prov:value
| - Examples of X1 and X2 include an acyl group such as acetyl, ethylcarbonyl, propylcarbonyl, butylcarbonyl, chloroacetyl, dichloroacetyl, trichloroacetyl, bromo-acetyl, dibromoacetyl, tribromoacetyl, fluoromoacetyl, difluoromoacetyl, trifluoromoacetyl, phenylacetyl, tolylacetyl, methoxyphenylacetyl, naphthylacetyl, benzoyl, methylbenzoyl, bromobenzoyl, p-(2-benzoxazole)-benzoyl, p-(2-benzimidazole)b
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