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-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5.alpha.-epoxymorphinan