Recent synthesis of ellipticine and its derivatives

The pyrido[4,3-b]carbazole alkaloid, ellipticine, was attracting considerable interest for many years due to its pronounced antitumor activity. We review the most important achievements in the field of ellipticine synthesis and its derivatives since 2012.


Introduction
The natural plant product ellipticine [1][2][3][4][5][6][7] was isolated in 1959 from the Australian evergreen tree, Ochrosia elliptica, of the Apocynaccae Apocynaceae. This compound was found to be a promising anticancer drug. So, the syntheses of ellipticine and its derivatives have been reported by many groups. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The planar polycyclic structure was found to interact with DNA through intercalation, exhibiting a high DNA binding affinity (10 6 M -1 ). The presence of protonatable ring nitrogens distinguished ellipticine from other simple intercalators. Both monocationic and uncharged species were found to be present under physiological conditions. The position charge stabilized the binding of ellipticine to nucleic acids, while the more lipophilic uncharged compound was shown to readily penetrate membrane barriers. The structural nature of these compounds offers a plausible basis for the implication of multiple modes of action, including DNA binding, interactions with membrane barriers, oxidative bioactivation and modification of enzyme function; most notably that of topoisomerase II and telomerase. Pharmaceutically, a number of toxic side effects have been shown to be problematic, but the amenability of ellipticine towards systematic structural modification has permitted the extensive application of rational drug design. A number of successful ellipticine analogs have been designed and synthesized with improved toxicities and anticancer activities. [28][29] More recently the synthetic focus has broadened to include the design of hybrid compounds, as well as drug delivery conjugates. Considerable research efforts have been directed towards gaining a greater understanding of the mechanism of action of these drugs that will aid further in the optimization of drug design.
This article provide an overview of the various syntheses of ellipticine from the years 2013 to 2017. Although a previous review by McCarthy et al. has appeared in 2012, 30 some reports were missing from their compilation. Thus, we have chosen to cover the literature under one section up to December 2017, omitting those works which have already been reported in the previous review.

Synthesis of Ellipticine and Derivatives
In 2014, Meesala et al. 31 have developed a simple and an efficient method to synthesize a new series of ellipticine analogues using the Vilsmeier-Haack reagent (Scheme 1). They described the synthesis of pyrido [3,2-b]carbazoles and pyrido [2,3-c]carbazoles by treating N-(carbazol-3-yl)acetamides 1a-e with DMF (2.5 equiv) and POCl 3 (10.0 equiv) at 70 °C for 12 hours. The reaction works well with different types of N-(carbazol-3-yl)acetamide derivatives and provides the corresponding linear and angular products. The angular products were obtained as the major isomers compared to the linear products.
Since the pyridocarbazoles contain the reactive substituents chlorine and aldehyde, these can be utilized for further heteroannulations to develop novel pyridocarbazole-based heterocyclic systems which may exhibit interesting biological properties. Nagarajan et al. 32 reported a simple and efficient route to the synthesis of ellipticine quinone 12 (Scheme 3) from isatin 4. The first key step is the synthesis of 7 from isatin using various alkylating reagents (Scheme 2). They performed reaction between sodium 2-(2-aminophenyl)-2-oxoacetate 5 and 2-bromo-1-(pyridin-4yl)ethanone hydrobromide in DMF at 70 °C for 5 h affording 7 in 17% yield. The next step is the rearrangement of 7 to carboxylic acid 9. Hydrolysis of compound 7 afforded 9 through an intermediate 8 along with easily separable decarboxilated product 10 in 76% and 24% yield respectively (Scheme 3).

