R1 and R2 are identical or different and represent hydrogen, C1 -C6 -alkyl, C5 -C7 -cycloalkyl, phenyl which is optionally substituted (disubstituted or especially monosubstituted) by C1 -C4 -alkyl, halogen, (chlorine or bromine) and/or C1 -C4 -alkoxy, or C7 -C9 -aralkyl, whose aryl radical is optionally substituted (monosubstituted or disubstituted, especially monosubstituted) by C1 -C4 -alkyl, h