The role of cyclobutenes in gold(I)-catalysed skeletal rearrangement of 1,6-enynes.

1,6-Enynes with electron-donating substituents at the alkyne undergo gold(I)-catalysed single cleavage skeletal rearrangement, whereas substrates with electron-withdrawing substituents evolve selectively to double cleavage rearrangement. Theoretical calculations provide a qualitative rationale for these effects, and suggest that bicyclo[3.2.0]hept-5-enes are involved as intermediates. We provide the first X-ray structural evidence for the formation of a product of this class in a cycloisomerisation of a 1,6-enyne.