Unified Total Synthesis of 3‐epi‐Ryanodol, Cinnzeylanol, Cinncassiols A and B, and Structural Revision of Natural Ryanodol and Cinnacasol

Ryanodane diterpenoids structurally share an extremely complex fused ring system, but differ in the substitution patterns of the hydroxy groups. Since these congeners exhibit various biologically important functions, their efficient chemical constructions have been greatly anticipated. We previously accomplished the total synthesis of ryanodine (1) using pentacycle 8 as the advanced intermediate. Here, we report the unified total syntheses of four distinct diterpenoids, 3-epi-ryanodol (3), cinnzeylanol (4), cinncassiols B (5), and A (6), from 8, all within 10 steps. A series of highly optimized chemo- and stereoselective reactions and protecting-group manipulations enabled assembly of the densely oxygenated structures of 3-6. Furthermore, the present synthetic studies established the C13S stereochemisty of 5-7 and revised the proposed structures of natural ryanodol (2) and cinnacasol (7) to be those of 3 and 6, respectively.