oxidative stability of lipid matrix in the presence of these compounds, measured by the Rancimat method, has shown that these derivatives retain the high protective capacity of free hydroxytyrosol and similar induction times, having higher induction times than butylhydroxytoluene (BHT) and -tocopherol . These results are in agreement with those obtained in the case of hydroxytyrosyl esters, covered in the previous section (Mateos et al., 2008). The antioxidant activity has been checked by the DPPH, FRAP and ABTS assays in a hydrophilic medium . The antioxidant activity of the lipophilic hydroxytyrosyl ethers was slightly lower in bulk oils and higher in hydrophilic media in comparison with their reference HT, supporting the polar paradox. The length of the alkyl chain did have a positive influence in hydrophilic medium for ethers with a short alkyl chain (methyl, ethyl, propyl), while ethers with longer alkyl chains (from butyl to octadecyl) maintained or decreased their antioxidant activity, probably due to the steric effect of the hydrocarbon chains.

Biological activity
In order to evaluate the safety and potential biological activity of these ethers, studies of their transport, absorption and metabolism in cellular and animal models have been developed (Pereira-Caro et al., 2010a, 2010b using a human hepatoma cell line (HepG2) as a model system of the human liver and human enterocyte-like Caco-2/TC7 cells, which are commonly used to characterize the intestinal absorption of a range of drugs, nutrients, and other xenobiotics.
The results showed a direct relationship between the lipophilic nature of each compound and the level of metabolization; as an example, hydroxytyrosyl butyl ether biotransformation was complete after 18 h, whereas small amounts of the others remained after the same time. Furthermore, an intestinal absorption increase was observed from methyl to n-butyl ethers.
Protective effects against oxidative stress have also been studied (Pereira-Caro et al., 2011) using HepG2 cells, the ones previously employed to assess the metabolism of the synthesized HT ethers. The results obtained show the potential to prevent cell damage induced by tert-butyl hydroperoxide (t-BuOOH) and the ability to maintain unaltered cellular redox status, partially after 2 hours of pretreatment and almost completely after 20 hours. These results are in accordance with those obtained with hydroxytyrosol (Martín et al., 2010), but they also show the relevance of the role of the lipophilic character of the phenolic compounds on their antioxidant potential against cell damage: HT methyl and ethyl ethers are less effective than HT propyl and butyl ethers.

Hydroxytyrosol-derived isochromans
Isochroman fragment is a ubiquitous scaffold that can be found in natural products, drugs and agrochemicals (Larghi & Kaufman, 2006). Access to dihydroxyisochromans derived f r o m H T c a n b e a c h i e v e d b y u s i n g t h e o x a -Pictet-Spengler reaction, by reaction of arylethanols with aldehydes, ketones or masked-carbonyl derivatives ). The reaction is highly regioselective, as intramolecular cyclization takes place mainly in the less hindered position, as it can be deduced from its reaction mechanism, shown in Scheme 9. Two of the synthetized isochromans (Fig. 4) have been detected in olive oil (Bianco et al., 2001). Hydroxytyrosol isochroman derivatives shown in Fig. 4 were effective free radical scavengers able to inhibit platelet aggregation and thromboxane release (Togna, 2003).

Hydroxytyrosol glucoronide derivatives
One of the major metabolic pathways found in vivo for dietary phenolic compounds such as hydroxytyrosol is O-conjugation via glucuronidation and sulfation. Therefore, it is of interest to study these metabolites and their biological activities.
Biocatalyzed syntheses of hydroxytyrosol and other phenolic glucuronides have been developed using porcine liver microsomes (Khymenets et al., 2006(Khymenets et al., , 2010. This type of glucuronides has also been synthesised stereoselectively  in the phenolic or aliphatic hydroxyl groups using efficient chemical method from O-partially protected hydroxytyrosol and glucuronosyl trichloroacetimidate donors (Scheme 10).
The antioxidant activities of hydroxytyrosol conjugates have been evaluated, concluding that none of these glucuronides displayed significant antioxidant activities at the concentration tested (Khymenets et al., 2010).

Hydroxytyrosol glucosides
The three isomers of hydroxytyrosol -D-glucopyranosides (Fig.5) have been reported to be present in olives (Bianco et al., 1998). The 4-glucoside (Romero et al., 2004) and the 1glucosides  have been found in table olive brines, and have been analysed as antimicrobial compounds against Lactobacillus pentosus with negative results. It has been recently shown that hydroxytyrosol 4-glucoside was the main phenolic compound in the aqueous phase of fresh alpeorujo, followed by hydroxytyrosol, and hydroxytyrosol 1glucoside.

Arylhydroxytyrosol derivatives
The synthesis of 2-arylhydroxytyrosols from 2-halohydroxytyrosol derivatives has been described (Bernini et al., 2008a). The reaction of the corresponding 2-chloro precursors via Suzuki-Miyaura cross-coupling reaction with arylboronic acids containing electrondonating, electron-withdrawing, as well as ortho substituents, yielded this family of compounds in high to excellent yields (Scheme 11).

Complexation of hydroxytyrosol with β-cyclodextrins
The complexation of hydroxytyrosol with commercially-available -cyclodextrin (  Moreover, the antioxidant activity of encapsulated HT, together with the photoprotection effect of -CD on HT, has been evaluated by scavenging of the stable DPPH radical. It has been proven that -Cyclodextrin acts as a secondary antioxidant and provides a moderate improvement of the radical scavenging activity of HT measured by the DPPH assay.
-Cyclodextrin exerts a strong photoprotection of HT upon UV irradiation, which could be deduced from the EC 50 values (Table 2). For equimolecular mixtures of HT and -CD at 1.2 mM, the observed degradation after 24 h and 48 h is similar to the degradation found for HT at the same concentration and time (entries 4 and 5) showing no protection at 24 h and only a slight protection after 48 h. However, using 1:4 mixtures of HT (1.2 mM) and -CD (4.8 mM), a remarkable reduction of the degradation rate was observed when compared with pure HT. In this way, the complexation of HT with cyclodextrins might enhance stability, improve its performance as antioxidant and extend its storage life (López-García et al., 2010

Conclusions
Hydroxytyrosol is a phenolic compound that can be isolated from olive oil mill wastewaters. The remarkable biological properties of this compound, mainly due to its strong antioxidant activity, has stimulated the synthesis of a series of derivatives, some of them are also naturally-occurring in the olive tree. Among these derivatives hydroxytyrosyl esters and ethers are of great interest, as some of them show strong antioxidant activity and improved bioavailability.