A Concise Total Synthesis of Dactylol via Ring Closing Metathesis

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Abstract
A straightforward total synthesis of the cyclooctenoid sesquiterpene dactylol (1) and of 3a-epi-dactylol (13) has been achieved in six synthetic operations. The unusual rearranged bicyclo[6.3.0]undecane isoprenoid skeleton of these target molecules has been formed via an initial three-component coupling triggered by 1,4-addition of a methylcopper reagent (MeLi, CuI, Bu3P) to cyclopentenone, followed by trapping of the enolate formed with 2,2-dimethyl-4-pentenal as the electrophile. The aldol 8 thus obtained was elaborated into the trans-disubstituted cyclopentanone derivative 10 which reacted with a methallylcerium reagent to afford a mixture of the tertiary alcohols 11a and 12a. Separation and O-silylation of these diastereoisomers, ring-closing metathesis (RCM) of the resulting dienes 11b and 12b to form the cyclooctene ring using Schrocks molybdenum carbene 5 as a precatalyst, and a final deprotection afforded the title compound and its epimer in excellent yields. This approach clearly surpasses previous ones in terms of efficiency, flexibility, accessibility of the substrates, number of steps, atom economy, and overall yield.