# Gold(I)-Catalyzed Domino Cyclization for the Construction of Trispirocyclic Cyclohexanes

Manon Genet, Jérôme Marrot, Isabelle Chataigner, Xavier Moreau

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Manon Genet, Jérôme Marrot, Isabelle Chataigner, Xavier Moreau. Gold(I)-Catalyzed Domino Cyclization for the Construction of Trispirocyclic Cyclohexanes. ChemistryEurope, 2025, 4 (2), pp.e202500362. 10.1002/ceur.202500362. hal-05503004

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# Gold(I)-Catalyzed Domino Cyclization for the Construction of Trispirocyclic Cyclohexanes

Manon Genet, Jérôme Marrot, Isabelle Chataigner, and Xavier Moreau*

An unprecedented gold(I)-catalyzed domino cyclization involving 2-ethynylbenzyl alcohol derivatives and heterocyclic α,β-unsaturated imines affords elaborated trispirocyclic cyclohexanes. This operationally simple protocol allows the one-pot generation of five bonds, three cycles, and four stereocentres

in a short amount of time. Twenty substrates are successfully engaged, affording the desired products with good diastereoselectivity. The formation of these trispirocyclic compounds over other possible products has been rationalized by (DFT) density functional theory calculations.

## 1. Introduction

Spirocyclic scaffolds are structural motifs found in countless natural products.[1] They have found applications in different fields including agrochemicals,[2] organic optoelectronics,[3,4] or in the chemistry of fragrances.[5] They have emerged as privileged structures for drug discovery as they define a rigid 3D space with specified physicochemical and pharmacological parameters.[6] These findings have prompted synthetic organic chemists to launch new research programs dedicated to the synthesis of new intricate spirocyclic architectures[7] or to the development of new building blocks used as promising isosteres for medicinal chemistry.[8–10] As a reliable strategy for accessing such molecular architectures, domino processes that involve consecutive cyclization reactions also have the advantage of being atom- and step-economic methods.[11] Among the diversity of strategies implemented, sequences initiated by a cycloisomerization reaction catalyzed by a gold complex leading to a transient reactive cyclic intermediate have emerged as a practical method for generating heterospirocyclic compounds.[12] Alkynes are relevant substrates for such transformations,[13] as they are readily activated in the presence of carbophilic π-Lewis acids to afford transient intermediates acting as partners in cycloaddition

reactions. Among others, this aspect has been clearly illustrated with the use of 2-ethynylbenzyl alcohol derivatives, paving the way for the formation of 5,6-benzannulated spiroketals. An early example described by Gong and coworkers involved a relay catalytic three-component reaction of alkynols, salicylaldehydes, and anilines with a catalytic system consisting of a Au(I) complex to trigger the cycloisomerization reaction, and a chiral phosphoric acid to promote spiroketalisation (Scheme 1a).[14] Soon after, Xu and coworkers developed a Gold/Lewis acid acid bimetallic relay catalysis strategy to construct spiroketal moieties. The reactive vinyl ether obtained from the Au(I)-catalyzed 5-exo-dig cyclization could react with unsaturated keto-esters (Scheme 1b)[15] or in situ generated o-quinone methides (Scheme 1c).[16] p-Quinone methide derivatives were reported by Shi[17] and Xu[18] as efficient partners in the Au(I)-promoted cycloisomerization/cycloaddition process (Scheme 1c). In 2018, Xu and coworkers detailed a gold(I)-mediated cascade reaction to synthesize 5,5-benzannulated spiroketals from alkynols and quinone monooimines (Scheme 1d).[19] More recently, Yang and Deng described an Au/Ir-catalyzed cycloisomerization/allylation/spiroketalization to afford enantioenriched spiroketals (Scheme 1e).[20] Different spiroheterocyclic scaffolds have also been synthesized using this strategy, but the number of examples is still very limited in the literature. Xu and coworkers reported a gold-catalyzed cascade reaction of alkynyl alcohols and alkynyl enones that afforded original spirocycles (Scheme 1f).[21] The synthesis of nitrogen-containing spiroheterocycles have also been targeted from alkynols and aziridines. Polycyclic edifices are obtained diastereo-[22] or enantioselectively,[23] depending on the catalyst involved in the reaction (Scheme 1g). In the present study, an unusual reactivity of cyclic enols obtained through gold-catalyzed cycloisomerization of 2-ethynylbenzyl alcohol derivatives and leading to novel trispiroheterocycles was investigated.

