Disclosed are compounds of the formula (I): ##STR1## wherein R.sup.3 is selected from the group consisting of C.sub.1-10 alkyl, C.sub.1-10 alkenyl, C.sub.3-10 cycloalkyl, C.sub.4-10 cycloalkylalkyl or C.sub.3-10 cycloalkenyl, wherein said alkyl, alkenyl, cycloalkyl, cycloalkylalkyl or cycloalkenyl is optionally substituted in one position with hydroxy; or benzyl, wherein said benzyl is optionally substituted in one or two positions with halogen, alkoxy, cycloalkoxy or polycycloalkyl, and wherein said alkyl moiety of said alkoxy or cycloalkoxy substituent is optionally substituted in one position with hydroxy; R.sup.8 is selected from the group consisting of hydrogen, C.sub.1-10 alkyl, C.sub.1-10 alkenyl, C.sub.3-10 cycloalkyl, C.sub.4-10 cycloalkylalkyl or C.sub.3-10 cycloalkenyl, wherein said alkyl, alkenyl, cycloalkyl, cycloalkylalkyl or cycloalkenyl is optionally substituted in one position with hydroxy; or benzyl, wherein said benzyl is optionally substituted in one or two positions with halogen, alkoxy, cycloalkoxy or polycycloalkyl, and wherein said alkyl moiety of said alkoxy or cycloalkoxy is optionally substituted in one position with hydroxy; R.sup.6a and R.sup.6b are independently selected from the group consisting of hydrogen, C.sub.1-10 alkyl, C.sub.1-10 alkenyl, C.sub.3-10 cycloalkyl, C.sub.4-10 cycloalkylalkyl, or C.sub.3-10 cycloalkenyl, wherein said alkyl, alkenyl, cycloalkyl, cycloalkylalkyl or cycloalkenyl is optionally substituted in one position with hydroxy; and pharmaceutically acceptable salts thereof.