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.