M. Genet, J. Marrot, X. Moreau

UVSQ

CNRS

UMR 8180 Institut Lavoisier de Versailles

Université Paris-Saclay

78035 Versailles Cedex, France

E-mail: xavier.moreau@uvsq.fr

I. Chataigner

INSA Rouen Normandie

Normandie Univ. COBRA

Université Rouen Normandie

76000 Rouen, France

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Supporting information for this article is available on the WWW under https://doi.org/10.1002/ceur.202500362

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© 2025 The Author(s). ChemistryEurope published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

## 2. Results and Discussion

Our laboratory has an extensive experience in the development of stereoselective cascade reactions yielding original polycyclic
<figure><img src="image_5.png" /></figure>

Previous works

<figure><img src="image_6.png" /><figcaption>(b) With unsaturated keto-esters</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(c) With quinone methides</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(d) With quinone imines</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>(e) With π-allyl Ir</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>(f) With alkynyl enones</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(g) With aziridines</figcaption></figure>

<figure><img src="image_12.png" /></figure>

This works: with α,β-unsaturated imines

<figure><img src="image_13.png" /></figure>

**Scheme 1.** Gold catalyzed cascade reaction with 2-ethynylbenzyl alcohol derivatives and: a) salicylaldehydes and anilines, b) unsaturated keto-esters, c) quinone methides, d) quinone imines, e) π-allyl Ir, f) alkynyl enones, g) aziridines.

<figure><img src="image_14.png" /><figcaption>**Table 1.** Optimization of the reaction conditions.</figcaption></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<table><thead><tr><th>Entry</th><th>Deviations from above</th><th>Yield<sup>a)</sup> of 3a [%]</th><th>d.r.<sup>b)</sup></th></tr></thead><tbody><tr><td>1</td><td>None</td><td>44</td><td>5/1</td></tr><tr><td>2</td><td>1.5 equiv. of 2</td><td>27</td><td>6/1</td></tr><tr><td>3</td><td>2.5 equiv. of 2</td><td>34</td><td>5/1</td></tr><tr><td>4</td><td>Ph<sub>3</sub>PAuCl/AgPF<sub>6</sub> instead of PPh<sub>3</sub>AuNTf<sub>2</sub></td><td>28</td><td>6/1</td></tr><tr><td>5</td><td>Me<sub>3</sub>PAuCl/AgPF<sub>6</sub> instead of PPh<sub>3</sub>AuNTf<sub>2</sub></td><td>31</td><td>6/1</td></tr><tr><td>6</td><td>Toluene instead of CH<sub>2</sub>Cl<sub>2</sub></td><td>18</td><td>7/1</td></tr><tr><td>7</td><td>THF instead of CH<sub>2</sub>Cl<sub>2</sub></td><td>38</td><td>5/1</td></tr><tr><td>8</td><td>5 mol% of PPh<sub>3</sub>AuNTf<sub>2</sub></td><td>38</td><td>6/1</td></tr><tr><td>9</td><td>2.5 equiv. of 2, 5 mol% of PPh<sub>3</sub>AuNTf<sub>2</sub>, addition of substrates over 10 min<sup>c)</sup></td><td>49</td><td>6/1</td></tr></tbody></table>

architectures, and we wondered whether cyclic unsaturated imines would be suitable partners in this scenario. We aimed to investigate the reactivity of indoline-derived azadienes **1** because of their ability to react in different ways depending on the reaction partners or the conditions used. These

electrophiles react as azadiene (C-4 synthon) in (4 + n) cyclization reactions<sup>[24–29]</sup> or as electrodeficient alkenes (C-2 building block) in (2 + n) cyclization reactions.<sup>[28–32]</sup> α-β-Unsaturated imine **1a** and 2-ethynylbenzyl alcohol **2a** were chosen as model substrates for the Au(I)-mediated cycloisomerization/cycloaddition

<footnote><sup>a)</sup>Yields of isolated pure major stereoisomer **3a** after column chromatography; <sup>b)</sup>The dr was determined by ¹H-NMR analysis of crude mixtures; <sup>c)</sup>A solution of **1a** (0.1 mmol) and **2a** (0.25 mmol) in CH<sub>2</sub>Cl<sub>2</sub> (0.8 mL) was added over 10 min to a solution of PPh<sub>3</sub>AuNTf<sub>2</sub> (5 mol%) in CH<sub>2</sub>Cl<sub>2</sub> (0.2 mL) at room temperature and the solution was stirred for further 10 min.</footnote>
<figure><img src="image_17.png" /><figcaption>**Scheme 2.** Scope and limitation of the transformation involving indoline-derived azadienes 1. Conditions: A solution of **1a** (0.1 mmol) and **2a** (0.25 mmol) in CH₂Cl₂ (0.8 mL) was added over 10 min to a solution of PPh₃AuNTf₂ (5 mol%) in CH₂Cl₂ (0.2 mL) at room temperature and the solution was stirred for further 10 min. The given yields referred to the isolated yield of the major diastereomer. The dr was determined by ¹H-NMR analysis of crude mixtures. [a] dr could not be determined.</figcaption></figure>
sequence. We first selected triphenylphosphine gold(I) bis-(trifluoromethanesulfonyl)imide (Ph₃AuNTf₂, 10 mol%) as catalyst and the reaction was conducted in dichloromethane at room temperature for 1 h (Table 1, entry 1). While we initially thought we were triggering a (4 + 2) cyclization reaction between **1a** and compound **2a**, we were stunned to observe the formation of the more complex polycyclic architecture **3a** containing 2 equivalents of 2-ethynylbenzyl alcohol **2a**. The trispirocyclic molecule **3a** was obtained as a 5/1 mixture of diastereomers and the major one was isolated in 44% yield.[33] The amount of **2a** involved in the reaction, whether increased (entry 2) or decreased (entry 3), had an impact on the efficiency of the reaction and led to lower yields, but had no impact on the stereoselectivity. The results were not improved either by the use of different types of gold catalyst (entries 4 and 5) nor by the use of other solvents (entries 6 and 7). Lowering the catalytic loading from 10 to 5 mol% only slightly affected the yield. Finally, optimum conditions were achieved with a dropwise addition of a solution of substrates

