The present disclosure relates to a process for the preparation of a compound of formula (I) wherein R is hydrogen, a formyl group, a straight, branched or cyclic C1-C6 alkylcarbonyl group or a phenylcarbonyl group, or a salt thereof, characterized by the steps of reacting a compound of formula (II) with a mixture of Grignard initiator and Mg in a solvent to form a Grignard reagent, reacting the Grignard reagent with paraformaldehyde or trioxane to obtain a compound of formula (III) and then further reacting the compound of formula (III) in a known manner to obtain a compound of formula (I) and optionally salt formation.