*3.4. Aryne-Acetal Substrates*

Arylalkynes containing acetal moieties as useful building blocks exhibited excellent reactivity in the gold-catalyzed syntheses of cyclopentene derivatives. In 2013, in pioneering work, the Toste group developed a gold-catalyzed strategy for the enantioselective syntheses of *β*-alkoxy indanone derivatives using this kind of substrate (Scheme 21) [54]. It was proposed that the activation of triple bonds by gold complexes triggered the migration of an alkoxy group to the alkyne, generating oxonium intermediates (**99**) via intermediates (**98**). An enantioselective annulation then occurred to form oxonium intermediates (**100**), which were transformed into products (**101**) after isomerization. The use of [Au]+ with chiral ligands ensured enantioselective cyclization with up to 98% *ee*. In addition, the *β*-alkoxy indanone derivatives could be further hydrolyzed to corresponding 3-methoxycyclopentenone derivatives under PTSA conditions with wet DCM.

In 2016, Liu et al., described a gold(I)-catalyzed hydrogen-bond-regulated tandem cyclization for the syntheses of indeno-chromen-4-one and indeno-quinolin-4-one derivatives by introducing a Michael acceptor in the substrates (Scheme 22) [55]. The double activation of a hydrogen bond and gold catalyst promoted methoxy migration to generate vinyl–gold intermediates (**103**), followed by an intramolecular annulation to produce intermediates (**104**) after isomerization. With conformational changing, intramolecular Michael addition occurred to yield indeno-chromen-4-one or indeno-quinolin-4-one derivatives (**105**) after the elimination of alkoxy groups.

In 2020, Sajiki and colleagues developed a gold(I)-catalyzed approach for the preparation of indenone derivatives using arylalkyne substrates containing cyclic acetals (Scheme 23) [56]. The triple bonds were first activated by the gold complex to produce vinyl–gold intermediates (**107**), which initiated the migration of benzylic hydride to generate oxonium cationic intermediates (**108**). Cyclized gold(I)–carbene intermediates (**109**) were then formed by

intramolecular nucleophilic addition. At this stage, when the system contained water, a carbene insert process occurred to yield intermediates (**110**), followed by a [Au]+-activated dehydration reaction to produce indenone derivatives (**112**). Alternatively, products (**112**) were generated directly from the cyclized gold(I)–carbene intermediates (**109**) through a *1,2-H* shift and elimination of gold species. The key 1,5-hydride shift was verified by deuterium-labeled experiments and 2D NMR analysis.

**Scheme 21.** Gold(I)-catalyzed syntheses of *β*-alkoxy indanone derivatives.

**Scheme 22.** Gold(I)-catalyzed syntheses of indeno-chromen-4-one and indeno-quinolin-4-one derivatives.

**Scheme 23.** Gold(I)-catalyzed syntheses of indenone derivatives.

In 2020, the Liu group reported a gold(I)-catalyzed domino reaction to construct benzo[*b*]indeno[1,2-*e*][1,4]diazepine derivatives using *o*-phenylenediamines and ynones (Scheme 24) [57]. The coordination of the gold species with a triple bond induced a series of transformations into intermediates (**115**), which was similar to the generation of intermediates (**104**) shown in Scheme 22. The intermediates (**115**) underwent Michael addition with exogenous *o*-phenylenediamine to produce intermediates (**117**) after the elimination of MeOH. Ultimately, benzo[*b*]indeno[1,2-*e*][1,4]diazepine derivatives (**118**) were synthesized by intramolecular condensation and aromatization accompanied by the elimination of MeOH and H2O. Controlled experiments were further conducted to determine the rationality of the above reaction.

**Scheme 24.** Gold(I)-catalyzed syntheses of benzo[*b*]indeno[1,2-*e*][1,4]diazepine derivatives.

Recently, the Liu group developed a synthetic strategy for 2,2 -spirobi[indene] derivatives using arylalkyne–acetal substrates based on their previous research (Schemes 22 and 24), mainly involving methoxylation and aldol condensation (Scheme 25) [58]. Intermediates (**120**) were easily produced by the activation of [Au]+/H+ and converted into intermediates (**121**) through an intramolecular aldol reaction. After releasing MeOH, 2,2 -spirobi[indene] derivatives were obtained. It should be noted that the reversible equilibrium of aldol/retro-aldol reactions led to the isomerization of the hydroxyl group.

**Scheme 25.** Gold(I)-catalyzed syntheses of 2,2 -spirobi[indene] derivatives.
