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We report novel insights into the cascade rearrangement of destabilized vinyl cations deriving from the BF3&middot, Et2O-induced decomposition of cyclic &alpha, diazo-&beta, hydroxy ketones in turn prepared by aldol-type condensation of cycloalkanones with diazoacetone. Complexation of the hydroxy group of the &alpha, hydroxy compound with the Lewis acid is the first event, followed by the generation of the cycloalkanylidenediazonium salt that, after nitrogen loss, produces the highly reactive vinyl cation. The subsequent ring expansion results in the formation of a cycloalkenyl vinyl cation that affords the allylic cation by 1,2-methylene shift and ring contraction. The cation can then trap the solvent, the fluoride or the hydroxide released from the [BF3OH]&minus, to afford different reaction products. The effect of both solvent and substrate ring size on products types and ratios were analyzed and discussed from a mechanistic point of view.

α-Triethylsilyl-α-diazoacetone underwent a sequential aldolisation–deprotection–aldolisation process to access the 2-diazo-3-oxo-1,5-dihydroxy skeleton with significant diversity. The diazo group could be efficiently removed by treatment with rhodium acetate to afford the corresponding 5-hydroxy-1,3-diketones.