One-Pot Catalytic Asymmetric Synthesis of Tetrahydrocarbazoles

Abstract

A one-pot asymmetric synthesis of 1,2,3,4-tetrahydrocarbazoles has been developed via an enantioselective [3 + 3] annulation of 2-alkynylindoles and donor–acceptor cyclopropanes. In the presence of chiral Lewis acids as catalysts, a series of optically active tetrahydrocarbazoles were furnished in high yields (63–87%) with good to excellent levels of enantioselectivity (up to 94% ee).

Graphical Abstract

Polycyclic indoles, such as carbazoles and their derivatives, are important heterocyclic compounds owing to their frequent occurrence as key motifs in various bioactive nature products and drug molecules (Figure 1).^1,2 It was found that the tetrahydrocarbazoles D, especially in their optically active forms, usually show high bioactivities, such as analgesic potency and anti-inflammatory potency.^2a Great efforts have been made to explore the effective methods for the enantioselective construction of this subunit, including Friedel–Crafts alkylation, the Diels–Alder reaction, and others.^3–5 In 2009, Kerr and co-workers reported an extraordinarily efficient approach that led to tetrahydrocarbazoles,⁶ which involved ring-opening of the donor–acceptor (D–A) cyclopropanes with indole^7,8 and Conia–ene cyclization.^9,10 Recently, we have developed a series of catalytic processes for enantioselective ring opening and annulation reactions of D–A cyclopropanes with versatile reagents such as nitrones, amines, enol silyl ethers, and indoles,¹¹ by Lewis acid based on side arm modified chiral bisoxazoline ligands.^12,13 Here, we report our recent results on the development of catalytic system for enantioselective [3 + 3] annulation of 2-alkynylindoles with donor–acceptor cyclopropanes, which provides a one-pot access to a variety of optically active tetrahydrocarbazoles.

Figure 1.

Natural products containing tetrahydrocarbazole.

Initially, the reaction was carried out with 1.0 equiv of 2-ethynylindole 1a and 2.0 equiv of cyclopropane 2a in 1,2-dichloroethane (Table 1). After evaluation of several Lewis acids,¹⁴ copper complexes were proven to be competent catalysts for the asymmetric ring-opening reaction. By using in situ generated Cu(SbF₆)₂/L1 as catalyst, the reaction proceeded very fast at 35 °C, affording the desired product 3a in 98% yield with 55% ee after 50 min (entry 1). When Cu(OTf)₂ was employed, it gave rise to a 91% yield with a moderate level of enantioselectivity (59% ee, entry 2). Although the reaction provided a similar yield and enantioselectivity in dichloromethane (entry 3), a dramatic increase of enantioselectivity (77% ee) was observed by employing toluene as solvent (entry 4). Aiming at improving both the reactivity and the enantioselectivity of the ring-opening reaction, a variety of chiral ligands were investigated. Importantly, it was found that bisoxazoline ligands bearing cyclohexyl backbones provided better enantioinduction. With ligand L2, the ring-opening product 3a was afforded in 72% yield with 83% ee (entry 5). Although trisoxazoline L3 led to a lower reactivity and enantioselectivity, SaBOX ligand L4 bearing a benzyl side arm could promote the reaction and gave a higher ee value (entries 6 and 7). Notably, SaBOX ligand L5 containing a sterically demanding side arm could promote the reaction efficiently, affording the ring-opening product 3a in 95% yield with 87% ee (entry 8). Since a further increase in steric hindrance of the side arm group resulted in no improvement of enantioselectivity of the reaction (entry 9), the SaBOX ligand L7 with two side arm groups was employed. To our delight, with L7, the product 3a was obtained in 97% yield with 90% ee (entry 10). In addition, when the ratio of 1a/2a was 1.5/1, both high yield and an excellent level of enantioselectivity could be afforded as well (entry 11).

Table 1.

Reaction Optimization^a

entry	metal precursor	L	solvent	yieldb (%)	eec (%)
1	Cu(SbF6)2	L1	DCE	98	55
2	Cu(OTf)2	L1	DCE	91	59
3	Cu(OTf)2	L1	DME	93	59
4	Cu(OTf)2	L1	toluene	72	77
5	Cu(OTf)2	L2	toluene	72	83
6	Cu(OTf)2	L3	toluene	44	77
7	Cu(OTf)2	L4	toluene	75	88
8	Cu(OTf)2	L5	toluene	95	87
9	Cu(OTf)2	L6	toluene	96	85
10	Cu(OTf)2	L7	toluene	97	90
11d	Cu(OTf)2	L7	toluene	90	90

^aPerformed with 0.2 mmol of 1a, 0.4 mmol of 2a, and 3 mL of solvent.

^bIsolated yields.

^cDetermined by chiral HPLC.

^d1a/2a = 1.5/1, at 40 °C.

Next, we turned our attention to investigate the one-pot [3 + 3] annulation reaction of 2-alkynylindole 1a with cyclopropane 2a (Table 2). With InCl₃ as catalyst, the cyclization reaction proceeded smoothly at 120 °C after 5 h, furnishing the desired product 4a in 73% yield but with a slight drop of the ee value (85% ee, entry 1). When 5 mol % of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) was added as additive, both the yield and enantioselectivity were improved (entry 2). Increasing the amount of the base to 10 mol %, the annulation product 4a was obtained in 82% yield with 89% ee (entry 3). However, a further increase of DBU to 15 mol % led to a dramatically sluggish reaction, delivering the [3 + 3] annulation product in 78% yield with 90% ee even after 3 days (entry 4).

