A pharmaceutical composition having as an essential ingredient a compound of the formula ##STR1## wherein one to three of R.sup.1, R.sup.2 and R.sup.3 represent an alkenyl group having from 4 to 8 carbon atoms, the other ones are alkyl having from 1 to 12 carbon atoms, and R.sup.1 may also be hydrogen, the compounds themselves and a process for preparing them.