The present invention provides compounds of the Formula I: ##STR00001## wherein R.sup.1 is alkyl, cycloalkyl, cycloallkylalkyl, or --CH.sub.2-alkenyl, X.sup.1 is O, NH, N(alkyl), S or --C(.dbd.O), Z is N or CH; and R.sup.2 and R.sup.3 are as defined herein, pharmaceutical compositions comprising same, and methods for their use.