AbstractAn enantioselective synthesis of the tricyclic ketone (+)‐5, which displays the carbon core of NGF‐inducing cyathane diterpenes, has been completed according to a strategy in which the key step was the intramolecular Heck reaction of the AC subunit 49 establishing the crucial anti stereochemical relationship between the two angular substituents. The C‐9 quaternary centre was set up by taking advantage of the enantioselective Michael addition involving chiral imines providing keto ester (R)‐10 in 91 % ee. After incorporation of the isopropyl group and iododecarboxylation of the propionate side chain, the iodo ketone 39 was condensed with the lithium enolate of methyl dihydrobenzoate to give the AC subunit 43 which was further elaborated to triflate (–)‐22. While direct Heck cyclisation of 22 was ineffective, oxidation of the bis(allylic) position of the 1,4‐cyclohexadiene moiety enhanced the reactivity, allowing the stereoselective formation of the hexahydro‐cyclopenta[a]naphthalene (+)‐50a. A key element of the construction was the final C ring enlargement, involving trimethylaluminum‐promoted one‐carbon expansion of ketone 52 with trimethylsilyldiazomethane which provided the tricyclic ketone (+)‐5. Furthermore, epoxidation of trienone (+)‐50a was shown to occur exclusively on the β face, giving rise to the C‐14 functionalised advanced intermediate 51 which has considerable potential for the synthesis of natural cyathins and analogues.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)