DOI: 10.1002/chem.201404769

Communication

& Total Synthesis

A General Diversified Synthesis of Carbazoles and the First Synthesis of Karapinchamine A Youai Qiu, Jing Zhou, Chunling Fu, and Shengming Ma*[a] Abstract: [IPrAuCl]/AgSbF6-catalyzed cyclization of the readily available 4-benzoxyl-1-(indol-2-yl)-2-alkynols occurred smoothly in 1,2-dichloroethane (DCE) in the presence of 4  MS to form a series of differently polysubstituted 2-oxygenated carbazole derivatives efficiently. Based on mechanistic study, a possible mechanism involving 1,3migration of a benzoate group to form the allene, Au + -mediated cyclization–elimination to form a gold–carbene intermediate, and subsequent highly selective 1,2-migration has been proposed for the formation of carbazoles. Highly selective 1,2-migration referring to the two substituents R3 and R4 (R4 = H, alkyl, and aryl group) was observed: (1) In the presence of both H and alkyl groups, 1,2-hydrogen migration is exclusive; (2) in the presence of a methyl group (R3), propyl, isopropyl, 4-methylphenyl, and 4-chlorophenyl groups (R4) migrate exclusively. Finally, the first total synthesis of the recently isolated naturally occurring carbazole alkaloid karapinchamine A in 5.2 g scale has been realized in 6 steps from commercially available chemicals without need for any protection–deprotection.

Scheme 1. Naturally occurring 2-oxygenated carbazole alkaloids and organic materials.

Carbazole alkaloids are attractive targets for organic synthesis, owing to their exceptional biological and pharmacological activities, as well as their special thermal, electrical, and optical properties (Scheme 1).[1] Among many useful synthetic procedures to prepare carbazoles,[2] methods forming a benzene ring to indoles are particularly attractive,[3] because the indole derivatives are widely available and studied in organic chemistry.[4] However, there remain some limitations, such as harsh reaction conditions, poor atom economy, substrates that are not readily available, and the diversity required to introduce the different substituent(s). Thus, the development of a mild, efficient, and regiocontrolled substituent-diversified method for preparation of polysubstituted carbazoles from indoles is still highly desirable.

[a] Y. Qiu, J. Zhou, Prof. Dr. C. Fu, Prof. Dr. S. Ma Laboratory of Molecular Recognition and Synthesis Department of Chemistry, Zhejiang University Hangzhou, 310027, Zhejiang (P. R. China) Fax: (+ 86) 21-6260-9305 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404769. Chem. Eur. J. 2014, 20, 14589 – 14593

Recently, we reported the AuCl3-catalyzed cyclization of 1(indol-2-yl)-3-alkynols to form carbazole skeletons by the connection of the indole C3 to C4 of the homopropyl alcohol [Eq. (1)].[5] When we tried the same reaction with 4-benzoxyl-1(indol-2-yl)-2-alkynol substrate 2 a,[6] surprisingly, the hydrogen at the 4-position and the benzoxyl group shifted to the 3-position and 2-position of carbazole, respectively, affording 3 a (Table S1, entry 5), whose structure was confirmed unambiguously by the X-ray single crystal diffraction study (Figure S1).[7] Herein we report such an unexpected gold-catalyzed reaction for the efficient and easy synthesis of polysubstituted 2-oxygenated carbazoles[8] involving the unique double transfer of the two 4-position substituents (Scheme 2). Based on the result described above, we began our investigation with the optimization of the reaction conditions for the conversion of the model substrate 2 a (for some typical results, see Table S1). No reaction occurred after 12 h when AuCl3, AuCl, or PtCl4 was employed as the catalyst (Table S1, entries 1–3). When [AuCl(PPh3)]/AgSbF6 was used as the catalyst, the yield of 3 a was only 7 % (Table S1, entry 4). When [IPrAuCl] was used instead of [AuCl(PPh3)], the yield of 3 a was improved to 28 % (Table S1, entry 5). Interestingly, the yield of 3 a was improved to 93 % when the reaction was carried out in the pres-

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Scheme 2. New approaches to carbazoles.

