Process for the preparation of a compound of formula I: wherein R1, R2, R3, R4, R5 and R6 have the meanings as defined in the specification, comprising the step of reacting an acrylic acid chloride compound of formula II': wherein R1, R2, R3, R4, R5 and R6 have the meanings as defined in the specification, with aqueous hydroxylamine and optionally converting the resulting compound into an acid addition salt thereof.
