α- and β-funaltrexamine (a- and β-FNA, la and lb) are naltrexone derivatives differing only in chirality at C(6). Both epimers bind to μ opioid receptors in GPI and MVD preparations, but only the β-epimer irreversibly blocks these receptors in both preparations. In an effort to investigate the reasons for this difference, we have determined the molecular structures of la and lb by X-ray diffraction techniques. The two epimers have almost identical conformations in the fused ring system except for ring C, which is observed in a twist-boat conformation in α-FNA and a chair in β-FNA. As a result the electrophilic fumaramate moieties are equatorial in both structures and orthogonal to one another when the fused rings are superimposed. In the crystal structure of β-FNA there is a close intermolecular contact between a phenolic oxygen and the fumaramate double bond that can serve as a model for nucleophilic attack on the fumaramate group. When la and lb are superimposed, the fumaramate double bond of la is more than 2 A away from that in its epimer lb and in the wrong orientation for nucleophilic attack from the proposed direction to take place. The results of this study are consistent with a model that postulates the involvement of two consecutive recognition steps leading to the irreversible blockage by β-FNA (Sayre, L. M.; Larson, D. L.; Fries, D. S.; Takemori, A. E.; Portoghese, P. S. J. Med. Chem. 1983, 26, 1229) and underscores the importance of the second recognition step in conferring selectivity in the Michael addition of a nucleophile to the fumaramate group.