prov:value
| - Preferably, R.sup.1 represents a hydrogen atom or analkyl group containing from 1 to 4 carbon atoms, i.e. methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl or tert-butyl; most preferably R.sup.1 represents a hydrogen atom or a methyl group.Examples of the group --R.sup.2 --R.sup.3 are methyl, ethyl, 1-methylethyl, propyl, 1-methylpropyl, 2-methylpropyl, 1-ethylpropyl, butyl, 1-methylbut
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