provided that:(i) if R6 is other than H, then neither R3 nor R4 can be H;(ii) if R1 and R2 are methyl, and R4 is H, then R3 cannot be NHR5 or N(R5)2; and(iii) if R1 and R2 are methyl, and R4 is H, then R3 cannot be imidazole, substituted imidazole, or an imidazole derivative.