ence of 4  molecular sieves (MS) for 1 h (Table S1, entry 6). Various other silver salts were screened but none was better than AgSbF6 (Table S1, entries 6–9). Several solvents were tested for this transformation at room temperature: CH2Cl2 was also effective affording 3 a in 84 % NMR yield (Table S1, entry 10). This reaction did not proceed in the absence of either AgSbF6 or [IPrAuCl] (Table S1, entries 13 and 14) ; no conversion was observed when the reaction was conducted with 4  MS alone in the absence of [IPrAuCl] and AgSbF6 (Table S1, entry 15). The reaction was proven to be quite broad in substrate scope and some typical results are listed in Table 1. R1 could be H (Table 1, entries 1, 4–7, and 11), methyl (Table 1, entries 2 and 3), methoxy (Table 1, entry 10), Cl (Table 1, entry 8), or Br, which could be easily transformed to many useful functional groups in the synthesis of biologically active carbazole alkaloids[9] (Table 1, entry 9). The R2 group connected to the nitro-

gen atom could be an alkyl (Table 1, entries 1–6 and 8–11) or a phenyl group (Table 1, entry 7). The propargylic R3 substituent may be H, n-heptyl, n-propyl, phenethyl, and even the more bulky subtituents, such as iso-propyl and cyclohexyl. The scope of the reaction with regard to 4,4-disubstituted-4benzoxyl-1-(indol-2-yl)-2-alkynols was also examined. R1 may be H (Table 2, entries 1–7, 10–15, and 17–19), methyl (Table 2, entries 8, 9), and methoxy group (Table 2, entry 16); R2 may be an alkyl (Table 2, entries 1, 2, and 4–19,) or a phenyl group (Table 2, entry 3). The reaction of substrate 2 s could also be easily conducted at a scale of 3.0 mmol (1.1260 g) in a slightly higher yield (Table 2, entry 9,); R5 could be H (Table 2, entries 1–4, 8–13, and 17–19), alkyl (Table 2, entries 5–6 and 14– 16) or phenyl (Table 2, entry 7). The alkyne substituents R3 and R4 may be methyl (Table 2, entries 1–9), ethyl (Table 2, entry 10), and propyl (Table 2, entry 11). In addition, a substrate with a 4-membered cycle underwent an interesting smooth cyclization with ring expansion to afford fused carbazole 3 v in 72 % yield (Table 2, entry 12). Notably, R4 (propyl group) migrates selectively over R3 (methyl group) (Table 2, entries 13– 16). Furthermore, in the presence of a methyl group (R3), isopropyl, 4-methylphenyl, and 4-chlorophenyl groups (R4) all migrates exclusively (Table 2, entries 17-19). This highly selective migration provides a method to introduce specific substituents to the 2- and 3-positions of the final carbazole skeleton from the two substituents at the 4-positions of the readily available

Table 1. Gold-catalyzed cyclization of 4-monosubstituted-4-benzoxyl-1(indol-2-yl)-2-alkynols 2.[a]

Entry

2 R1/R2/R3

Time [h]

Yield of 3 [%][b]

1 2 3 4 5 6 7 8 9 10 11

H/Et/C7H15 (2 a) 5-CH3/Et/C7H15 (2 b) 5-CH3/Et/Pr (2 c) H/Et/CH2CH2Ph (2 d) H/Et/iPr (2 e) H/Et/Cy (2 f) H/Ph/Pr (2 g) 5-Cl/Bn/Pr (2 h) 5-Br/Bn/Pr (2 i) 6-OMe/Bn/Pr (2 j) H/Et/H (2 k)

1 1.5 1.5 3 3 3 4 4 2 5 4

86 85 88 76 75 77 78 84 78 53 78

(3 a) (3 b) (3 c) (3 d) (3 e) (3 f) (3 g) (3 h) (3 i) (3 j) (3 k)

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of

4,4-disubstitued-4-benzoxyl-1-

Entry

2 R1/R2/R3/R4/R5

Time [h]

Yield of 3 [%][b]

1 2 3 4 5 6 7 8 9[c] 10 11 12 13[d] 14[e] 15[e] 16[d] 17 18[f] 19[f]

