| http://www.w3.org/ns/prov#value | - A process for the preparation of a novel compound of formula I according to claim 1 or a salt thereof, which comprises reacting a compound of formula II, as racemate or in chiral form ##STR6## wherein R3 and R4 are hydrogen, or at least one of the two substituents R3 and R4 is a protective group or both substituents R3 and R4 are a protective group, with a compound of formula III ##STR7## in free
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