Five-membered carbocycles Pauson-Khand reaction

Asymmetric Pauson-Khand reaction in syntheses of heterocycles fused with five-member carbocyclic fragment 980PP121. 

The Pauson-Khand reaction (PKR) [96] consists of the synthesis of cyclopen-tenones by reaction of an alkene with a dicobalthexacarbonyl complexed alkyne (Scheme 57) and has recently emerged as one of the methods of choice for the obtainment of five-membered carbocyclic rings [97]. Its unique atom connectivity, which involves the two unsaturated carbons of the reagents and the carbon atom of a carbon monoxide ligand of cobalt usually in a regioselective manner (Scheme 57), has brought to refer to PKR as a [2 -I- 2 + 1] cycloaddition. 

In the laboratory of S.L. Schreiber, the total synthesis of (+)-epoxydictymene was accomplished by the tandem use of cobalt-mediated reactions as key steps. The eight-membered carbocycle was formed via a Nicholas reaction, while the five-membered ring was annulated by the Pauson-Khand reaction. Several P.-K. conditions were explored and the best diastereoselectivity was observed when NMO was used as a promoter. The annulated product was isolated as an 11 1 mixture of diastereomers. 

---