R is hydrogen, halogen, preferably chlorine, straight-chain or branched alkyl having 1 to 12 carbon atoms, aralkyl, e.g. benzyl or phenethyl, ???NO2, ???NH2 or NHR111, where R111 is alkyl having 1 to 8 carbon atoms, unsubstituted or C1-C4-alkyl-substituted phenyl or ???Si(CH3)3.