**1a** (1 mmol) and **2a** (2.5 equiv.) in dichloromethane to a solution containing catalyst PPh₃AuNTf₂ (5 mol%). This procedure afforded **3a** in 49% isolated yield of the major diastereomer. It should be pointed out that the moderate yields obtained under these different conditions can be ascribed to the fact that the crude reaction was an intricate mixture, and the separation of the diastereomers was not trivial. We can consider that these yields are acceptable regarding the complexity and originality of the obtained polycycles. Moreover, it is rather complicated to design an alternative route following traditional disconnections.

With the best conditions in hands, the scope of the transformation regarding the substituents (R group) bore by α,β-unsaturated imines **1** was explored (Scheme 2). In all cases, only two diastereomers were identified in the crude mixture, but for some examples we were unable to determine the diastereomeric ratio. Nevertheless, the major diastereomer has always been separated from the reaction mixture and isolated. We initially focused on the reactivity of α,β-unsaturated imine **1b-1d** decorated with a

<figure><img src="image_18.png" /><figcaption>**Scheme 3.** Scope and limitation of the transformation involving the benzofuran-derived azadienes **4**. Conditions: A solution of **1a** (0.1 mmol) and **2a** (0.25 mmol) in CH₂Cl₂ (0.8 mL) was added over 10 min to a solution of PPh₃AuNTf₂ (5 mol%) in CH₂Cl₂ (0.2 mL) at room temperature and the solution was stirred for further 10 min. The given yields referred to the isolated yield of the major diastereomer. The dr could not be determined.</figcaption></figure>
substituted aromatic ring on the benzylidene moiety. Regardless of the position (ortho, meta, para-position), the introduction of a methyl group did not influence the selectivity and the major stereoisomer of polycyclic scaffolds **3b**, **3c**, and **3d** were isolated in 33%, 63%, and 38% yield, respectively. We were not able to rationalize this difference, except that the purification of compound **3c** was easier compared to **3b** and **3d**. α,β-Unsaturated imines **1e**, **1f**, or **1g** bearing respectively a bromine atom, a trifluoromethyl group, or a methoxy group at the *para*-position of the aromatic ring were also amenable to the sequence by providing compounds **3e**, **3f**, or **3g** with a good level of diastereoselectivity (3:1 to 4:1 dr). In each case, the major diastereomer was isolated in acceptable isolated yield ranging from 34% to 43% which indicated that substituents with electronic effects of different nature led to similar results. Excellent results were observed when imines bearing a naphthyl substituent (**1h**) or a furan (**1i**) underwent the transformation and the corresponding products **3h** and **3i** were isolated with 60% and 65%, respectively.

These convincing results should not overshadow certain limitations. Replacing the aryl group by an alkyne unit afforded the expected product **3j** with a disappointingly low yield (13%). The functionalization of the indoline moiety is mandatory as no trace of product **3k** were detected when imine **1k** with a N-H indoline moiety were engaged in the reaction. Similarly, no reactivity was observed when the tosyl group on the imine nitrogen atom was replaced by a nosyl group. Finally, the substrate scope of 2-ethynylbenzyl alcohol derivatives **2** was also explored. When the alkyne was substituted with a methyl group in the 4-position, the reaction proceeded smoothly and the major diastereomer of the spirocyclic compound **3m** was obtained in an excellent yield of 81% (5:1 dr). In contrast, with a chlorine atom in the 4-position, the yield dropped to 21% concomitantly with disappointing diastereoselectivity (**3n**, 1.5:1 dr). Finally, we were pleased to see that a methoxy substitution in the 5-position was also very well tolerated, with the product **3o** being obtained in 63% yield.

