Abstract: The compounds of the formula: ##STR1## wherein R.sub.1 and R.sub.2, independent from each other, are selected from the following: C.sub.1-6 perfluoroalkoxy, C.sub.1-6 perfluoroalkyl, C.sub.1-6 alkyl, C.sub.1-6 alkoxy, hydroxyl, C.sub.2-6 alkoxycarbonyl, nitro, cyano, halogeno, C.sub.1-6 alkylsulfonamido, C.sub.1-6 perfluoroalkylsulfonamido, amino, C.sub.2-6 alkanoylamino, C.sub.2-6 perfluoroalkanoylamino, C.sub.1-12 mono- or di-alkylamino, C.sub.1-6 alkylsulfonyl, C.sub.6-12 arylsulfonyl, carboxyl, C.sub.2-12 mono- or di-alkylcarbamoyl, or hydrogen; a and b together form an --O-- linkage or a direct bond; R.sub.3 and R.sub.4, independent from each other, are C.sub.1-6 alkyl when a and b form an --O-- linkage or, H or C.sub.1-6 alkyl when a and b form a direct bond; either R.sub.5 is hydrogen, hydroxyl, C.sub.2-6 alkanoyloxy, C.sub.7-12 aroyloxy, carbamoyloxy, formyloxy, C.sub.2-6 alkoxycarbonyloxy, mono or di C.sub.2-12 alkylcarbamoyloxy, and R.sub.6 is hydrogen, or R.sub.5 and R.sub.6 together are a bond; R.