Stereoselective Total Syntheses of Guanacastepenes N and O

Total
syntheses of (±)-guanacastepenes N and O were accomplished
with 2-cycloheptenone as starting material. The six- and five-membered
rings of the core [5, 7, 6] ring skeleton were constructed with an
intramolecular Diels–Alder reaction and α-carbonyl radical
cyclization. The quaternary centers and their stereochemistry were
established with sequential Cu­(I)-mediated conjugate additions. A
sequence with dihydroxylation, conjugate addition, and β-elimination
was devised to incorporate all oxygen functionalities at positions.
The total synthesis is adaptable for the synthesis of enantiopure
guanacastepenes N and O using chiral intermediate (R)-3-vinyl-2-cycloheptenol obtained from lipase-catalyzed kinetic
resolution.