Stereochemical and electronic features of the [3 + 2] cycloaddition of pentafulvenes with acylnitrones

Gold‐Catalyzed Cyclization–Cycloaddition Cascade Reactions of Allenyl Acetals with Nitrones

“…The stereochemical outcome is consistent with a report on a [3+2]cycloaddition reaction of fulvene and nitrone. [12] This reaction together with previous reports show that these allenyl acetal substrates can undergo two different cascade reactions catalyzed by similar gold catalysts.…”

“…In summary, we have developed a gold‐catalyzed cyclization–cycloaddition cascade reaction between allenyl acetals14 and nitrones. A key intermediate in the cascade reaction is postulated to be a 1‐methoxyfulvene species 12. These reactions reveal that the allenyl acetal substrates act as 1,2‐dipole equivalents, a behavior that is in contrast with the “dication behavior” described in our previous investigation 5b.…”

“…A key intermediate in the cascade reaction is postulated to be a 1-methoxyfulvene species. [12] These reactions reveal that the allenyl acetal substrates act as 1,2dipole equivalents, a behavior that is in contrast with the "dication behavior" described in our previous investigation. [5b] Although we often obtained two diastereomeric products in this cascade reaction, HOAc-catalyzed hydroly- www.chemeurj.org sis of these isomeric mixtures afforded isopropylidene cyclopentanones as single products.…”

“…In the presence of a cationic gold species, the acetal moiety of species 1 is converted into an oxonium ion moiety, thus giving intermediate A , which then undergoes an intramolecular cyclization to give allylic cation B . We envisage that the released Au–OMe species then assists in the deprotonation of species B to give highly nucleophilic 1‐methoxyfulvene C ,9–12 which reacts with nitrone through an exo cycloaddition reaction to afford diastereomeric cycloadducts 3 and 4 as the major products. The stereochemical outcome is consistent with a report on a [3+2]‐cycloaddition reaction of fulvene and nitrone 12.…”

“…We envisage that the released Au–OMe species then assists in the deprotonation of species B to give highly nucleophilic 1‐methoxyfulvene C ,9–12 which reacts with nitrone through an exo cycloaddition reaction to afford diastereomeric cycloadducts 3 and 4 as the major products. The stereochemical outcome is consistent with a report on a [3+2]‐cycloaddition reaction of fulvene and nitrone 12. This reaction together with previous reports show that these allenyl acetal substrates can undergo two different cascade reactions catalyzed by similar gold catalysts.…”