Probes for narcotic receptor mediated phenomena. 18. Epimeric 6 alpha- and 6 beta-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinans as potential ligands for opioid receptor single photon emission computed tomography: synthesis, evaluation, and radiochemistry of [125I]-6 beta-iodo-