It is particularly preferred if the 2???- or 3???-position is protected by a protective group which is base-labile or can be removed with metal catalysis, preferably by an acyl group, in particular by an acetyl, benzoyl, nitrobenzoyl and/or methoxybenzoyl group, and/or the 4???-position is protected by an acid- or base-labile protective group, preferably by a trityl and/or Fmoc group, in particula