AbstractThe synthesis of the sequiterpenoid tricyclic hydrocarbon (±)‐cleovene (1) by application of the α‐alkynone cyclisation is described. The starting bicyclic carboxylic acid2was obtained from ethyl 3‐methyl‐2‐oxocyclohexane‐1‐carboxylate by modified known methods (24%) and converted to the α‐alkynone3(86%). The thermolysis of3in the gas phase at 620° selectively produced the tricyclo[6.3.1.01,5] dodecenone4(80%) which was converted to1(37%) by conventional procedures. The selectivity of the α‐alkynone cyclisation is discussed in terms of the stereoelectronic requirements (coplanarity factor) of the carbene insertion. In order to throw further light on the importance of this factor, the (1‐adamantyl) alkynone16was synthesised from adamantane‐1‐carboxylic acid (78%) and subjected to thermolysis at 620°. Since this led to the tetracyclo[6.3.1.1.13,10.03,7]tridecenones17and18(together 72%), we conclude that the planar carbene insertion transition state, while preferred, i