prov:value
| - R4 : alkyl of 1 to 4 carbon atoms, e.g. methyl, ethyl, propyl and butyl and substituted alkyl of 1 to 4 carbon atoms, e.g. methoxymethyl, ??-methylthioethyl, benzyloxymethyl, cyanomethyl, carbomethoxymethyl, carbethoxymethyl, p-bromobenzoylmethyl, benzyl and substituted benzyl, as specified for R3, e.g. 3,4-dimethoxybenzyl, p-methoxybenzyl, p-nitrobenzyl, o- and p-chlorobenzyl, p-bromobenzyl, 3,4-
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