H/Bu/CH3/CH3/H (2 l) H/Allyl/CH3/CH3/H (2 m) H/Ph/CH3/CH3/H (2 n) H/Bn/CH3/CH3/H (2 o) H/Et/CH3/CH3/CH3 (2 p) H/Et/CH3/CH3/Bu (2 q) H/Et/CH3/CH3/Ph (2 r) 5-CH3/Et/CH3/CH3/H (2 s) 5-CH3/Et/CH3/CH3/H (2 s) H/Et/Et/Et/H (2 t) H/Et/Pr/Pr/H (2 u) H/Et/-(CH2)3-/H (2 v) H/Et/CH3/Pr/H (2 w) H/Et/CH3/Pr/CH3 (2 x) H/Et/CH3/Pr/Bu (2 y) 5-OMe/Et/CH3/Pr/Bu (2 z) H/Et/CH3/iPr/H (2 aa) H/Et/CH3/4-MeC6H4/H (2 ab) H/Et/CH3/4-ClC6H4/H (2 ac)

2 4 5 4 2 2 2 3 3 4 5 3 12 4 12 4 12 24 48

81 52 31 50 77 90 89 82 86 70 45 72 91 88 89 85 44 45 55

(3 l) (3 m) (3 n) (3 o) (3 p) (3 q) (3 r) (3 s) (3 s) (3 t) (3 u) (3 v) (3 w) (3 x) (3 y) (3 z) (3 aa) (3 ab) (3 ac)

[a] Reaction conditions: 2 (1.0 mmol), [IPrAuCl]/AgSbF6 (5 mol %), 4  MS (500 mg), DCE (10 mL). [b] Yield of isolated product. [c] The reaction was conducted with 3.0 mmol (1.1260 g) of 2 s. [d] The ratio of 3:3’ was 97:3. [e] The ratio of 3:3’ was 96:4. [f] T = 80 8C.

[a] Reaction conditions: 2 a (1.0 mmol), [IPrAuCl]/AgSbF6 (5 mol %) 4  MS (500 mg), DCE (10 mL), RT. [b] Yield of isolated product.

Chem. Eur. J. 2014, 20, 14589 – 14593

Table 2. Gold-catalyzed cyclization (indol-2-yl)-2-alkynols 2.[a]

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Communication starting materials. Finally, diversified polysubstituted carbazoles could be afforded by simply installing different R3, R4, and R5 groups in starting alkynols and R1 and R2 groups in the original indole unit (Table 2, entries 15 and 16). To probe the mechanism, we prepared the 4-position deuterium-labeled 1-(indol-2-yl)-2-alkynol 2 a-D (97 % D), which, under the standard reaction conditions, provided 3 a-D in 81 % yield with 93 % D-incorporation at the 3-position of the newly formed phenyl ring [Eq. (3)]. This result indicated that the reaction may occur by a 1,2-D shift process. In addition, we prepared 1-(1-ethyl-1H-indol-2-yl)4-methoxybut-2-ynol 4. However, the corresponding carbazole 5 was not formed, although 4 decomposed gradually, which shows the importance of the benzoate group in this transformation (Scheme 3). Scheme 4. Proposed mechanism.

Scheme 3. Role of the benzoxyl group.

A mechanism for this reaction was then proposed (Scheme 4), whereby the propargylic benzoate unit in substrate 2 may undergo a 1,3-shift of the benzoate functionality by the nucleophilic attack of the carbonyl oxygen mediated by Au(I), leading to the formation of the allene intermediate M2.[10] Subsequent nucleophilic attack of the indolyl C3 at the coordinated C=C bond in the allene unit would afford the vinyl–gold species M3. The gold–carbene intermediate M4[11] may be afforded by protonation of the hydroxy group in M3 followed by elimination of H2O,[3d, 12] as suggested by the effect of 4  MS (Table S1, entry 6). Finally 1,2-R4 (R4 = H, aryl, and alkyl) migration[13] of intermediate M4 would afford the 2-oxygenated carbazoles 3, with regeneration of the gold catalyst into the catalytic cycle (Scheme 4). A major advantage of this method is the diversity it allows, even with regards to the N-substituent. In 2013, karapinchamiChem. Eur. J. 2014, 20, 14589 – 14593

