Acid‐Catalysed Rearrangements of α‐Vinylcyclobutanones

The BF 3 · Et 2 O‐ and the CH 3 SO 3 H‐catalysed rearrangements of 10 α‐vinylcyclobutanones have been examined. With little acid, the β,β‐dialkyl derivatives 1 were transformed into linear dienones 2 and 3 ; with more acid, they were converted into cyclopentenones 4 by Nazarov cyclisation of initially formed 2/3 . The β‐monoalkyl (including the β,γ‐dialkyl) derivatives 7 rearranged only with a high acid concentration to afford the cyclopentenones 8 by 1,2‐acyl migration. In the case of 7a , the cyclopentenone 8a was accompanied by the unexpected constitutional isomer 9a , which is explained by a reversible interconversion of the cyclobutanone 7a with its isomer 19 via a cyclopropane intermediate like 18 . In the case of the β,β‐dialkyl derivative 5 , which contains an α‐isobutenyl (instead of an α‐vinyl) group, the acid‐catalysed rearrangement product was the bicyclo[3. 1. 0]hexanone derivative 6 .