Synthese und thermisches Verhalten von 6,8‐Dimethylentricyclo[3.2.1.0 2,7 ]oct‐3‐enen; [3 s 3 s ]‐sigmatropische Umlagerungen von 6‐Allenyl‐ und 6‐Propargly‐1‐ Methylen‐cyclohexa‐2,4‐dienen in aromatische Kohlenwasserstoffe als Beispiele für konzertierte Semibenzol → Benzol‐Umlagerungen

The tricyclic dimethylene hydrocarbons 5 , 6 , 7 , 8 and d 2 ‐ 5 , (Scheme 2), which are prepared by Wittig ‐reaction from the corresponding ketones, are rearranged, by heating, to 4‐aryl‐but‐1‐yne derivatives via the unstable 6‐allenyl‐1‐methylene‐cyclohexa‐2, 4‐diene intermediates ( e.g. Scheme 14). Using the deuterium‐labelled compound d 2 ‐ 5 , it was shown that the allenyl moiety, formed by a retro‐ Diels ‐ Alder reaction (cycloreversion) of the tricyclic dimethylene compound, migrates with complete inversion in the final o ‐semibenzene‐benzene rearrangement (Schemes 11 and 14). Reaction of 6‐propargyl‐cyclohexa‐2, 4‐dien‐1‐ones with triphenylphosphonium methylide gives 6‐propargyl‐1‐methylene‐cyclohexa‐2 4‐dienes, which immediately undergo a [3 s , 3 s ]‐rearrangement to form 4‐aryl‐buta‐1, 2‐dienes (Scheme 9). In contrast, the rearrangement of the corresponding 4‐propargyl‐1‐methylene‐cyclohexa‐2, 5‐ dienes proceeds by a radical mechanism (Schemes 10 and 13).