A method for producing a compound represented by the formulae ##STR25##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.2 is acyl in