Broad scope gold(I)-catalysed polyenyne cyclisations for the formation of up to four carbon–carbon bonds

Abstract

The polycyclisation of polyenynes catalyzed by gold(I) has been extended for the first time to the simultaneous formation of up to four carbon–carbon bonds, leading to steroid-like molecules with high stereoselectivity in a single step with low catalyst loadings. In addition to terminal alkynes, bromoalkynes can also be used as initiators of polyene cyclisations, giving rise to synthetically useful cyclic bromoalkenes.

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Gold(I)-catalyzed cycloisomerisations of 1,n-enynes as well as the reactions of these substrates with many nucleophiles allow the construction of complex carbo- and heterocyclic compounds by the selective activation of the alkyne in the presence of many other functional groups.¹ These transformations have been used as the key steps in the total synthesis of diverse natural products.^2,3 In most cases, such as in substrates 1a,⁴ nucleophilic additions to 1,5-enynes proceed by an overall 6-endo-dig/endo-trig process leading to the formation of cyclohexenes 2avia a bicyclic gold(I) carbene intermediate.^5,6 However, hydroxy-1,5-enyne 1b reacts in the presence of AuCl₃ to exclusively form cyclopentene 2b by a 5-endo-dig/exo-dig cyclisation in which the alcohol adds to the alkene with an anti-Markovnikov regioselectivity (Scheme 1).⁴ Similar transformations have been reported with 1,6-enynes bearing hydroxyl⁷ or carboxylic acid groups at the alkenyl chain.^8,9

Scheme 1 Gold(I)-catalysed intramolecular heterocyclisation of 1,5-enynes 1a–b.⁴

Remarkable examples of gold(I)-catalysed cyclisations in which up to 2–3 carbon–carbon bonds were formed had been reported by Toste,⁹ Michelet,¹⁰ and by other groups.¹¹ Thus, the 6-exo-dig/endo-trig cyclisation of 3 with a chiral gold(I) catalyst leads to tetracyclic compound 4 in a highly enantioselective process (Scheme 2).^9,12,13 Similar intriguing is the 6-endo-dig/endo-trig cyclisation of 5, which is terminated by trapping of the cationic intermediate by the phenol to form 6.^10a Although highly ordered, concerted mechanisms have been proposed for these polycyclisations,⁸ step-wise processes have been suggested for reactions involving external nucleophiles and in other processes.^1j,3d,11a,14

Scheme 2 Gold(I)-catalysed polycyclisations of dienynes 3 and 5.

Recently, the group of Gagné reported the polycyclisation of pentanene 7 with a pincer-platinum(II) catalyst to give steroid-like product 8 (Scheme 3).¹⁵ This fascinating transformation, clearly reminiscent of sterol biosynthesis from squalene in bacteria,¹⁶ allows the formation of four C–C bonds in a single step.¹⁷

Scheme 3 Platinum(II)-catalysed polycyclisation of pentanene 7.

We decided to explore the possibility of performing gold(I)-catalysed cyclisations analogous to that of pentanene 7 but with a terminal alkyne instead of an alkene to form steroid-like products functionalised with an alkene at the A ring. As a first step towards this goal, we studied a set of 1,5-enynes substituted with different alcohols, phenols, arenes, and heteroarenes as potential nucleophiles. In addition to terminal alkynes, which were not broadly studied previously,^10,18 we decided to employ 1-bromo-1,5-enynes as the initiators of cyclisation. Surprisingly, bromoalkynes have seldom been used in gold-catalysed cyclisations.^19,20

At the outset, we examined the cyclisation of (E)-2,6-dimethyldeca-1,5-dien-9-yne (9a) with gold(I) catalysts A–F bearing electronically different bulky groups (Table 1). In all cases, trans-fused hexahydronaphthalene 10a was cleanly obtained as the major product after 1 h by using just 1 mol% catalyst. As we have observed before in other contexts, the best yields were obtained with cationic gold(I) complexes bearing very bulky biphenylphosphine ligands (Buchwald ligands).^1j,21 In this particular instance, cationic dicyclohexylphosphinobiphenyl gold(I) complex C outperforms Johnphos, t-BuXphos, and Xphos complexes A, B, and D (Table 1, entries 1–4).

Table 1 Gold(I)-catalysed cyclisation of dienyne 9a^a

Entry	Catalyst	Yield^b (%)
a Reactions carried out with 1a (0.3 mmol), catalyst (3 μmol) in CH2Cl2 (3 mL) at 23 °C for 1 h. b Isolated yields.
1	A	84
2	B	45
3	C	90
4	D	41
5	E	61
6	F	45

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Complex C (3 mol%) was used as the catalyst in the cyclisation of aryl substituted 1,5-enynes (Table 2). The reaction of substrates 9b–f bearing electron-rich aromatic and heteroaromatic rings as cyclisation terminators proceeds to give products 10b–f as single diastereomers in good yields in all cases, with the exception of 10c, which was obtained in 54% yield (Table 2, entries 1–5). Similar results have been obtained with different metal catalysts using 1,5-enynes analogous to 9b–c with a methyl substituent at the terminal alkyne.^10b,12b,13 The trans-relative configuration was confirmed by X-ray diffraction in the case of indole derivative 10f (Fig. 1), which has a carbon skeleton somewhat related to that of the alkaloids aristomakinine and aristomakine, although for these natural products a cis-hexalin structure has been assigned.²² The gold(I)-catalysed cyclisation of 1,5-enynes 9g–h substituted at C-5, gave spirocyclic derivatives 10g–h in good to excellent yields (Table 2, entries 6 and 7). The structure of 10h was confirmed by X-ray diffraction (Fig. 1).

