A reinvestigation of the Rh2(OAc)4-catalysed decomposition of 1-diazo-4-(2-methoxyphenyl)alkan-2-ones: evidence for ionic ‘ring-walk’ rearrangement in norcaradiene derivatives

On Rh2(OAc)4-catalysed decomposition of 1-diazo-4-(2-methoxyphenyl)butan-2-one (9) and its methyl-substituted analogue (18), norcaradienones 10 and 19, respectively, were observed as the only regioisomers resulting from intramolecular cyclopropanation, indicative of attack by the metal carbene away from the methoxy group; to explain the transformation of 10 and 19 into dihydroazulenones 15 and 23, interconversion of 10 and 19 into their isomers 14 and 22 must be invoked, and an ionic ‘ring-walk’ rearrangement is suggested for such an isomerization.