Scheme 2. Synthesis of 7.
The key intermediate 9 was subjected to esterification with ethanol to give corresponding ester 11 in 92% yield. The ortho-lithiation of 11 by utilizing of LiHMDS/TMEDA produced the target ellipticine quinone 12 in good yield (Scheme 4).
In 2014, Nagarajan et al. 33 reported an expedient synthesis of the pyrido[4,3-b]carbazole alkaloids, ellipticine 29 and 9-methoxyellipticine 30 (Scheme 7) over seven steps from known 1,4-dimethylcarbazoles 13 They treated commercially available ethyl 1H-indole-2-carboxylate 31 with pyridine-3-carboxaldehyde 32 in presence of AlCl 3 followed by oxidation using IBX in DMSO, giving ketone 34 in 94% yield. The carbinol formed 33 was not isolated from the reaction mixture, and subsequently they carried out oxidation after the reaction work up. The ketone 34 was then subjected to directed o-lithiation reaction using LiTMP (lithium tetramethylpiperidide) as a base to afford a single regioisomer 12 in 72% yield. Thus, ellipticine quinone 12 was obtained in 3 steps and 67.6% overall yield (Scheme 8).
Similarly, isoellipticine quinones 39 and 40 can be obtained by varying pyridine part 36 as shown in Scheme 9. Also the other isomer of ellipticine quinone 43 was synthesized by using pyridine-2-carboxaldehyde 41.
They have successfully applied their synthetic route to the synthesis of olivacine 48 and calothrixin B 51. Treatment of 31 with 2-methylnicotinaldehyde in the presence of 1,1,3,3-tetramethylguanidine (TMG) in MeOH at room temperature followed by oxidation using IBX afforded the ketone 45 in 84% yield. Wolff−Kishner reduction of the ketone 45 gave reduced compound 46. The cyclized compound 47 was obtained by treating 46 with LDA/HMPA at −78 °C. Finally, the addition of MeMgI into 47 followed by treatment upon NaBH 4 /AlCl 3 (3:1) in dry THF at room temperature produced 48 in 58% yield. Thus, olivacine 48 was obtained in 6 steps and 15.6% overall yield as shown in Scheme 10.

Scheme 10. Synthesis of olivacine 48
The synthesis of calothrixin B 50 is outlined in Scheme 11. The reaction of 31 with quinoline-3carboxaldehyde in the presence of TMG in MeOH followed by oxidation with Dess−Martin periodinane (DMP) in DCM/ AcOH (9:1) at room temperature gave the ketone 49 in 80% yield. Then intramolecular directed olithiation reaction of 49 in the presence of LiTMP afforded 50 in 48% yield. Thus, calothrixin B 50 was obtained in three steps and 38.4% overall yield.
In 2014, Konakahara et al. 35 developed a simple and efficient synthetic method of novel four ellipticine derivatives in good to high yields. Moreover three kinds of novel pyridocarbazole-5-carboxylate derivatives were synthesized. All these new compounds exhibited higher solubility in water than ellipticine itself. To construct a pyridocarbazole ring, the compound 57 was treated with 58 and 59 in the presence of 3ethoxycarbonyl-1-methylpyridinium chloride 62 leading to 2-alkylpyridocarbazolium derivatives 64a,b in a yield of 10% (Scheme 13). Alternatively, the 2-alkylpyridocarbazolium derivatives 64a, 64b and 64d were prepared in good yields in two steps via treatment of 57 with 61a, 61b and 61d respectively, in the presence of NaOMe and 62 followed by the action of Amberlite IRA-900 (Scheme 13). The stability of the quaternary salts of these molecules increases by converting them into the corresponding tosylate and chloride salts.
In 2016, Ergün et al. 37 reported a new synthetic route for the synthesis of 5-methyl-6H-pyrido [4,3b]carbazole 96, so called 11-demethylellipticine (Scheme 18). They have used tetrahydrocarbazole acid 89 as a starting material and synthesized according to the literature. 38 Then, acid 89 was converted to glycine 90 derivative using ethyl chloroformate and methyl glycinate. The reduction of glycine 90 with lithium aluminium hydride gave amine alcohol 91. Amine alcohol 91 was reacted with benzene sulfonyl chloride and gave protected compound 92. The oxidation of the compound 92 at position 1 with periodic acid yielded tetrahydrocarbazolone 93. Then, reaction of 93 in the presence of sodium hydride led to the tetracyclic structure 94. Finally, pyridocarbazole 96 was synthesized by aromatization of compound 95, which was obtained from reaction between compound 94 and methyl lithium. One of the syntheses of pyridocarbazole alkaloid olivacine had been achieved via the reaction between pyridocarbazole 96 and methyl lithium in the literature previously. 39 Tetracyclic structure 94 can also allow the synthesis of several ellipticine derivatives.
These compounds exhibited potent antitumor activity. The introduction of glucose conjugaison at the 9position enhanced its solubility in water compared with those of ellipticine alone. This is the first report of the synthesis and evaluation of the antitumor activity of the uncharged glucose-conjugates of 9-hydroxyellipticine with increased water solubility.
An efficient and simple Ni-catalyzed C(aryl)-OMe bond cleavage and subsequent C(aryl)-Me bond formation by treating carbazoles with MeMgBr has been developed in 2016 by Das et al. 42 This protocol was successfully applied to the synthesis of the natural product ellipticine from readily available starting materials.

