R5 is hydrogen or alkyl of 1 to 8 carbon atoms; R6 is hydrogen, alkyl of 1 to 8 carbon atoms, benzyl, or halo; provided that R6 is other than hydrogen if X and Y are C???O and (i) each of R1, R2, R3, and R4 is fluoro or (ii) one of R1, R2, R3, or R4 is amino; and (b) the acid addition salts of said compounds which contain a nitrogen atom capable of being protonated.