Shigeo Yasuda

The first Csp³–Csp² cross-coupling using aldehydes as latent α-alkoxyalkyl anion equivalents has been achieved.

Benzylation and allylation of aldehyde acyl anions were enabled by the merger of a thiazolium N‐heterocyclic carbene (NHC) catalyst and a palladium/bisphosphine catalyst in a synergistic manner. Owing to the mildness of the reaction conditions, various functional groups were tolerated in the substrates.

Treatment of the benzylallene-internal alkynes with [RhCl(CO)₂]₂ effected a cycloisomerization via a C_sp2–H bond activation to produce the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton. The reaction mechanism via formation of the rhodabicyclo[4.3.0] intermediates and σ-bond metathesis between the C_sp2–H bond on the benzene ring and the C_sp2–Rh^III bond was proposed. In addition, a plausible alternative mechanism for the previously reported cycloisomerization of the benzylallene-terminal alkynes could also be proposed.

The first total synthesis of (+)-sieboldine A was completed starting from 5-(p-methoxybenzyloxy)pentyne in 19 steps. The enantioselective Keck allylation provided the dienyne derivative, which was exposed to the Pauson–Khand conditions to afford the bicyclo[4.3.0]nonenone derivative with high stereoselectivity with an ee value of 93%. The following Ueno–Stork reaction formed the cis-hydrindane core with a quaternary carbon center. The late-stage Schmidt glycosylation led to the formation of the N-hydroxyazacyclononane ring.

The treatment of benzylallene‐substituted internal alkynes with [RhCl(CO)₂]₂ effects a novel cycloisomerization by C(sp²)−H bond activation to produce hexahydrophenanthrene derivatives. The reaction likely proceeds through consecutive formation of a rhodabicyclo[4.3.0] intermediate, σ‐bond metathesis between the C(sp²)−H bond on the benzene ring and the C(sp²)−Rh^III bond, and isomerization between three σ‐, π‐, and σ‐allylrhodium(III) species, which was proposed based on experiments with deuterated substrates.

A highly chemo‐ and regioselective partially intramolecular rhodium(I)‐catalyzed [2+2+2] cycloaddition of allenynes with alkynes is described. A range of diverse polysubstituted benzene derivatives could be synthesized in good to excellent yields, in which the allenynes served as synthetic equivalent to the diynes. A high regioselectivity could be observed when allenynes were treated with unsymmetrical alkynes.

Treatment of the allenylazetidine-alkynes with a catalytic amount of [RhCl(CO)dppp]2 (dppp: 1,3-bis(diphenylphosphino)propane) effected the intramolecular hetero-[6+2]-type ring-closing reaction via the C-C bond cleavage of the azetidine ring to produce azabicyclo[6.4.0]dodecatriene derivatives in good to excellent yields. The formation of the oxa analogue could also be achieved.

The synthesis of a carbon analogue of scytonemin was accomplished on the basis of molybdenum-mediated intramolecular double Pauson-Khand type reaction of bis(allenyne), followed by the double aldol condensation of the formed double Pauson-Khand type adduct.

A novel Rh‐catalyzed cycloisomerization of a variety of benzylallene‐alkyne substrates giving tricyclic products in moderate to good yields is presented.