| http://www.w3.org/ns/prov#value | - In the above specific embodiments, those wherein R1 and R3 are a methyl group and R2 and R4 are an isobutyl group in the general formula [2???] and R6 to R9 are a methyl group or three of R6 to R9 are a methyl group and the rest is a 2-hydroxyethyl group in the general formula [5???] are particularly preferable.
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