| http://www.w3.org/ns/prov#value | - A process for producing a compound of formula I represented by the formulae ##STR27##wherein each R.sub.1 is independently an alkyl group provided that at least one R.sub.1 is an alkyl group containing at least two carbon atoms; and each R.sub.2 is independently selected from the group consisting of H, acyl and alkyl having atleast two carbon atoms or R.sub.2 can be methyl in position 4 when R.sub
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