Salvinorin B methoxymethyl ether

The title compound [MOM-SalB; systematic name: methyl (2S,4aR,6aR,7R,9S,10aS,10bR)-2-(3-furyl)-9-methoxymethoxy-6a,10b-dimethyl-4,10-dioxo-2,4a,5,6,7,8,9,10a-octahydro-1H-benzo[f]isochromene-7-carboxylate], C₂₃H₃₀O₈, is a derivative of the κ-opioid salvinorin A with enhanced potency, selectivity, and duration of action. Superimposition of their crystal structures reveals, surprisingly, that the terminal C and O atoms of the MOM group overlap with the corresponding atoms in salvinorin A, which are separated by an additional bond. This counter-intuitive isosterism is possible because the MOM ether adopts the 'classic anomeric' conformation (gauche-gauche), tracing a helix around the planar acetate of salvinorin A. This overlap is not seen in the recently reported structure of the tetrahydropyranyl ether, which is less potent. The classic anomeric conformation is strongly favoured in alkoxymethyl ethers, but not in substituted acetals, which may contribute to their reduced potency. This structure may prove useful in evaluating models of the activated κ-opioid receptor.