First [4+2] Cycloadditions Involving the Olefinic Bond of an ‘Aldoketeniminium Salt’ (=N‐Alk‐1‐enylideneaminium Salt)

‘Keteniminium triflates’ (=N‐alk‐1‐enylideneaminium trifluoromethanesulfonates; [MeN(Ts)CHC NR¹(R²)]⁺TfO⁻) generated in situ from N^α‐tosylsarcosinamides MeN(Ts)CH₂CONR¹(R²) unexpectedly react with cyclopenta‐1,3‐diene and cyclohexa‐1,3‐diene to give Diels–Alder reactions across the CC bond of the cumulene. The use of N^α‐tosylsarcosinamides derived from chiral pyrrolidines allows the direct preparation of trinorbornenone derivative 6 in high enantiomer purity.