Abstract: A compound of general formula I wherein A is selected from the group of an unbranched or branched alky group with 1 to 12 carbon atoms, an —R1—O—R2 group, an —R1—Si(R3R4R5) group, an —R1—O—Si(R3R4R5) group, a —C(O)—O—R9—Si(R3R4R5) group, a —CH(O—R6)(O—R7) group, an —R1—CH(O—R6)(O—R7) group, or an —R1—O—C(O)—O—R8 group; SG is a protective group; R1 is a divalent hydrocarbon residue with 1 to 12 carbon atoms; R2 is a monovalent hydrocarbon residue with 1 to 12 carbon atoms; R3, R4 and R5 each independently are a monovalent hydrocarbon residue with 1 to 12 carbon atoms; R6 and R7 each independently are a monovalent hydrocarbon residue with 1 to 12 carbon atoms; R8 is a monovalent hydrocarbon residue with 1 to 12 carbon atoms; and R9 is a divalent hydrocarbon residue with 1 to 12 carbon atoms.