Supervisors info:
Προκόπης Μαγιάτης, Αναπληρωτής Καθηγητής, Τμήμα Φαρμακευτικής ΕΚΠΑ,
Αθανάσιο Γκιμήσης Καθηγητής Τμήμα Φαρμακευτικής ΕΚΠΑ,
Νεκτάριο Αληγιάννης Αναπλ. Καθηγητής Τμήμα Φαρμακευτικής ΕΚΠΑ
Summary:
Based on the results of previous postgraduate research (Karkoula, 2013, and Tsolakou, 2019) the present study aimed to synthesize three acetal analogs consisting of the valuable secoiridoids, which are found in high quantities in extra virgin olive oil, oleocanthal and oleacein, attached to different 1,2-diol substrates. The concerning problems to be resolved were first the unwanted loss of a large percentage of dialdehydes from olive oils that had been oxidized, while they were initially rich in them. In addition, during the isolation of dialdehydes by column chromatography with silica gel, a large percentage of the dialdehydes has getting lost during their collection, meaning that only a small portion of them was recovered. Also, it was examined in which ways oleocanthal and oleacein do interact with propylene glycol and glycerol, to conclude the appropriate way of administering these dialdehydes as active substances in pharmaceutical preparations with different excipients, referring to/meaning propylene glycol and glycerol respectively.
In the synthetic part, the first substrate was a very lipophilic 1,2-diol, erythro-9,10-dihydroxystearic acid, while the other two were smaller hydrophilic molecules, propylene glycol and glycerol molecules. Subsequently, the ways of deprotection of the corresponding acetals were studied, by using a lot of different reagents and conditions. The first acetal OLC-DHSA was synthesized with a relatively high heating at 80oC, with Amberlyst 15 resin in two hours while the cleavage to its original form was accomplished by adding THF/H2O 2:8, after 90 minutes reaction time. In contrast, oleocanthal and oleacein acetals with propylene glycol and glycerol were synthesized almost immediately in pure form upon contact, with optimal conditions being heating at 50oC for three hours, prolonged contact for one day, or addition of Amberlyst 15 resin and heating at 60oC for 90 minutes. On the other hand, when the dialdehydes are in extracts or in the olive oil itself, they do not react like their pure forms do, except by only heating to 60 oC under acidic conditions (Amberlyst 15 resin), for 90 minutes. Analogs of oleocanthal and oleacein acetals with propylene glycol and glycerol can be completely deprotected in the presence of water by the effect of an ideally strong acid in excess, such as pTsOH or TFA and an extended reaction time of one day. All products were identified by one and two dimensional NMR spectra.
The following conclusions are drawn from this work: (1) The confirmation of the binding model of oleocanthal with lipophilic diols resulting from the oxidation of olive oil. Specifically, dialdehydes such as oleocanthal, react with the lipophilic 1,2-diol forming the acetal analog, which is very lipophilic and so its recovery is impossible with standard extraction methods. (2) Synthesis of acetals with propylene glycol and glycerol is rapid and direct, meaning they do form quite easily. After the acetal was synthesized, the recovery rate of OLC-PG acetal and free oleocanthal were compared by using column chromatography and silica, introducing equal amounts and of the same purity. The recovery rate of OLC-PG acetal was 50% higher than that of free oleocanthal. Therefore, the acetal forms of dialdehydes with propylene glycol can be used for more efficient and higher recovery by column chromatography and silica gel. (3) Acetal analogs with glycerol and propylene glycol are difficult to be deprotected compared to the first lipophilic acetal derivative, with ideal conditions being an excess in a strong acid and prolonged reaction time. Under these conditions, oleocanthal and oleacein can return to their original free form, allowing their higher recovery. (4) Based on the above data, we conclude that oleocanthal and oleacein dialdehydes are formed immediately when they come into contact with glycerol and propylene glycol, while at the same time when they are converted into acetals, they cannot be deprotected and return to their original form. This suggests that oleocanthal and oleacein cannot be used as active substances in pharmaceutical preparations that include propylene glycol and glycerol as excipients respectively, because react with the dialdehydes, forming different molecules than from the active substances listed on the product itself.
Keywords:
lipophilic acetal, acetals secoiridoids olive oil, synthesis, deprotection, extraction, chromatography
