Process for the preparation of hydroxytyrosol

A process for preparing hydroxytyrosol, wherein a compound where X is CH2OH or CH2OM (M=Li, Na, K, Mg, Ca), R1 and R2 are identical or different and are C1-C8 alkyl radical, benzyl radical, alkyl- or halogen-substituted benzyl radical or arylalkyl radical, where R1 and R2 can also be linked via to give a ring, R3, R4, R5 and R6 are identical or different and are hydrogen or C1-C6 alkyl radical, aryl radical, alkyl-substituted aryl radical, where R5 and R6 can also be linked via -(CH2)4-, -(CH2)5- or -(CH2)6- to give a ring, is reacted with an aluminum compound AlR7R8R9 (2), where R7, R8 and R9 are identical or different and are H or C1-C8 alkyl radical, and then an aqueous solution of a hydroxycarboxylic acid is added to form a clear homogeneous acidic solution with a pH