AbstractThe thermal cycloisomerization of some alkylacetylenes was investigated in a tubular quartz reactor. At 570°C 1‐hexyne (1) rearranges to 3‐methyl‐1‐cyclopentene (5) with a selectivity of about 27 by a reaction sequence including an acetylene‐vinylidene rearrangement and 1,5‐C,H insertion of the intermediately formed alkylidenecarbene species. 5‐methyl‐1‐hexyne (2) behaves analogously forming 3,3‐dimethyl‐1‐cyclopentene (6), while 2‐hexyne (3) provides 1‐methyl‐1‐cyclopentene (7) indicating that the acetylene‐vinylidene rearrangement is obviously not restricted to 1,2‐H shifts. The mechanism of the cycloisomerization of alkylacetylenes is investigated by means of D‐labeled parent alkynes. The results show that the unimolecular cycloisomerization via alkylidenecarbenes obviously can be an important channel despite t