Evaluation of an octahydroisochromene scaffold used as a novel SARS 3CL protease inhibitor

An octahydroisochromene scaffold has been introduced into a known SARS 3CL protease inhibitor as a novel hydrophobic core to interact with the S2 pocket of the protease. An alkyl or aryl substituent was also introduced at the 1-position of the octahydroisochromene scaffold and expected to introduce additional interactions with the protease. Sharpless-Katsuki asymmetric epoxidation and Sharpless asymmetric dihydroxylation were employed to construct the octahydroisochromene scaffold. The introductions of the P1 site His-al and the substituent at 1-position was achieved using successive reductive amination reactions. Our initial evaluations of the diastereo-isomeric mixtures (16a-d) revealed that the octahydroisochromene moiety functions as a core hydrophobic scaffold for the S2 pocket of the protease and the substituent at the 1-position may form additional interactions with the protease. The inhibitory activities of the diastereoisomerically-pure inhibitors (3a-d) strongly suggest that a specific stereo-isomer of the octahydroisochromene scaffold, (1S, 3S) 3b, directs the P1 site imidazole, the warhead aldehyde, and substituent at the 1-position of the fused ring to their appropriate pockets in the protease.