Synthesis of chiral spiro-indenes via Pd-catalyzed asymmetric (4 + 2) dipolar cyclization
Abstract
Chiral indene skeletons are widely found in biologically active natural products and pharmaceutical molecules, making indene synthesis an ongoing research hotspot in organic synthetic chemistry. However, the construction of chiral spiro-indenes bearing all-carbon quaternary stereocenters via catalytic asymmetric synthesis remains challenging due to their inherent rigidity and hindrance. Herein, we present a solution to this unmet challenge through palladium-catalyzed asymmetric (4 + 2) dipolar cyclization by trapping π-allyl-Pd 1,4-dipoles with indene-involved ketenes generated in situ from 1-diazonaphthalene-2(1H)-ones via visible light-induced Wolff rearrangement. This protocol features mild reaction conditions, wide substrate scope, and high enantio- and diastereoselectivities [31 examples, up to 86% yield, 97% enantiomer excess (ee) and 19:1 diastereoisomer ratio (dr)].
Keywords
INTRODUCTION
Indenes bearing chiral quaternary stereocenters with unique carbocyclic structures serve as the key structural unit for many biologically active natural products and pharmaceutical agents[1-5]. As shown in Figure 1A, Dalesconol A and B display immunosuppressive activities comparable to the clinically used Cyclosporine A[6-8]. Cyanosporaside A and B from the marine actinomycete genus Salinispora show significant antimicrobial activity[9,10], and Dichroanal B is reported to have anti-inflammatory and anti-tumor activities[11]. In addition, indenes with unique three-dimensional structures and rigid skeletons are widely used as novel pharmacophores, e.g., in Chemokine[5]. Accordingly, many efforts have been devoted to constructing this important scaffold, which could be divided into two synthetic strategies. One is the construction of a chiral indene ring using aromatic compounds as substrates, including intramolecular cyclization[12-15] and intermolecular alkyne insertion[16-22] [Figure 1B]. Several elegant examples have been reported for accessing chiral indenes bearing quaternary stereocenters via this method through transition-metal-catalyzed domino reaction[23-29]. The other strategy involves direct enantioselective functionalization of the indene ring, such as benzofulvenes[30-32] and other indene derivatives[33,34]. However, generating chiral spiro-indenes bearing all-carbon quaternary stereocenters remains challenging, and only a few examples have been reported[23,24,29-33]. Some of them even suffer from the harsh reaction conditions, low efficiency and structural limitation of the substrate, which could be attributed to the rigid ring and steric hindrance of the indene skeleton, making chiral indene synthesis difficult. Furthermore, it is difficult to control the spiro-quaternary stereocenters.
Palladium-catalyzed dipolar cyclizations have proven to be one of the most unique and powerful strategies for synthesizing various heterocycles in a highly stereoselective manner. Since 2017, we have launched a program on developing asymmetric dipolar cyclizations for efficient construction of structurally diverse heterocycles by combining transition metal catalysis and photo-Wolff rearrangements[35]. Starting from readily available linear α-diazoketones, 5-10 membered oxa- and aza-heterocyclic products bearing chiral quaternary stereocenters have been constructed[36]. Following these successes, it is hypothesized that
EXPERIMENTAL
Under argon atmosphere, a flame-dried 10 mL Schlenk tube was charged with Pd2(dba)3·CHCl3
RESULTS AND DISCUSSION
First, we determined the ultraviolet-visible (UV-vis) absorption spectra of 1-diazonaphthalen-2(1H)-ones with different electronic substituents (see Supplementary Figure 1 for details). Optically, this kind of cyclic
Figure 2. Condition optimization. Conditions: 1a (0.1 mmol), 2a (0.2 mmol), Pd2(dba)3·CHCl3 (5 mol%) and chiral ligand (20 mol%) in 2.0 mL anhydrous DCM at rt under the irradiation of 6 W blue LEDs for 24 h. aDetermined by analyzing the 1H NMR of reaction mixture with 1,3,5-trimethoxybenzene as an internal standard. bDetermined by chiral HPLC analysis of the purified products. cDetermined by 1H NMR analysis of the reaction mixture. dLigand (10 mol%) was used instead. eIsolated yield. dba: Dibenzylideneacetone; rt: room temperature; LEDs: light-emitting diodes; dr: diastereoisomer ratio; ee: enantiomer excess; DCM: dichloromethane; ND: not detected; DMF: N,N-dimethylformamide; THF: tetrahydrofuran; NMR: nuclear magnetic resonance; HPLC: high performance liquid chromatography.
With the optimal conditions in hand, we first explored the substrate scope of vinylbenzoxazinanones for this reaction. As summarized in Figure 3, the electronically varied substituents at the 6- or 7-position of the benzene ring, such as H, Me, F, Cl, and Br, were found to be applicable to generate chiral spiro-indenes in good yields with no obvious influence on the enantio- and diastereoselectivities (3aa-3ag, 76%-84% yields, 93%-97% ee and 9:1-19:1 dr). It could be speculated that vinylbenzoxazinones bearing an electron-donating or electron-withdrawing group display comparable reactivity at the 6- or 7-position of the benzene ring. The absolute configuration of the resulting product 3aa was assigned to 3S,4S by X-ray single-crystal diffraction analysis. In addition, the substrate with fluorine at the 5-position was successfully converted to the desired product 3ah, while the one with Me was infeasible, possibly due to steric hindrance. Next, we briefly investigated the effect of sulfonyls on the nitrogen atom and found that the replacement of p-toluene with 2-nitro-, 4-nitro-, and 4-bromobenzenes can also afford products 3ai-3ak in 74%-79% yields with 92% ee and 10:1-14:1 dr.
