Oxygenated Theonellastrols: Interpretation of Unusual Chemical Behaviors Using Quantum Mechanical Calculations and Stereochemical Reassignment of 7-Hydroxytheonellasterol.

A total of eight new oxygenated 4--methylene sterols, -, together with one artifact and six known sterols -, were isolated from the marine sponge collected from the Bohol province in Philippines. Structures of sterols - were determined from 1D and 2D NMR data. Among the sterols, 8-hydroxytheonellasterol () spontaneously underwent an allylic 1,3-hydroxyl shift to produce 15-hydroxytheonellasterol () as an artifact; this was rationalized by quantum mechanical calculations of the transition state. In addition, the 1,2-epoxy alcohol subunit of 8-hydroxy-14,15--epoxytheonellasterol () was assigned using the Gauge-Independent Atomic Orbital (GIAO) NMR chemical shift calculations and subsequent DP4+ analysis. Finally, comparison of the C chemical shifts of isolated 7-hydroxytheonellasterol () with the reported values revealed significant discrepancies at C-6, C-7, C-8, and C-14, leading to reassignment of the C-7 stereochemistry in the known structure.