*4.1. 1,5-Enyne Substrates*

1,5-enyne is an important building block in the gold(I)-catalyzed construction of smallmolecule heterocycles. In 2016, Liu and colleagues reported a gold(I)-catalyzed tandem strategy for the syntheses of furopyran derivatives involving Claisen rearrangement and 6-*endo*-trig cyclization, the regioselectivity of which was mainly controlled by the angle strain of propargyl *γ*-butyrolactone-2-enol ethers (**139**) (Scheme 29) [63]. A 6-*endo*-dig cyclization was initiated by the coordination of the gold catalyst to the triple bond to form intermediates (**140**) that were rearranged into *β*-allenic ketones (**141**). Intermediates (**143**) were produced by keto–enol tautomerism, and angle strain controlled 6-*endo*-dig cyclization. After demetallation, furopyran derivatives (**144**) were successfully obtained. The reason for the change in regioselectivity from 5-*exo*-trig to 6-*endo*-trig was explained by DFT calculation.

In the same year, Liu and colleagues achieved the syntheses of multisubstituted furofuran derivatives based on the studies represented in Scheme 29 by trapping key intermediates (**141**) (Scheme 30a) [64]. Alkynes of substrates (**145**) were activated by gold species to induce a rearrangement reaction and yield allene intermediates (**147**), consistent with the generation of intermediates (**141**). The terminal alkene of the allene was coordinated by a [Au]<sup>+</sup> complex to enable the attack of nucleophiles, generating *σ*allyl gold species (**148**). After SE'-type protodeauration of **148**, intermediates (**149**) were

accessed, the enolether units of which were activated by gold species to trigger a 5-*exo*-trig cyclization. Finally, furofuran products (**150**) were delivered after protodeauration. In addition, multisubstituted furopyran derivatives were successfully produced when the propargyl terminal was substituted with thiophene or furan (Scheme 30b). Substrates (**151**) were converted to intermediates (**152**) under the activation of a gold catalyst, which was similar to the formation of intermediates (**140**) shown in Scheme 30. Intermediates (**152**) were not rearranged to *β*-allenic ketones due to the chelation of the heteroatom to the gold complex but were transformed to intermediates (**153**). Ultimately, furopyran products (**154**) were obtained via the nucleophilic addition of oxonium moiety after protodeauration. Thus, the authors achieved the syntheses of furofuran and furopyran derivatives by substituent modulation using propargyl vinyl ethers in the catalysis of gold(I) catalysts.

**Scheme 29.** Gold(I)-catalyzed syntheses of furopyran derivatives.

**Scheme 30.** Gold(I)-catalyzed syntheses of multisubstituted (**a**) furofuran and (**b**) furopyran derivatives.

In 2016, Jiang et al., developed a gold(I)-catalyzed, ligand-regulated cyclization for the syntheses furopyran or dihydroquinoline derivatives using 1,5-enyne substrates containing directing groups (Scheme 31) [65]. When using tris(2,4-di-*tert*-butylphenyl) phosphite (L1) in combination with trifluoromethanesulfonate (OTf−), gold(I)–π-alkyne intermediates (**156**) were formed by three coordinations, which were attributed to the increased electrophilicity of the gold center. The activated triple bond was attacked by the ortho position of the left aromatic ring, which overcame the steric hindrance. After protodeauration, furopyran and dihydroquinoline derivatives (**158**) were accessed (Scheme 31a). When a combination of Xphos ligand (L2) and NTf2 − was used, intermediates (**160**) were produced, which were attributed to the decreased electrophilicity of the gold center. Next, the activated alkyne was attacked by the para position of the left aromatic ring, which yielded products (**162**) after protodeauration (Scheme 31b). The above regiodivergent cyclization depended mainly on the electronic and steric effects of the ligand in gold species. The authors systematically examined the scope of the above switchable strategy with moderate to excellent yields.

**Scheme 31.** Gold(I)-catalyzed syntheses of (**a**) furopyran and (**b**) dihydroquinoline derivatives.
