alkyl in a preferred embodiment the present invention is concerned with compounds of formula i wherein r 1 r 2 r 3 and r 4 independently of each other represent hydrogen halogen perhalomethyl hydroxy cyano or c 1 7 alkyl c 4 7 alkenynyl c 2 7 alkenyl c 2 7 alkynyl c 1 7 alkoxy aryl aryloxy aralkyl aralkoxy heterocyclyl heteroaryl heteroaralkyl heteroaryloxy heteroaralkoxy acyl acyloxy hydroxyc 1 7 alkyl amino acylamino c 1 7 alkyl amino arylamino aralkylamino aminoc 1 7 alkyl c 1 7 alkoxycarbonyl aryloxycarbonyl aralkoxycarbonyl c 1 7 alkoxyc 1 7 alkyl aryloxyc 1 7 alkyl aralkoxyc 1 7 alkyl c 1 7 alkylthio thioc 1 7 alkyl c 1 7 alkoxycarbonylamino aryloxycarbonylamino aralkoxycarbonylamino u2014 cor 11 or u2014 so 2 r 12 wherein r 11 and r 12 independently of each other are selected from hydroxy perhalomethyl c 1 6 alkoxy or amino optionally substituted with one or more c 1 6 alkyl perhalomethyl or aryl optionally substituted with one or more halogen perhalomethyl hydroxy nitro or cyano or r 1 and r 2 r 2 and r 3 and or r 3 and r 4 may form a cyclic ring containing from 5 to 7 carbon atoms optionally substituted.