Table 2.

One-Pot [3 + 3] Annulation Reaction^a

entry	DBU (mol %)	time (h)	yieldb (%)	eec (%)
1	0	5	73	85
2	5	6	80	87
3	10	6	82	89
4	15	72	78	90

^aPerformed with 0.3 mmol of 1a and 0.2 mmol of 2a in 3 mL of toluene for the first step; an additional 1 mL of toluene was added for the second step.

^bIsolated yields.

^cDetermined by chiral HPLC.

Under the optimized reaction conditions, the substrate scope of the enantioselective one-pot [3 + 3] annulation reaction of 2-alkynylindoles with cyclopropanes was examined. As shown in Table 3, electron-rich indole derivatives, such as those bearing a methyl group at the 4-, 5-, 6-, and 7-positions (1b–e), could react smoothly with cyclopropane 2a, giving the corresponding [3 + 3] products 4b–e in 71–87% yields with 88–93% ee (entries 2–5). Similar product yields (77–85% yields) and slightly higher levels of enantioselectivity (91–94% ee) were obtained for the reactions of 2a with indoles 1f and 1g, which contain methoxy groups at the 4- and 5-positions, respectively (entries 6 and 7). Halo-substituted indole substrates 1h–j were also tolerated in the current catalyst system, furnishing the desired products 4h–j in good yields with up to 84% ee (entries 8–10). In addition to 2a, a variety of D–A cyclopropanes were compatible with this asymmetric [3 + 3] annulation reaction. For example, the reactions of cyclopropanes bearing heterocyclic substituents such as 2-furyl (2b) and 2-thienyl (2c) with 1a resulted in the desired products in good yields with moderate to good enantioselectivities (entries 11 and 12).

Table 3.

Substrate Scope of Annulation Reaction^a

entry	R3	R4	4	yieldb (%)	eec (%)
1	H (1a)	PMP (2a)	4a	82	89
2	4-Me (1b)	PMP (2a)	4b	86	93
3	5-Me (1c)	PMP (2a)	4c	87	90
4	6-Me (1d)	PMP (2a)	4d	76	89
5	7-Me (1e)	PMP (2a)	4e	71	88
6	4-OMe (1f)	PMP (2a)	4f	85	94
7	5-OMe (1g)	PMP (2a)	4g	77	91
8	5-Cl (1h)	PMP (2a)	4h	77	77
9	6-Cl (1i)	PMP (2a)	4i	69	77
10	5-F (1j)	PMP (2a)	4j	79	84
11	H (1a)	2-furyl (2b)	4k	68	57
12	H (1a)	2-thienyl (2c)	4l	71	77
13	H (1a)	4-TBSOC6H4 (2d)	4m	86	82
14	H (1a)	3,4-(MeO)2C6H3 (2e)	4n	77	87
15	H (1a)	3,4,5-(MeO)3C6H2 (2f)	4o	78	90
16	H (1a)	CH=CHPh (2g)	4p	63	86

^aPerformed with 0.6 mmol of 1a and 0.4 mmol of 2a in 6 mL of toluene for the first step; an additional 2 mL of toluene was added for the second step.

^bIsolated yields.

^cDetermined by chiral HPLC.

D–A cyclopropanes with other aromatic substituents, such as 2d–f containing electron-donating groups on the phenyl ring, could also perform well in the asymmetric [3 + 3] annulation reaction, affording the corresponding products 4m–o in high yields with high levels of enantioselectivity (entries 13–15). In addition, styrenyl-substituted D–A cyclopropane 2g was also a suitable substrate, delivering the corresponding product 4p in 63% yield with 86% ee (entry 16). The absolute configuration of 4h was established as S by X-ray crystallography (Figure 2).

Figure 2.

X-ray crystal structure of annulation product 4h.

Previous studies on the crystal structures of SaBOX/Cu(II) complexes indicated that the aryl groups of the side arms always bend toward the metal center. Accordingly, an asymmetric induction model was proposed to explain the observed enantioselectivity (Scheme 1). In this model, it is evident that the upper and lower sides of the Cu(II)/oxazoline ring square are blocked by the side arms while the top-left and low-right corners are sterically hindered by the cyclohexyl group. Upon interacting with the Lewis acidic Cu center through its two ester groups, the D–A cyclopropane is further activated to give rise to positive charge at the carbon center bearing electron-rich substituent. In combination with the stabilization effect from the donor–acceptor synergistic system, the reaction undergo an asymmetric transformation of D–A cyclopropanes through a chiral Lewis acid-catalyzed ring opening with indoles. Due to the steric demand of the nucleophilic attack of C3-position of indole, the Si face of the indole is preferred to approach the transient (R)-cyclopropane (left), suffering less steric interaction with the ligand cyclohexyl substituent.

Scheme 1.

Proposed Stereochemical Model

In summary, we have developed a one-pot catalytic system for asymmetric synthesis of 1,2,3,4-tetrahydrocarbazoles via an enantioselective [3 + 3] annulation of 2-alkynyl indoles with donor–acceptor cyclopropanes. In the presence of Lewis acids as catalysts, a series of chiral tetrahydrocarbazoles were furnished in high yields (63–87%) with good to excellent levels of enantioselectivity (up to 94% ee). In addition, a rational model for stereoinduction was provided. Together with the ready accessibility of both the catalysts and starting materials, the simple operation and the broad substrate scope should make this current protocol synthetically useful.