The invention provides compounds of the formula: or a pharmaceutically acceptable salt thereof, where m, n, R.sup.1, R.sup.2, R.sup.3 R.sup.4, R.sup.5, R.sup.6, R.sup.7 are those defined herein. The invention also provides pharmaceutical compositions comprising a compound of the invention, methods for using compounds and/or pharmaceutical compositions of the invention, and methods for synthesizing compounds of the invention. ##STR00001##