prov:value
| - C.sub.1 -C.sub.4 alkyl R.sub.13, R'.sub.14 and R.sub.15 independently of one another are, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl or isobutyl, preferably methyl or ethyl, and in particular methyl.
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