*2.4. Other Arylalkyne Substrates*

In 2013, Ye et al., described a gold(I)/acid-catalyzed methodology for the syntheses of anthracenes using *o*-alkynyl diarylmethanes with 21 examples and 58–80% yields (Scheme 11) [32]. Coordination of alkynes by gold catalysts triggered the attack of electronrich benzene rings to furnish vinyl–gold intermediates (**43**) via 6-*exo*-dig cyclization. After protodeauration and [Au]+/H+ promoted isomerization, anthracenes (**45**) were obtained. An alternative pathway was also proposed; the alkyne of the substrates (**42**) was hydrolyzed under gold-catalyzed conditions to yield intermediates (**44**) that were converted to products (**45**) by an acid-mediated cyclodehydration. In addition, the products (**45**) were further modified into a variety of potentially valuable anthracene derivatives.

**Scheme 11.** Gold(I)-catalyzed syntheses of anthracene derivatives.

In 2017, a gold(I)-catalyzed tandem cycloisomerization, Diels–Alder, and retro-Diels– Alder reactions were reported by the Liu lab (Scheme 12) [33]. Activation of alkyne in substrates (**46**) initiated the first cycloisomerization to yield furopyran intermediates (**47**). A subsequent Diels–Alder reaction of dienes (**47**) and dienophiles occurred to form highly strained intermediates (**48**), which underwent a retro-Diels–Alder reaction to provide biaryl products (**49**) by releasing acetaldehyde (HCHO). The above pathways were reasonably explained by density functional theory (DFT).

**Scheme 12.** Gold(I)-catalyzed syntheses of biaryl derivatives.

In 2021, the Hashmi group reported the syntheses of *meta*- and *para*dihydroxynaph thalenes based on diazoalkynes through a regiodivergent gold-catalyzed cyclization (Scheme 13) [34]. The activated triple bonds of substrates (**50**) were attacked by diazocarbon to generate intermediates (**51**), followed by the formation of quinoid gold carbene intermediates (**52**) via the release of nitrogen. At this stage, two different reaction paths occurred via the addition of water or Et3N(HF)3. Under the condition of water as an additive (path a), *meta*dihydroxynaphthalenes (**54**) were produced via carbene insertion of water after protodeauration. When H2O and Et3N(HF)3 were used as additives, *para*dihydroxynaphthalenes (**56**) were obtained via Michael-type addition of quinoid carbene species, 1,2-phenyl migration, and protodeauration. Moreover, when only Et3N(HF)3 was used as an additive, "F−" was inserted instead of water for gold carbene to generate the α-fluoronaphthalenes.

**Scheme 13.** Gold(I)-catalyzed syntheses of meta- and paradihydroxynaphthalene derivatives.

Gold(I)-catalyzed arylalkyne annulations provide abundant strategies for the syntheses of benzene derivatives, including the strategies shown in this chapter and several other intramolecular or intermolecular strategies [35–47].