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>**Scheme 4.** a) Proposed mechanism of the domino reaction and b) DFT calculations (ΔG⁺ and ΔGr in kcal mol⁻¹).</figcaption></figure>
<figure><img src="image_21.png" /></figure>

To extend the scope of this transformation, we investigated whether benzofuran-derived azadienes 4, commonly used as C-4 synthons in the construction of novel heterocyclic scaffolds,[34] could also serve as C-2 building blocks under the optimized reaction conditions. We were delighted to observe that the α,β-unsaturated imine 4a was amenable to the reaction conditions and gave rise to the polyspirocyclic compound 5a in 60% isolated yield as a single diastereomer (Scheme 3). It should be mentioned that the crude reaction was a complex mixture. Consequently, we were unable to determine the diastereomeric ratio of products 5. Nevertheless, during the study of the scope of the reaction, we always succeeded in isolating each polycyclic compound as a single diastereomer α,β-Unsaturated imines 4 bearing an electron-donating group (methyl or methoxy substituents) were well tolerated and afforded the expected compounds 5b and 5c in moderate to good yields (respectively 26% and 40% yield). Similarly, the polyspirocyclic compound 5d decorated with a bromine atom was isolated in 36% yield. However, using the α,β-unsaturated imine 4e substituted by an electron-withdrawing methyl ester strongly affected the outcome of the reaction as we were unable to isolate the expected product 5e. The modification of the dihydrobenzofuran skeleton with different substituents at different positions was also considered. Compound 5f bearing de methoxy group at the 5-position was obtained in 33% yield. α,β-Unsaturated imine 4g having a methyl group at the 7-position afforded 5g in 37% yield. The resulting crystalline solid was suitable for X-ray diffraction and confirmed the structure and the relative configuration of this intricate framework.[33] Finally, compound 5h with a methyl group in the 6-position was synthesized in a disappointing yield of 15%.

In this transformation, the anticipated (4 + 2) cyclization reaction mechanism appears to be outweighed by a domino reaction involving the addition of a second equivalent of the intermediate A, resulting from the cycloisomerization reaction of compound 2, followed by a cyclization step leading to the trispirocyclic cyclohexane motif (Scheme 4a). To get insights into this unforeseen reaction pathway, DFT calculations were undertaken considering N–Ms azadiene as a realistic model of azadiene 1 in the presence of the intermediate A resulting from the cycloisomerization reaction of compound 2 (Scheme 4b). The nucleophilic addition of the transient enol ether A to the N–Ms azadiene leading to intermediate B is characterized by a free activation energy of 22.8 kcal mol⁻¹ and an endergonicity of 11.2 kcal mol⁻¹. At this stage, two routes can be considered. On the one hand, intermediate B undergoes cyclization to form spirocyclic compound 6 (−4.2 kcal mol⁻¹) with a low energy cost of 1.2 kcal mol⁻¹ (TS₂, 12.4 kcal mol⁻¹). On the other hand, a second nucleophilic addition of enol-ether A to intermediate B affords intermediate C (8.8 kcal mol⁻¹) with a higher energy cost of 13.9 kcal mol⁻¹ (TS₃, 25.1 kcal mol⁻¹) and this step is less exergonic than the previous one leading to compound 6. The subsequent cyclization is straightforward. This step is almost barrierless and only requires 0.8 kcal mol⁻¹ of free energy of activation (TS₄, 9.6 kcal mol⁻¹) and is highly exergonic by 29.4 kcal mol⁻¹. In view of these DFT computational evidence, we can postulate that the step leading to the more kinetically

favored compound 6 is reversible and allows the formation of the more thermodynamically stable compound 3.

### 3. Conclusion

We have explored an unprecedented domino polycyclization from readily available 2-ethynylbenzyl alcohol derivatives and heterocyclic α,β-unsaturated imines. This gold(I)-catalyzed reaction enables the concomitant formation of five bonds, three cycles, and four stereocentres to yield complex architectures in a single operation. A range of trispirocyclic cyclohexanes scaffolds is obtained in diastereoselective manner. DFT calculations have shed the light on a plausible mechanism for this transformation.

### Supporting Information

The authors have cited additional references within the Supporting Information.[35–40]

### Acknowledgements

This work was funded by the French Ministère de l'Enseignement Supérieur et de la Recherche (M.G.). The authors also thank Université de Versailles-Saint Quentin-en-Yvelines, Université Rouen Normandie, and CNRS for financial support. Criann is also acknowledge for allocation of computer time. Aurélie Damond and Flavien Bourdreux are warmly acknowledged for their support with mass spectrometry and NMR spectroscopy.

### Conflict of Interest

The authors declare no conflict of interest.

### Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** benzofuran-derives azadienes · domino reactions · gold catalysis · indoline-derived azadiene · spirocyclic compounds

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Manuscript received: September 5, 2025

Revised manuscript received: October 1, 2025

Version of record online: