When R.sup.a and/or R.sup.b represents an aralkyl group, it may suitably be one having 7 or 8 carbon atoms, e.g. a benzyl or phenethyl group.R.sup.c and R.sup.d in formula (I) each represents a lower alkyl group and suitably one having from 1 to 5 carbon atoms, e.g. methyl, ethyl, propyl, butyl, isobutyl, pentyl or isopentyl.When R.sup.e and/or R.sup.f represents an alkyl group, it may suitably be