*3.5. Other Arylalkyne Substrates*

In 2021, Xu and colleagues achieved a cascade strategy for the syntheses of indene derivatives involving gold(I)-catalyzed Wolff rearrangement and ketene C=C dual functionalization (Scheme 26) [59]. Diazoketone substrates (**123**) were activated by a gold complex to form gold carbine, which as converted to ketene intermediates (**124**) by Wolff rearrangement. The ketene units of **124** were then attacked by nucleophiles (ROH) to form enol intermediates (**125**). Activation of a triple bond by gold(I) species initiated a C-5-*endo*-dig cyclization to obtain indene products (**126**). In addition, when nucleophiles such as indoles

or pyrroles were used, O-7-*endo*-dig cyclization occurred to generate benzo[*d*]oxepine derivatives. The scope of the above strategy was studied in detail with 46 examples and up to 88% yield, and the related reaction pathways were explained by DFT calculations.

**Scheme 26.** Gold(I)-catalyzed syntheses of indene derivatives.

In 2021, a strategy for the syntheses of indene derivatives based on the cyclization of ynamides was developed by the Evano group (Scheme 27) [60]. Gold–keteniminium ions (**128**) were formed upon the coordination of [Au]+ to the triple bond in ynamide, followed by a *1,5-H* shift, resulting in carbocation intermediates (**129**). Subsequently, the carbocations of **129** were trapped by vinyl–gold to trigger a cyclization, producing intermediates (**130**). After a *1,2-H* shift and elimination of [Au]+, indene products (**131**) were achieved. Alternatively, indene products (**131**) could be formed by the elimination of a proton and protodeauration. This method is associated with a wide range of substrates and was systematically studied using 20 examples with 40–96% yields.

**Scheme 27.** Gold(I)-catalyzed syntheses of polysubstituted indene derivatives.

Recently, the Ohno lab reported a gold(I)-catalyzed cascade acetylenic Schmidt reaction/ *1,5-H* shift/*N*- or *C*-cyclization method producing indole[*a*]- and [*b*]-fused polycycle derivatives (Scheme 28) [61]. The isotopic labeling experiment showed that the reaction started with an acetylenic Schmidt reaction activated by gold species, which resulted in the formation of *α*-imino gold carbenes (**133**), followed by a *1,5-H* shift to yield carbocationic intermediates (**134**), which were in reversible equilibrium with aromatized intermediates (**135** and **137**). Finally, *C*-cyclization products (**136**) were generated via aromatized intermediates (**135**), and the *N*-cyclization products (13b) were yielded via aromatized intermediates (**137**) with a bond rotation. Notably, the selectivity of the *N* and *C*-cyclization products could be tuned through the electron density of the left benzene ring, the stability of the carbocation, and the effect of the counterion. Moreover, the above strategy is excellent example of benzylation of benzylic C(sp3)-H functionalizations, providing a concise method for the syntheses of indole[*a*]- and [*b*]-fused polycycle derivatives.

**Scheme 28.** Gold(I)-catalyzed syntheses of indole[*a*]- and [*b*]-fused polycycle derivatives.

Based on the cases summarized in this chapter, it seems that the gold(I)-catalyzed tandem approach using a variety of arylalkyne substrates could be used to synthesize corresponding cyclopentene derivatives, such as benzofulvenes, dibenzopentalenes, 2,2 spirobi[indene], indenes, etc. These structurally diverse cyclopentene derivatives can provide further possibilities for the discovery of bioactive lead compounds and provide strategic support for the syntheses of related bioactive molecules.
