prov:value
| - preferably each R1, R2, R3, R4 and R5 is a hydrogen atom or a linear or branched, cyclic or acyclic, C1-C40-alkyl, C2-C40 alkenyl, C2-C40 alkynyl, C6-C40-aryl, C7-C40-alkylaryl or C7-C40-arylalkyl radical, optionally containing one or more heteroatoms belonging to groups 13-17 of the Periodic Table of the Elements; or two adjacent R1, R2, R3, R4 and R5 form one or more 3-7 membered ring optional c
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