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ne A,[14] with a N-geranyl substituent, was isolated from Sri Lankan curry-leaf (Murraya Koenigii), which is a medicinal plant used in southern Asia, and showed interesting melanogenesis inhibitory activity. The first total synthesis of karapinchamine A was further demonstrated to show the synthetic potential of the current method, especially the elaboration ability of the 2benzoxyl group in the product. 1-Geranyl-5-methyl-1H-indole2-carbaldehyde 11 was afforded from the commercially available starting materials ethyl 5-methyl-1H-indole-2-carboxylate and (2E)-geranyl bromide through the direct introduction of the geranyl group to the N atom, followed by reduction with LiAlH4 and oxidation with MnO2. The required propargylic alcohol 2 ad was then easily afforded in 83 % yield by the reaction of 11 and (3-(benzoyloxy)prop-1-yn-1-yl)lithium conveniently. Luckily, carbazole 3 ad was formed exclusively in 80 % yield under the catalysis of [IPrAuCl]/AgSbF6 in the presence of 4  MS. Finally, deprotection of benzoates with potassium carbonate led to karapinchamine A in 85 % yield (5.2011 g) (Scheme 5). Furthermore, easy deprotection of benzoates with potassium carbonate led to 6 with a free hydroxy group in 88 % yield. Protection of the OH group as a triflate, followed by Suzuki coupling with phenylboronic acid using LB-Phos·HBF4[15] as ligand then afforded 8 in 90 % combined yield, demonstrating the easy possibility of replacing the original 2-oxygenated substituent (Scheme 6). In conclusion, we have developed a simple and efficient [IPrAuCl]/AgSbF6-catalyzed reaction of 4-benzoxyl-1-(indol-2yl)-2-alkynols, which are readily available from the reaction of indole-carbaldehydes with the related terminal alkynes in the presence of 4  MS, providing differently substituted carbazoles isolated in good yields under very mild conditions. The diversity has been established simply by introducing substituents to the phenyl ring, the N-atom of the indole unit, and the 1-position and two 4-positions of the alkynols. A possible mechanism has been proposed for the formation of carbazoles involving 1,3-migration of the benzoate group forming an

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Communication Acknowledgements Financial support from the National Natural Science Foundation of China (21232006) and the National Basic Research Program (2011CB808700) is greatly appreciated. Shengming Ma is a Qiu Shi Adjunct Professor at Zhejiang University. We thank Mr. Weilong Lin in our group for reproducing the results presented in entry 8 of Table 1; entries 6 and 17 of Table 2.

Scheme 5. The first total synthesis of karapinchamine A. Reaction conditions: a) i) NaH (1.5 equivalents), DMF, RT, 15 min + 1 h; ii) geranyl bromide, RT, 10 min + 2 h, ca. 65 %; b) LiAlH4 (1.0 equiv), THF, 0 8C, 15 min, RT, 1 h, ca. 65 %; c) MnO2 (4.0 equivalents), CH3CN, RT, 12 h, 82 %; d) 11 was added in 78 8C, 10 min; 78 8C!RT, 3 h, 83 %; e) [IPrAuCl]/AgSbF6 (2.5 mol %), 4  MS, DCE, 80 8C, 20 min + 18 h, 80 %; f) K2CO3 (1.6 equivalents), NMP, 150 8C, 10 h, 85 %.

[2]

[3]

Scheme 6. Synthetic application of 3 c.

allene, which was followed by a nucleophilic attack of the Au + -coordinated C=C bond, and elimination of H2O affording a gold–carbene intermediate. Subsequent selective 1,2-migration of R4 (R4 = H, aryl, and alkyl) afforded polysubstituted carbazoles. Highly selective 1,2-migration took place involving the two substitutents R3 and R4 : (1) In the presence of both H and alkyl groups, 1,2-hydrogen migration was exclusive; (2) in the presence of methyl group (R3), propyl, isopropyl, and aryl groups (R4) migrated exclusively. Finally, this methodology has been applied to the first total synthesis of recently isolated naturally occurring carbazole alkaloid karapinchamine A. Further studies of this reaction are being carried out in our laboratory.

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[4]

[5]

[6] [7]

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Keywords: carbazoles · carbenes · gold · migration · selectivity

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Received: August 8, 2014 Published online on September 26, 2014

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A general diversified synthesis of carbazoles and the first synthesis of karapinchamine A.

[IPrAuCl]/AgSbF6 -catalyzed cyclization of the readily available 4-benzoxyl-1-(indol-2-yl)-2-alkynols occurred smoothly in 1,2-dichloroethane (DCE) in...
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