*2.2. Arene-Enyne Substrates*

Gold-catalyzed cyclization of arene-enynes is an important strategy for building smallmolecule carbocyclic skeletons that has inspired many excellent methods to be reported. In 2017, Shi et al., developed a gold(I)-catalyzed tandem cyclization–oxidation strategy to access aryl acetaldehyde derivatives using alkylidene–cyclopropane and pyridine *N*-oxide (Scheme 3) [24]. First, coordination of the triple bond by [Au]<sup>+</sup> triggered the 6-*endo*-dig cyclization to form intermediates (**13**), and benzylic carbocation was stabilized by electronrich cyclopropane and the benzene ring. Subsequently, the 3,5-dibromo-pyridine *N*-oxide acted as a nucleophile to attack cyclopropane and produce intermediates (**14**). Finally, aryl acetaldehyde derivatives (**15**) were generated by Kornblum-type oxidation with the simultaneous release of 3,5-dibromo-pyridine. The scope of application of the above strategy was examined using 27 examples with 36–93% yields. It is worth noting that when R<sup>2</sup> was a substituted phenyl group, the aryl acetaldehyde derivative could be further modified to polycyclic aromatic hydrocarbons (PAHs) under the catalysis of In(OTf)3.

**Scheme 3.** Gold(I)-catalyzed syntheses of aryl acetaldehyde derivatives.

In 2019, the Ohno lab described a gold(I)-catalyzed cascade cyclization strategy for the syntheses of cyclopropanes derivatives, with 11 examples and yields of up to 96% (Scheme 4) [25]. The activation of the allenyl moiety of 1-allenyl-2-ethynyl-3-alkylbenzene substrates (**16**) by the gold complex induced a nucleophilic attack of alkyne to yield vinyl cationic intermediates (**17**). Then, a *1,5-H* shift occurred to generate benzylic carbocation intermediates (**18**). Subsequent carbocation cyclization provided acenaphthene derivatives (**19**) after aromatization and protodeauration. In addition, a series of 1-(naphth-1-yl)cyclopropa-[*b*]benzofuran derivatives was successfully prepared when phenylenetethered allenynes and benzofurans were subjected to the same gold-catalyzed conditions.

In 2021, the Ohno lab reported the syntheses of benzo[*cd*]indole skeletons by goldcatalyzed tandem cyclization based on their previous work (Scheme 5) [26]. In this approach, a series of amino-allenyne substrates (**20**) were designed and prepared. First, the activated allene was attacked by the electron-rich alkyne to form vinyl cationic intermediates (**21**). The vinyl cation was captured by the neighboring amine group to yield tricyclic

fused indoles (**22**), which underwent an isomerization to furnish pyrrolonaphthalenes (**23**). The resulting tricyclic derivatives could be transformed into nitrogen-containing polycyclic aromatic compounds (*N*-PACs) with special photophysical properties through *N*-arylation or Friedel–Crafts acylation.

**Scheme 4.** Gold(I)-catalyzed syntheses of acenaphthene derivatives.

**Scheme 5.** Gold(I)-catalyzed syntheses of pyrrolonaphthalene derivatives.

Recently, Liu and colleagues achieved the gold(I)-catalyzed construction of benzene derivatives using arene–enyne substrates, which was applied to the total syntheses of eight natural products (Scheme 6) [27]. Coordination of alkyne in substrates (**24**) promoted a 6-*endo*-dig cyclization to yield intermediates (**25**) that were converted into iodonaphthalenes (**26**) in situ in the presence of *N*-iodosuccinimide (NIS). The intermediates (**26**) were used as key moieties to synthesize benzo[*c*]phenanthridine alkaloids in a pot-economic approach. Moreover, the cytotoxicities of these alkaloids were investigated, indicating the future potential of these molecules for anticancer research.

**Scheme 6.** Gold(I)-catalyzed syntheses of iodonaphthalene derivatives.

## *2.3. Aryne-Enolether Substrates*

Enolether showed versatile properties in gold-catalyzed reactions, making it suitable for use not only as a nucleophile but also as an electrophile to be coordinated by [Au]+. The combination of enolether with alkyne derivatives to form building blocks containing 1,5 enyne showed unique advantages in gold-catalyzed tandem cyclization for the syntheses of benzene derivatives. Accordingly, the Liu lab has reported a number of such studies in recent years.

In 2014, a gold(I)-catalyzed cycloisomerization of arylalkyne-enolether for the construction of multisubstituted naphthalenes was developed by the Liu group (Scheme 7) [28]. First, the triple bonds of the substrates (**27**) were activated by the gold species, which induced an intermolecular nucleophilic addition by alcohol to yield dienol ether intermediates (**28**). Coordination of [Au]+ to the electron-rich enolether promoted cycloisomerization to provide multisubstituted naphthalenes (**29**) via the release of methanol and protodeauration. The scope of this strategy was examined by synthesizing 20 alkyne–enolether substrates with 38–88% yields.

**Scheme 7.** Gold(I)-catalyzed syntheses of multisubstituted naphthalenes.

In 2017, Liu and colleagues achieved gold(I)-catalyzed tandem cyclization for the syntheses of benzo[*a*]carbazole derivatives using arylalkyne-enolether substrates (Scheme 8) [29]. The authors modulated the electronic properties of the triple bond through the substituent of the right benzene ring, which further tuned the cyclization order. When there were sulfonamide substituents on the appropriate benzenes, the *α*-position of the alkyne activated by [Au]<sup>+</sup> induced a 5-*endo*-dig cyclization to produce indole intermediates (**31**). The enolether was then activated by a gold(I) complex and attacked by the electron-rich indole to promote the second cyclization, yielding benzo[*a*]carbazoles (**32**) by elimination of methanol and protodeauration. The above reaction mechanism was verified by capturing the intermediates and further supported by DFT calculations. Notably, when the appropriate benzene rings of the substrates were substituted with amine groups, the order of cyclization was changed to yield indeno-[1,2-*c*]quinoline derivatives, which are described in detail in later sections.

**Scheme 8.** Gold(I)-catalyzed syntheses of benzo[*a*]carbazole derivatives.

One year later, another cascade cyclization strategy was reported by the Liu group as an ongoing study on the gold-catalyzed cyclization of enolether-involved substrates in the construction of small-molecule scaffolds. The authors achieved the syntheses of xanthone and acridone derivatives by designing a series of alkyne–enolether substrates with 25 examples and up to 98% yield (Scheme 9) [30]. Initially, the triple bonds of the substrates (**33**) were chelated by the gold(I) species, which promoted an intramolecular Michael addition to obtain intermediates (**34**) after protodeauration. Then, gold(I)-activated enolether was attacked by newly generated enolethers or enamines to undergo a 6-*endo*-trig cyclization. Finally, xanthone or acridone derivatives (**35**) were achieved via a similar pathway as reported previously.

**Scheme 9.** Gold(I)-catalyzed syntheses of xanthone and acridone derivatives.

In 2022, Liu and colleagues reported a gold(I)-catalyzed 6-*endo*-dig cyclization of arylalkyne–enolethers (**36**) to construct 2-(naphthalen-2-yl)aniline derivatives (Scheme 10) [31]. The authors found that the amine group on the right-hand benzene ring benefited 6-*endo*-dig cyclization via an electron-donating effect to generate naphthalenes (**37**) after isomerization and protodeauration. In addition, several important heterocycles (**38–41**) were synthesized in a divergent manner from that of naphthalene derivatives (**37**).

**Scheme 10.** Gold(I)-catalyzed syntheses of 2-(naphthalen-2-yl)aniline derivatives.
