Intramolecular cyclization by facilitated by functional group rearrangement offers superior atom economy and selectivity compared to intermolecular coupling and nucleophilic substitution. Notably, this transformation enables precise control over the product’s ring size and proceeds without the need for stoichiometric quantities of acids and bases. π-Lewis acids, such as gold and silver catalysts, exhibit a strong affinity for alkynes, thereby promoting their selective activation under mild conditions. In this presentation, we report a Lewis acid-catalyzed cyclization/rearrangement of cyclic acetals 1 and 3 bearing an alkynyl moiety, affording indenones 2 and phenanthrenes 4. 2-Alkynylbenzaldehyde derivatives 1 were transformed into indenone derivatives 2 using a gold catalyst, IPrAuCl, in combination with AgSbF₆ (Scheme 1A). Mechanistic studies suggest that the reaction of 1 is initiated by 1,5-hydride shift from the ipso position of the cyclic acetal to the alkyne carbon, which is coordinated and activated by the gold catalyst. In contrast, biphenyl derivatives 3, bearing a cyclic acetal and an alkyne unit at the 2- and 2'-positions, were converted into phenanthrenes 4 using either AgOTf or BF₃OEt₂as catalysts (Scheme 1B). Our experimental and computational investigations indicate that the transformation of 3 proceeds via a 1,6-hydride shift initiated by alkyne activation through coordination with the silver catalyst. Alternatively, the oxygen atoms of cyclic acetals may coordinate to the vacant orbital of BF₃ at the onset of the reaction, thereby promoting subsequent rearrangement to afford the same phenanthrenes 4. The substrate scope can be broadened by selecting an appropriate catalyst tailored to the structure features of the substrate 3.

Tsuyoshi Yamada

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