Enantiospecific synthesis of pacifigorgianes

Pacifigorgianes 1 are a small group of sesquiterpenes, containing bicyclo[4.3.0]nonane moiety, found to be present in both marine and terrestrial sources. The first member of this group, ichthyotoxic (+)pacifigorgiol 2 was isolated in 1982 by the research groups of Fenical and Clardy 1 from the pacific gorgonian coral Pacifigorgia cf. adamssi. Later in 1986, Simpa and coworkers 2 reported the isolation of (–)-pacifigorgiol 2 from the essential oil of Valeriana officinalis, which was found to be optical antipode of that isolated by Fenical and Clardy from the marine sources. In 1984 Conolly and coworkers 3 reported the isolation of the second member of the family, (+)tamariscol 3 from the liverwort Frullania tamarisci. The absolute configuration of (–)-tamariscol 3 was assigned by Asakawa and coworkers 4 via degradation of tamariscol 3 in to the bicyclic ketone 4, which was found to be enantiomeric to a synthetic sample obtained from (R)-carvone. In 2001 Konig and coworkers 5 reported the isolation of five new pacifigorgianes, pacifigorgia-1(9),10-diene 5, pacifigorgia-1,10-diene 6, pacifigorgia-1(6),10-diene 6, pacifigorgia-2(10),11-diene 7, pacifigorgia-2,10-diene 8, from the liverwort Frullania fragilifolia collected at the Hinanger Wasserfall (Sonthofen, Allgau, Germany). Subsequently, French and coworkers 6 have reported the isolation of pacifigorgia-7,10-diene 10 from the root extracts of Aristolochia salvadorensis (Figure 1). From the synthesis point of view, so far only one report each on the synthesis of pacifigorgiol 2 and tamariscol 3 (along with its C-2 epimer) have been appeared in the literature prior to the initiation of the research work described here 7 . In 1982, Clardy and Martin 8 reported the synthesis of (±)-pacifigorgiol 2 starting from commercially available 3-methoxyindanone using Birch reduction and cyclopropanation as key steps. In 1990, Asakawa and coworkers 9 reported the synthesis of (±)-2-epitamariscol and tamariscol 3 starting from 3-methoxyindanone. So far there is no report in the literature on the synthesis of any pacifigorgiadienes either in racemic or enantiopure form. Last fifteen years had witnessed an exponential growth in the application of the olefin metathesis 10