Furthermore, in addition to phenylalkyl having 1 to 4 carbon atoms in the alkyl moiety, for example benzyl, R1 can also be cycloalkyl having 5 to 8 carbon atoms, for example cyclopentyl, cyclohexyl and cyclooctyl, or a radical of the formula in which R4 and R5 independently of one another are alkyl having in each case 1 to 5 carbon atoms, in particular methyl, or R4, together with the CnH2n+1???m