Construction of quaternary centres for natural polycycles: The Pauson–Khand approach

Asymmetric Total Synthesis of (-)-Spirochensilide A, Part 1: Diastereoselective Synthesis of the ABCD Ring and Stereoselective Total Synthesis of 13(R)-Demethyl Spirochensilide A

A concise and diastereoselective construction of the ABCD ring system of spirochensilide A is described. The key steps of this synthesis are a semipinacol rearrangement reaction to stereoselectively construct the AB ring system bearing two vicinal quaternary chiral centers and a Co-mediated Pauson-Khand reaction to form the spiro-based bicyclic CD ring system. This chemistry leads to the stereoselective synthesis of 13(R)-demethyl spirochensilide A, paving the way for the first asymmetric total synthesis of (-)-spirochensilide A.

Synthetic Studies toward the Hamigerans with a 6-7-5 Tricyclic Core

An approach toward the 6-7-5 tricyclic carbon skeleton of the hamigeran natural products was developed. The key steps include a benzyne-β-ketoester annulative ring expansion to form the 7-membered ring, a Nazarov reaction to form the 5-membered ring, a Ni-catalyzed conjugate methyl addition or a Corey-Chaykovsky reaction to install the all-carbon quaternary center, and a Suzuki cross coupling followed by reduction to introduce the isopropyl group.

A Total Synthesis of (-)-Hamigeran B and (-)-4-Bromohamigeran B

A concise synthesis of (-)-hamigeran B and (-)-4-bromohamigeran B is presented. The key reactions include a Suzuki coupling of enol triflate 15 with arylboronic ester for efficient synthesis of the densely 1,2,3-trisubstituted cyclopentene 23, a coordination-controlled intramolecular Friedel-Crafts cyclization of free phenol 13 for highly regioselective construction of tricyclic core 12, and a LiOH/O₂-promoted hydrolysis and concomitant aerobic oxidation of 31 for atom- and step-economic accessing of diketone 32. The application of these key transformations allowed for a rapid and efficient synthesis of (-)-hamigeran B and (-)-4-bromohamigeran B in 13 steps from the readily available chiral material 18.

A catalytic, asymmetric formal synthesis of (+)-hamigeran B

A concise asymmetric, formal synthesis of (+)-hamigeran B is reported. A Pd-catalyzed, decarboxylative allylic alkylation, employing a trifluoromethylated derivative of t-BuPHOX, is utilized as the enantioselective step to form the critical quaternary carbon center in excellent yield and enantioselectivity. The product is converted in three steps to a late-stage intermediate previously used in the synthesis of hamigeran B.