The intramolecular tiiels-Alder reaction of several substituted cyclopentadienes to functionalized tricyclic products, suitable for transformation to polyquinane natural products, is described. We have embarked on what promises to be an extremely versatile strategy for the synthesis 'of natural products containing more tnan one 5-membered carbocyclic ring (a sampling of which are depicted below). The basic strategy involves an intramolecular Diels-Alder4 reaction with cyclopentadiene (to 2) followed by cleavage of the double bond to form a [3.3.0]-bicyclooctane -PENTALENENE SILPHINENE : ; \ e-ISOCOMENE ring system (eq. 1). Three points about this strategy are noteworthy. First, up to three dudternary centers could be formed in the cyclization, depending on the substitution at the dienophile. Second, the stereochemistry of the four chiral centers shown in 3 is well defined because of the exe nature of the intramolecular Diels-Alder reaction (vide infra). Third, all of the ring carbons in 2 except for C-7 may bear substituents depending on the choice of the bridging chain. eq. 1 In this iqtral;lolecuIar Diels-Alder Reaction the cliene dnd the dienophile are tethered by a three cdrbon clidirl. In silaildr ring systems it has been determined that the bridging chdin of t;lis length dl'i+ays occupies the fxo position. 5 This has been interpreted as d consequence of the cis[3.3.D]oicyslooctane ring system that is formed in the exo approach (outlined in boldface
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