Pinacol Approach to the Eunicellane Skeleton

A short route to a partially aromatic version of the eunicellane skeleton, which occurs in cytostatic marine diterpenoids, was developed. The key step is the pinacol cyclization of a p ‐cymene‐derived C 20 keto aldehyde to form the 10‐membered ring of the [8.4.0] bicycle. We observed different reactivities upon employment of the SmI 2 /THF or TiCl 3 /Zn–Cu/DME reagent system. Upon treatment with SmI 2 in THF, the arylalkynes cyclized to novel [4.2.0] cyclohexylidene benzocyclobutanols, whereas the corresponding arylalkenes reacted to the desired [8.4.0] system. Under McMurry conditions, the alkynes afforded the [8.4.0] system, and this represents the first de novo synthesis of the complete eunicellane skeleton with a benzene partial structure.