Figure 3. The scope of vinylbenzoxazinanones. Standard conditions: 1 (0.1 mmol), 2 (0.2 mmol), Pd2(dba)3·CHCl3 (5 mol%) and L8
Subsequently, the substrate range of 1-diazonaphthalen-2(1H)-ones, which were easily prepared from
Figure 4. The scope of 1-diazonaphthalen-2(1H)-ones. Standard conditions: 1 (0.1 mmol), 2 (0.2 mmol), Pd2(dba)3·CHCl3 (5 mol%) and L8
To demonstrate the utility of this methodology, a gram-scale reaction was carried out under the irradiation of two Kessil lamps (456 nm) and comparable enantio- and diastereoselectivities were observed (Figure 5A, 68% yield, 94% ee and 14:1 dr). Additionally, the reaction directly using sunlight as a light source worked well, yielding the corresponding product comparable to that in the laboratory [Figure 5B]. Furthermore, four additional synthetic transformations of 3aa were carried out [Figure 5C]. Firstly, regioselective olefin epoxidation was successfully conducted, giving the product 4a with excellent enantio- and diastereoselectivities. Reduction of the amide group with lithium tetrahydridoaluminate allowed for the smooth production of 4b. Then, the p-tosyl group was easily removed under reductive conditions to afford compound 4c in excellent yield with no loss of enantiopurity. Last, the spiro-indene product 3aa underwent a facile hydrogenation reaction in the presence of Pd/C and H2, which easily afforded the target product spiro-indane 4d.
Figure 5. Demonstration of the synthetic utility of methodology. ee: Enantiomer excess; dr: diastereoisomer ratio; DCM: dichloromethane; THF: tetrahydrofuran.
Quantum chemical calculations were carried out to explore the origin of regioselectivity in the reductive elimination processes (L6 was employed in the calculations; the process involving two ligands was calculated based on the result of the nonlinear effect experiments in Section 7.1 of the Supplementary Materials). The calculated energy profiles [Figure 6A] indicate that TS1aRR, the transition state (TS) leading to chiral spiro-indenes, was predicted to be most stable compared with TSs in other pathways, agreeing with the experimentally observed good regio- and chemo-selectivity. Intriguingly, we found that the trend of relative stability of TSs was generally consistent with the trend of thermodynamics of the cyclization products. Herein, we examined the geometries of different plausible products to understand the regio- and chemoselectivity according to the Evans-Polanyi principle. As shown in Figure 6B, compared with 3aa, the lone pair of the tosyl N atom of 3b loses conjugated stabilization with the phenyl group. In both 3c and 3d, the double bonds were highly distorted due to the ring strain, resulting in a significant increase in energy.
CONCLUSIONS
In summary, we have developed a novel route to chiral spiro-indenes via the Pd-catalyzed asymmetric (4 + 2) dipolar cyclization of vinylbenzoxazinanones and 1-diazonaphthalene-2(1H)-ones. This reaction allows the one-pot preparation of a range of chiral spiro-indenes bearing all-carbon quaternary stereocenters in moderate to good yields and with generally high enantio- and diastereoselectivities. A key factor contributing to the success of this method is the formation of indene-involved ketenes by photo-Wolff rearrangement, which serves as efficient acceptors for Pd-containing 1,4-dipoles to proceed with the subsequent asymmetric dipolar cyclization. We believe that the strategy of combining asymmetric palladium catalysis with photo-Wolff rearrangement would find more potential applications in the chiral carbo- and heterocycle synthesis.
DECLARATIONS
Authors’ contributions
Conceiving the work: He L
Designing and conducting the experiments: He L, Qu BL, Yu JQ, Yu J
DFT calculations: Xiao M, Zhang Z
Discussion and manuscript preparation: Cheng Y, Xiao WJ
Supervising and directing the research: Lu LQ
Availability of data and materials
Detailed experimental procedures and spectroscopic data can be found in Supplementary Materials. Deposition Number 2271263 (for 3aa) contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (22271113 and 92256301), the National Key R&D Program of China (2022YFA1506100 and 2023YFA1507203), Central China Normal University, and Wuhan Institute of Photochemistry and Technology.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
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Cite This Article
How to Cite
He, L.; Qu B. L.; Xiao M.; Yu J. Q.; Yu J.; Cheng Y.; Zhang Z.; Xiao W. J.; Lu L. Q. Synthesis of chiral spiro-indenes via Pd-catalyzed asymmetric (4 + 2) dipolar cyclization. Chem. Synth. 2024, 4, 53. http://dx.doi.org/10.20517/cs.2024.39
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