An Enantioselective Approach for the Structure Revision of Isolophanthin E and Syntheses of Proposed Structures of Isolophanthins A, B, and C

The
first enantioselective total synthesis and structure revision
of isolophanthin E and syntheses of proposed structures of isolophanthins
A, B, and C are demonstrated. Natural product 3β-hydroxy-8,11,13,15-abietatetraene
was directly synthesized utilizing an efficient cationic polyene cyclization,
and it is used as a common intermediate in the synthesis of isolophanthins
and related abietatriene natural products. Two distinct synthetic
routes were established for the synthesis of different stereoisomers
of isolophanthin E. Spectroscopic analysis and structural assignment
of isolophanthin E stereoisomers provide valuable insights into the
relative configuration of the C-2, C-3-dihydroxy A ring of similar
terpenoids, aiding in the identification of their configuration. A
total of seven diterpenoids were obtained using regioselective chloromethylation,
Sharpless dihydroxylation, Cu(II) catalyzed allyl-benzyl coupling,
epoxide-initiated polyene cyclization, Rubottom oxidation, and additive-controlled
dihydroxylation as key synthetic steps.