The invention provides compounds of formula I: wherein R.sup.1 to R.sup.4 and n have any of the meanings defined in the specification and their pharmaceutically acceptable salts. The invention also provides pharmaceutical compositions comprising a compound of formula I, processes for preparing compounds of formula I, intermediates useful for preparing compounds of formula I, and therapeutic methods for treating pain and treating other conditions which involve, for example, binding opioid receptors using compounds of formula I.