. . . " amino group, a hydroxyl group, or a cyano group; or R2 and R3 together form a group of the formula ???O(CH2)nO??? wherein n represents the integer 1 or 2; R4 is F, a C2-3 alkenyl group, a C2-3 alkynyl group, or a C1-3 alkoxyl group; R5 is a C1-10 alkyl group, or a propargyl group; and R6, R7 and R8 are independently a C1-10 alkyl group, a C2-10 alkenyl group, a C2-10 alkynyl group, an aryl group" .