Fig. 1 ORTEP plot (50% thermal ellipsoids) of the crystal structure of 10f (left) and 10h (right).

Table 2 Gold(I)-catalysed cyclisation of aryl or heteroaryl 1,5-enynes 9b–h^a

Entry	Substrate	Product	Yield^b (%)
a Reactions carried out with catalyst C (3 mol%) in CH2Cl2 (0.1 M) at 23 °C for 1 h. b Isolated yields.
1			95%
2			54%
3			79%
4			80%
5			95%
6			75%
7			98%

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We also tested several chiral gold(I) catalysts in the cyclisation of enynes 9d and 9f.^23,24 However, despite the excellent cyclisation yields, the enantioselectivities achieved with the dinuclear gold(I) complex of [MeO–DTB–BIPHEP] in the presence of AgNTf₂ were only moderate (54 and 48% ee, respectively) (Scheme 4).²⁵

Scheme 4 Gold(I)-catalysed enantioselective cyclisation of 9d and 9f.

The intramolecular addition of alcohols and phenols was briefly studied with substrates 9i–k (Table 3). As expected considering the precedents,¹⁰ products 10i–k were obtained in good to excellent yields. Spirocyclisations similar to that of 9i to form 10i could be applied for the synthesis of analogues of the natural product filifolinol and other more complex, biologically active compounds with a spirobenzofuran structure.²⁶

Table 3 Gold(I)-catalysed cyclisation hydroxyl-cyclisation of 9i–k^a

Entry	Substrate	Product	Yield^b (%)
a Reactions carried out with catalyst C (3 mol%) in CH2Cl2 (0.1 M) at 23 °C for 1 h. b Isolated yields. c 1 mol% catalyst C.
1			96%
2^c			75%
3			89%

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The cyclisation of 1-bromo-1,5-enynes 9l–n and 1-bromo-1,5,9-dienyne 9o with catalyst C took place uneventfully under the usual reaction conditions to give products 10l–o in good yields (Table 4). These results show that bromoalkynes are perfectly suitable initiators of gold(I)-catalysed polycyclisations. The final products are bromoalkenes, which could be further functionalized by metal-catalysed cross-couplings, carbonylations, or by other methods.

Table 4 Gold(I)-catalysed cyclisation of 1-bromo-1,5-enynes 9l–o^a

Entry	Substrate	Product	Yield^b (%)
a Reactions carried out with catalyst C (3 mol%) in CH2Cl2 (0.1 M) at 23 °C for 1 h. b Isolated yields.
1			92%
2			88%
3			68%
4			78%

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The polycyclisation of trienynes 9p–q and tetraenynes 9r–s was similarly performed with catalyst C (1–3 mol%) to give tri- and tetracyclic compounds 10p–s (Table 5). Considering that four C–C bonds are formed in a single step, the catalytic transformations of tetraenynes 9r–s into 10r–s are quite remarkable and comparable to that achieved by Gagné in the Pt(II)-catalysed polycyclisation of pentanene 7 (Scheme 3).^15,27 However, in our case a lower catalyst loading is required and the final tetracyclic derivatives 10r–s feature two differently substituted double bonds. In 10s, the alkenyl bromide offers a handle for further functionalisation of the A-ring.

Table 5 Gold(I)-catalysed cyclisation of tri- and tetraenynes 9p–s^a

Entry	Substrate	Product	Yield^b (%)
a Reactions carried out with catalyst C (3 mol%) in CH2Cl2 (0.1 M) at 23 °C for 1 h. b Isolated yields. c 1 mol% catalyst C. d Reaction at 0 °C.
1^c			90%
2			84%
3^c			59%
4^c^,^d			51%

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Presumably, the cyclisation of 9s to give 10s proceeds by the initial formation of gold(I)-carbene intermediate^5,11 which triggers a cascade process to form secondary carbocation 12 (Scheme 5). The final tetracyclic compound 10s is then formed by Wagner–Meerwein 1,2 H and Me migrations,^15,28 followed by the proton elimination and protonolysis of the alkenyl-gold(I) bond.

Scheme 5 Mechanism for the formation of 10s.

In summary, building upon previous studies,^9,10 we have extended the polycyclisation of polyenynes up to the simultaneous formation of four C–C bonds. These reactions are performed under mild conditions with low catalyst loadings (1–3 mol%) of a cationic dicyclohexylphosphinobiphenyl gold(I)complex with a weakly coordinating acetonitrile ligand. In addition to terminal alkynes, we have also found that bromoalkynes can be used as the initiators of polyene cyclisations, leading to synthetically useful cyclic bromoalkenes. Further work on the development of broad scope and practical solutions of the asymmetric polycyclisation of polyeneynes is underway.

Acknowledgements

We acknowledge funding from MINECO/FEDER, UE (CTQ2016-75960-P), MINECO-Severo Ochoa Excellence Accreditation 2014–2018, (SEV-2013-0319), the European Research Council (Advanced Grant No. 321066), the AGAUR (2014 SGR 818), and CERCA Programme/Generalitat de Catalunya. We also thank the ICIQ X-ray diffraction unit and CELLEX-ICIQ HTE laboratory.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1530340 and 1530341. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ob00235a

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