Enantioselective synthesis of chiral multicyclic γ-lactones via dynamic kinetic resolution of racemic γ-keto carboxylic acids

Abstract

Ru-Catalyzed asymmetric transfer hydrogenation of γ-keto carboxylic acids has been achieved by using the formic acid–triethylamine azeotrope as the hydrogen source, affording chiral multicyclic γ-lactones in high yields with excellent diastereo- and enantioselectivities (up to 92% yield, >20 : 1 dr and 99% ee). This method provides a highly efficient approach to obtain valuable multicyclic γ-lactones through a reduction/lactonization sequence. Moreover, a concise synthetic route to obtain bioactive molecules (+)-GR24 and (+)-epi-GR24 has also been developed by using this methodology as a key step.

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Multicyclic lactones are prestigious scaffolds widely occurring in natural products,¹ drugs,² and synthetic bioactive molecules,³ and have exhibited some important bioactivities.⁴ Particularly, strigolactones, important plant signalling molecules containing a tricyclic γ-lactone core, widely exist in the roots of plants (Fig. 1), and play important roles in the mediation of vital biological functions in plants.⁵ The typical bioactivities include seed germination,⁶ inhibition of shoot-branching,⁷ regulation of plant architecture and the response to abiotic factors such as nutrient availability and light,⁸ and root-colonization by symbiotic arbuscular mycorrhizal fungi.⁹ However, the natural abundance of strigolactones in plants is very low, which greatly limits the studies and applications of strigolactones. Therefore, the synthesis of strigolactones and their derivatives has attracted wide attention, and several approaches have been developed.¹⁰ Nevertheless, asymmetric synthetic routes are still rare, and the development of efficient methods for the construction of chiral molecules with a tricyclic γ-lactone core is urgent.

Fig. 1 Representative strigolactones and their analogues.

Owing to the high efficiency in the preparation of optically pure molecules from racemic compounds, dynamic kinetic resolution (DKR) has emerged as a powerful strategy for the construction of chiral molecules.^11,12 In particular, dynamic kinetic asymmetric hydrogenation of racemic α-substituted ketones has proved to be an efficient and reliable approach to obtain chiral alcohols with two contiguous stereocenters.¹³ In this context, the dynamic kinetic resolution of α-keto esters and β-keto esters has been widely investigated to prepare functionalized α-/β-hydroxyl esters or their derivatives.¹⁴ Among them, Johnson reported his pioneering studies on dynamic kinetic asymmetric transfer hydrogenation of the well-designed α-keto esters to synthesize multiply substituted γ-butyrolactones through a reduction/lactonization sequence (Scheme 1-1).^14f,g Unfortunately, the substrate scope of this method is limited to β-aryl α-keto esters, which greatly restricted its applications in the synthesis of multicyclic γ-lactones. Recently, McErlean reported one example of dynamic kinetic resolution of cyclic γ-keto esters to synthesize tricyclic γ-lactones.^10d Nevertheless, this approach needs two steps to achieve this transformation, and the additive pyridinium para-toluenesulfonate (PPTS) is essential in the lactonization step. Therefore, the direct construction of tricyclic γ-lactones by dynamic kinetic resolution is still a challenge. As alternative substrates of γ-keto esters, the dynamic kinetic resolution of γ-keto carboxylic acids provides a potential access to γ-lactones. However, in comparison with keto esters, the analogous resolution of keto carboxylic acids has remained essentially undeveloped, despite its extensive potential synthetic utility in organic synthesis. Herein, we report the highly enantioselective synthesis of chiral multicyclic γ-lactones via Ru-catalyzed asymmetric transfer hydrogenation of racemic γ-keto carboxylic acids (Scheme 1-2).

Scheme 1 . Asymmetric synthesis of γ-lactones by dynamic kinetic resolution.

Initially, we chose asymmetric transfer hydrogenation of 2-(1-oxo-2,3-dihydro-1H-inden-2-yl)acetic acid (1a) as a model reaction to optimize the reaction conditions (Table 1). When the reaction was conducted in THF at 60 °C using the HCOOH–Et₃N (5 : 2) azeotrope as the hydrogen source, a series of catalysts were tested. To our delight, the dynamic kinetic resolution of 1a proceeded very well in the presence of Cat. A, giving tricyclic γ-lactone 2a in moderate yield with high enantioselectivity (entry 1). Then Cat. B with substituents on the aromatic ring was tested, and the yield and enantioselectivity increased slightly (entry 2). Subsequently, Cat. C, a tethered catalyst developed by Wills’ group,¹⁵ was employed in the dynamic kinetic resolution of 1a, and the target product was obtained with stereospecific syn-selectivity in high yield with excellent enantioselectivity (entry 3). When a Rh or Ir catalyst was used instead of a Ru catalyst for this transformation, the yield decreased to some extent, suggesting that the Ru catalyst was more suitable for this kind of reduction (entries 4 and 5). We also tested the variation in the molar ratio of formic acid/trimethylamine, and the results showed that reducing the ratio of formic acid is detrimental to the yield (from 5 : 2 to 3 : 2 and 1 : 1).

