Preferred alkyl substituents of B or B??? are C1-C4-alkyl, in particular C1-C2-alkyl, which is substituted by one or more substituents selected from the group consisting of ???OH and ???NR4R4???, wherein R4 and R4??? are each independently of another hydrogen or C1-C4-alkyl, preferably hydrogen, methyl or ethyl and particularly preferably hydrogen or methyl, for example ???CH2???NH2, ???CH2???N(CH