Scheme 22. Total synthesis of ellipticine.
They used commercially available 2,5-dimethoxybenzaldehyde 102 as the starting material and introduced the methyl group through a Ni-catalyzed Kumada-type coupling reactions at a late stage of the synthesis. They began the synthesis with the nitration reaction (CuSO 4 ·6H 2 O/HNO 3 ) of 2,5-dimethoxybenzaldehyde 102 to afford nitration product 103 in 80% yield (Scheme 22). The condensation reaction of 103 with aminoacetaldehydediethyl acetal in dry benzene gave imine product 104. The subsequent reduction of 104 with sodiumborohydride in methanol afforded amine 105 in 85% yield. The protection of amine 105 with a tosyl group gave protected compound 106, which underwent cyclization in an acidic medium to furnish isoquinoline 107 in 54% yield (two steps). In the next step, the reduction of the nitro group was achieved by using Pd/C in methanol, and amine 108 was isolated in 80% yield. This product was then subjected to a Cu-catalyzed Chan-Lam-type coupling with phenylboronic acid 109 to afford N-arylated product 110. 43 Subsequently, the preparation of carbazole 111 was achieved in 70% yield by using a Pd-catalyzed crossdehydrogenative (CDC) coupling reaction. Finally, they applied their optimized Ni-catalyzed protocol to replace the methoxy with a methyl group to afford ellipticine in 85% yield.
This protocol demonstrates that the lipophilicity of bioactive carbazoles can be easily modified by replacing a methoxy with a methyl group, which is important in the regulation of drug properties such as bioavailability and metabolic stability.
ET-1 and ET-2 were generated using a nine-step synthetic pathway with a 12% overall yield. First, 4,9dimethyl-9H-carbazole-3-carbaldehyde 112 45 was treated with aminoacetaldehyde diethylacetal to yield imine 113. The imine was reduced with sodium borohydride to produce amine 114, which was treated with benzene sulfonyl chloride to produce sulfonamide 115. Finally, cyclization of ET-1 was achieved by treating sulfonamide 115 with hydrochloric acid. ET-2 was obtained by treating ET-1 with iodomethane in DMF. ET-1 and ET-2 were more soluble than ellipticine. 3-Aminodibenzofurans 116a-b were used as starting materials (Scheme 24). The amino derivatives 116a-b were reacted with ethoxycarbonylisothiocyanate to give the thiourea intermediates 117, followed by the addition of the appropriate alkylamine or dialkylamine and HgCl 2 to give the ethoxycarbonylguanidine intermediates 118. The latter intermediates were subjected to thermal cyclization followed by filtration of the HgS-by-product to give the 3-(alkyl)(dialkyl)amino)benzofuro[2,3-f]quinazolin-1(2H)-ones 119, respectively.
9-methoxyellipticine 30 was synthesized starting from carbazole 122b (Scheme 26). The N-Cbz group was removed by catalytic hydrogenation, and resulting amine 123 was subjected, without further purification, to oxidation with MnO 2 in dioxane at 100 °C, to afford 124 in 70% yield. Removal of the N-Boc group with BBr 3 afforded 30 in 75% yield.