Synthesis of chiral spiro-indenes via Pd-catalyzed asymmetric (4 + 2) dipolar cyclization

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.

Figure 1. Significance and synthesis of chiral indenes. AAA: Asymmetric allylic alkylation.

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 1-diazonaphthalene-2(1H)-ones^[37-39] could be used as efficient cyclic ketene precursors under visible light irradiation, which would subsequently trap π-allyl-Pd 1,4-dipoles to form the chiral spiro-indenes bearing all-carbon quaternary stereocenters [Figure 1C]. Although feasible in principle, challenges remain due to the rigidity and hindrance of the spiro-indene skeleton, which is difficult in the enantioselective formation of all-carbon quaternary stereocenters.

EXPERIMENTAL

Under argon atmosphere, a flame-dried 10 mL Schlenk tube was charged with Pd₂(dba)₃·CHCl₃(0.005 mmol, 5 mol%, “dba”: dibenzylideneacetone), L8 (0.02 mmol, 20 mol%) and anhydrous dichloromethane (DCM) (1.0 mL), and the resulting solution was stirred for 30 min at room temperature (rt). Then, vinylbenzoxazinanone 1a (0.1 mmol, 1.0 equiv.), 1-diazonaphthalen-2(1H)-one 2a (0.2 mmol, 2.0 equiv.) and anhydrous DCM (1.0 mL) were added to the reaction mixture. After that, the reaction solution was stirred under the irradiation of 6 W blue light-emitting diodes (LEDs) for 24 h at rt. The combined solution was concentrated under vacuum, and the residue was purified by flash column chromatography on silica gel (petrol ether/ethyl acetate = 20/1 to 15/1) to afford the desired product 3aa.

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 α-diazoketone shows absorption around 450 nm. We then tested the feasibility of the above idea using vinylbenzoxazinanone 1a and 1-diazonaphthalen-2(1H)-ones 2a as model substrates under 6 W blue LEDs. As shown in Figure 2, using Pd₂(dba)₃·CHCl₃ as a precatalyst, the chiral diphosphine ligand L1, Trost’s ligand L2^[40], Zi’s chiral ligand L3^[41] and Carreira’s ligand L4^[42], which are widely used in Pd-catalyzed dipolar cyclizations, failed to give the desired product. Our chiral P,S ligand L5^[35] was found to promote the reaction with good enantioselectivity but in low yield [22% yield, 90% enantiomer excess (ee) and 3:1 diastereoisomer ratio (dr)]. Subsequently, we turned our attention to chiral phosphoramidite ligands^[43] by extensively exploring different substituents on the N atom (L6-L8). It is noted that chiral ligands L6 and L7, which have two identical substituent groups, e.g., Me, Et on the N atom, gave the desired products with the improved results and dr values and the comparable enantioselectivity (L6: 46% yield, -93% ee and 5:1 dr; L7: 72% yield, -84% ee and 10:1 dr). When using Feringa’s ligand L8, a satisfactory result with a higher yield and similar enantio- and diastereoselectivities was observed (84% yield, 94% ee and 14:1 dr). Furthermore, a number of solvents were screened, and the results showed that the use of DCM led to the highest reaction efficiency. Replacement of the 456 nm light source with other light sources, such as 390 or 370 nm, all promoted the formation of 3aa with similar overall enantio- and diastereoselectivities but with moderate yields (entries 16 and 17). In the absence of the light source, no product was determined, so, as expected, the control experiment demonstrated that light irradiation is crucial to the reaction (entry 18).

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(20 mol%) in 2.0 mL anhydrous DCM at rt under the irradiation of 6 W blue LEDs for 24 h. Isolated yields. ee: Enantiomer excess; dr: diastereoisomer ratio; dba: dibenzylideneacetone; DCM: dichloromethane; rt: room temperature; LEDs: light-emitting diodes.

Subsequently, the substrate range of 1-diazonaphthalen-2(1H)-ones, which were easily prepared from 2-naphthol derivatives, was investigated. As shown in Figure 4, the introduction of electron-donating (e.g., Me and OMe) and electron-withdrawing (e.g., Br) groups at the 6- or 7-position of the naphthol ring of 1-diazonaphthalen-2(1H)-one was well tolerated in this reaction, and corresponding spiro-indenes 3ba-3ja were obtained in 72%-86% yields with up to 94% ee and 14:1 dr. In addition, aryl substituents, such as phenyl and thiophenyl, were accommodated at the 6-position, giving the products of 3ka and 3la with good results. Significantly, the success of these transformations was further extended to substrates with different functional groups at the 6-position, such as cyclohexanyl (3ma, 82% yield, 90% ee and 12:1 dr), CN (3na, 73% yield, 91% ee and 6:1 dr) and Bpin (3oa, 75% yield, 94% ee and 10:1 dr). These results proved the good tolerance of functional groups with this asymmetric dipolar cyclization. The reactivity was slightly affected by the substituents at the 3-, 4- and 8-positions of the naphthol ring and the target products 3pa-3ra were obtained in the moderate yields even with extended reaction time. 1-Diazonaphthalen-2(1H)-one with an extended fused ring system was also an efficient substrate for conversion to the product 3ta. Dihydrodiazonaphthoquinone was also suitable for this reaction and was successfully converted to spiro-indane product 3ua in 79% yield with 93% ee and 12:1 dr.

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(20 mol%) in 2.0 mL anhydrous DCM at rt under the irradiation of 6 W blue LEDs for 24 h. Isolated yields. ^aChiral ligand L9(10 mol%) was used instead of L8. ee: Enantiomer excess; dr: diastereoisomer ratio; dba: dibenzylideneacetone; DCM: dichloromethane; rt: room temperature; LEDs: light-emitting diodes.

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 TS1a_RR, 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.

Figure 6. DFT Calculations. Energies are given in kcal/mol. DFT: density functional theory.

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.