prov:value
| - R1 and R2 are identical or different and each is hydrogen, alkyl, for example of from 1 to 6 carbon atoms, cycloalkyl, eg. cyclohexyl, or aryl, eg. phenyl or naphthyl, and R3 is alkoxy, for example of from 1 to 6 carbon atoms, or R3 combines with R2 via the group --O--(CH2)n where n is from 3 to 6, preferably n is 4, to form a ring, or
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