The present invention can utilize compounds of the general structural formula: ##STR113## and the pharmaceutically acceptable salts thereof, wherein: A is: ##STR114## except when R2 equals H, Y equals O, Z equals N, and there is a 5?? H, R6 and R3 cannot be independently selected from H, C1-8 alkyl, C3-6 cycloalkyl, phenyl or when R6 and R3 are taken together with the adjacent N to form a 5-6 memb