The invention can be used to double the yield of a Krapcho reaction by means of which oleacein is obtained from oleuropein, present in olive leaves. Oleacein, a dialdehyde secoiridoid present in virgin and extra virgin olive oil, has very interesting biological properties, such as anti-inflammatory and antiasthmatic properties. For the first time, the Krapcho reaction has been extended to the conversion of monoaldehyde aglycones of oleuropein and ligstroside, present in phenolic extracts of EVOO, into the corresponding oleocanthal and oleacein dialdehyde derivatives, thereby providing a very effective method for enriching phenolic extracts with dialdehyde secoiridoids, with high added value. The invention also relates to a new method for stabilising and increasing the bioavailability of oleacein and oleocanthal using acetalation and acetylation reactions.In terms of production, the invention falls within the scope of the agriculture, food, chemical and pharmaceutical fields.The invention is suitable for use in the activity sector of the pharmaceutical and food industries.