Process for the preparation of a compound of formula I in the form of a single stereoisomer in crystalline form, comprising deprotecting the amine group in a compound of formula IIa or in a mixture of a compound of formula IIa with a compound of formula IIb and isolating a compound of formula I from the reaction mixture; compounds and salts of compounds of formula I in crystalline form; pharmaceutical compositions comprising such salts; processes for the preparation of intermediates and intermediates in a process for the preparation of a compound of formula I.