8 taken together are oxo u2550 o and x is u2014 o u2014 a specific intermediate that is useful for preparing other compounds of the invention is a compound of formula i or ii wherein r 6 is carboxy a specific intermediate that is useful for preparing other compounds of the invention is a compound of formula i ii iii or iv wherein r 3 is halo azido or amino specific compounds of the invention also include compounds of formula i that comprise or that are linked to one or more detectable groups or isotopes such detectable compounds may be used as imaging agents or as probes for evaluating opioid receptor structure and function for example one or more detectable groups can be incorporated into the core of the compound or can be attached to the compound directly through a linking group or through a chelating group suitable detectable groups include deuterium tritium iodine 125 iodine 131 iodine 123 astatine 210 carbon 11 carbon 14 nitrogen 13 or fluorine 18 additionally groups such as tc 99m and re 186 can be attached to a linking group or bound by a chelating group which is then attached to the compound of formula i directly or by means of a linker suitable radiolabeling techniques are routinely used in radiopharmaceutical chemistry in cases where compounds are sufficiently basic or acidic to form stable nontoxic acid or base salts administration of the compounds as salts may be appropriate examples of pharmaceutically acceptable salts are organic acid addition salts formed with acids which form a physiological acceptable anion for example tosylate methanesulfonate acetate citrate malonate tartarate succinate benzoate ascorbate u03b1 ketoglutarate and u03b1 glycerophosphate.