New solvates of the drug naltrexone: protonation, conformation and interplay of synthons

Abstract

Naltrexone [systematic name: (4R,4aS,7aR,12bS)-3-cyclopropylmethyl-4a,9-dihydroxy-2,4,5,6,7a,13-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7-one] is an important morphine-related drug used for combating alcoholism and opioid dependence. Of the eight crystal forms of naltrexone known thus far, only one exists in the neutral form and it crystallizes as a monohydrate. We have isolated the naltrexone free base as two new solvate forms,i.e.the ethyl acetate 0.33-solvate, C20H23NO4·0.33C4H8O2, (I), and the diethyl ether hemisolvate, C20H23NO4·0.5C4H10O, (II). While just one solvent molecule is present in the asymmetric unit of each solvate, there are three drug molecules (Z′ = 3) in ethyl acetate solvate (I) and two (Z′ = 2) in diethyl ether solvate (II). In (I), one of the three crystallographically independent drug molecules is present with its cyclopropyl group disordered over two sets of positions, as is the whole diethyl ether solvent molecule in (II). In all known forms, including the title forms, the naltrexone molecule exhibits the same conformation of the fused rings. The only conformational variability of naltrexone is in the cyclopropylmethyl group. Two conformations can be found around the bond connecting this group to the N-heterocycle, which is directly related to drug protonation. We have calculated, at the B3LYP/6-31G** level of theory, the minimum energy conformations of protonated and neutral naltrexone molecules for a chosen torsion angle about this bond. The lowest energy conformers depend on the protonation state and are in agreement with those found in the solid state. Within the cyclopropylmethyl group, the bond joining the methylene C atom to the cyclopropyl fragment also evidences conformational variability. In the literature, there are two well defined conformations around this bond. A third cyclopropyl conformation around this second bond is observed in the title solvates. Concerning the supramolecular features of the previously reported crystal structures, only one classical hydrogen bond between naltrexone molecules and oneC(8) homosynthon is known, pointing to the robustness of this synthon and the difficulty in disrupting it. NewR22(7) andC22(10) homosynthons are found in both (I) and (II), suggesting that their occurrence derives from crystallization of the neutral drug from nonpolar solvents.