(3R,4R,4aS,7aR,12bS)-3-Cyclo­propyl­methyl-4a,9-dihy­droxy-3-methyl-7-oxo-2,3,4,4a,5,6,7,7a-octa­hydro-1H-4,12-methano­benzofuro[3,2-e]isoquinolin-3-ium bromide

The title compound, C21H26NO4 +·Br−, also known as R-methyl­naltrexone (MNTX) bromide, is a selective peripher­ally acting μ-opioid receptor antagonist with a oroxymorphone skeleton, synthesized by hydroxyl protection, N-methyl­ation, deprotection and anion exchange of naltrexone. It comprises a five-ring system A/B/C/D/E. Rings C and E adopt distorted chair conformations, whereas ring D is in half-chair conformation. The C/E ring junctions are trans fused. The dihedral angle between rings D and E is 82.3 (1)°, while the dihedral angles between the planes of rings C and A, and rings D and E are respectively 81.7 (1), 75.9 (1) and 12.2 (1)°. In the crystal, mol­ecules are linked by O—H⋯Br hydrogen bonds