. . . "Of particular interest, are compounds represented by graphic formula IA, wherein R1 is a C1 -C4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, secondary butyl, isobutyl and tertiary butyl; R2 and R3 are each methyl, ethyl or phenyl; R4 is selected from C1 -C4 alkyl, C1 -C4 alkoxy, chloro, fluoro or cyano; R5 is selected from hydrogen, fluoro, trifluoromethyl and C1 -C5 alkyl; R6 is se" .