Table 1 Catalyst screening^a

Entry	Catalyst	Yield^b (%)	ee^c(%)	dr^d
a All reactions were carried out by using 1a (0.16 mmol), catalyst (0.0032 mmol), 5 : 2 HCO2H/Et3N (1 mL) under argon protection at 60 °C. b Isolated yield. c Determined by chiral HPLC analysis using a Chiralpak AD-H column. d Determined by NMR. e 3 : 2 HCO2H/Et3N was used. f 1 : 1 HCO2H/Et3N was used.
1	Cat. A	65	90	>20 : 1
2	Cat. B	70	95	>20 : 1
3	Cat. C	90	98	>20 : 1
4	Cat. D	58	95	>20 : 1
5	Cat. E	65	93	>20 : 1
6^e	Cat. C	45	95	>20 : 1
7^f	Cat. C	30	93	>20 : 1

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Subsequently, the solvent effect was investigated, and the results disclosed that the solvents have no influence on the diastereo- and enantioselectivities (Table 2), and all solvents tested in the experiments afforded the target products with excellent diastereo- and enantioselectivities (>20 : 1 dr, 98–99% ee). However, the yield of γ-lactone was greatly affected by the solvents. When the reaction was conducted in alcohol or ether solvents, it proceeded very smoothly, affording multicyclic lactone 2a with high yields (entries 1–4). When toluene or acetonitrile was employed as a solvent, only moderate yields were obtained (entries 5 and 6). N,N-Dimethylformamide was also a good solvent for this reaction (entry 7). Interestingly, the azeotropic mixture of HCOOH/Et₃N, the hydrogen source of this reaction, was employed as a solvent, and the reaction worked very well, furnishing the desired product with high yield and excellent stereoselectivity (entry 8). Considering the reaction results and easy operation, the azeotropic mixture of HCOOH/Et₃N (molar ratio = 5 : 2) was chosen as the best solvent. Subsequently, the reaction temperature was evaluated, and the results disclosed that the lower temperature resulted in the yield dropping significantly, even if the reaction time was prolonged.

Table 2 Solvent screening^a

Entry	Solvent	Yield^b (%)	ee^c (%)	dr^d
a Unless otherwise noted, all reactions were carried out with a substrate/catalyst ratio of 50 : 1 at 60 °C for 12 h. b Isolated yield. c Determined by HPLC analysis using a chiral stationary phase. d Determined by ^1H NMR spectroscopy. e 1 mL HCO2H/Et3N (5 : 2) mixture was used. f Room temperature, 24 h. g 40 °C, 18 h.
1	MeOH	83	98	>20 : 1
2	EtOH	75	98	>20 : 1
3	THF	85	98	>20 : 1
4	Dioxane	82	98	>20 : 1
5	CH3CN	40	98	>20 : 1
6	Toluene	44	98	>20 : 1
7	DMF	89	98	>20 : 1
8^e	—	90	98	>20 : 1
9^e^,^f	—	65	99	>20 : 1
10^e^,^g	—	80	98	>20 : 1

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Under the optimal reaction conditions, a series of γ-keto carboxylic acids were hydrogenated to evaluate the substrate scope of this reaction. As shown in Table 3, the reaction has a good substrate generality, and the electronic properties and the position of the substituent on the benzene ring have no effect on the yield and diastereo- and enantioselectivities; all of them furnished the desired product with high yields and excellent stereoselectivities (2a–2f).¹⁶ Changing the size of the cycloketone to a six- or seven-membered ring, the reaction proceeded smoothly, giving the target products 2g–2l in good yields with excellent enantio- and diastereoselectivities.

Table 3 Substrate scope^a

a All reactions were carried out with a substrate/catalyst ratio of 50 : 1 at 60 °C for 12 h. b Isolated yield. c Determined by HPLC analysis using a chiral stationary phase. d Determined by ¹H NMR spectroscopy.

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To demonstrate the utility of this protocol, a gram-scale hydrogenation of 1a was conducted in the presence of 1 mol% catalyst loading, and it proceeded very efficiently, giving 2a in high yield and excellent stereoselectivity (Scheme 2a, 85% yield, >20 : 1 dr, 98% ee). In addition, (+)-GR24 and (+)-epi-GR24, the analogues of strigolactones but with higher germination activity and better stability than natural strigolactones, were efficiently synthesized. As shown in Scheme 2b, 2a reacted with HCOOMe smoothly in the presence of KO^tBu, and then reacted with bromobutenolide 3, delivering (+)-GR24 and (+)-epi-GR24 in 35% yield respectively (Scheme 2b).

Scheme 2 Gram-scale reaction and derivative reaction.

In summary, we have developed an efficient and practical methodology for the synthesis of chiral multicyclic γ-lactones by a tethered Ru(II) complex catalyzed asymmetric transfer hydrogenation of γ-keto carboxylic acids. The reaction featured a wide substrate scope, high yields, and excellent enantio- and diastereoselectivities. Furthermore, the gram-scale reaction and efficient synthesis of (+)-GR24 and (+)-epi-GR24 disclosed that the method has potential application in the synthesis of biologically active compounds bearing a multicyclic lactone core. Further investigations on asymmetric hydrogenation initiated cascade reactions are underway in our laboratory.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (Grant No. 21871212, 21402145), the Natural Science Foundation of Hubei Province (2018CFB430), the Open fund of CAS Key Laboratory of Molecular Recognition and Function (2017LMRF002), and the “111” Project of the Ministry of Education of China, JSGG (20160608140847864).

Notes and references

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16. The absolute configuration of compound 2a has been established by comparison with the literature data (for 2a in ref. 10d: ([α]²⁰_D = 110 (c 0.985, CHCl₃), 99% ee, our experiment result of 2a: [α]²⁰_D = 100 (c = 0.985, CHCl₃) (98% ee)). All the other configurations are uncertain and based on the assumption that the configuration follows that of 2a.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and NMR spectra of compounds. See DOI: 10.1039/c9qo01047e

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