A Facile Synthesis of 3-Substituted 9H-Pyrido[3,4-b]indol-1(2H)-one Derivatives from 3-Substituted β-Carbolines

A mild and efficient two-step synthesis of 3-substituted β-carbolinone derivatives from 3-substituted β-carboline in good yields is described. A possible reaction mechanism for the formation of the skeleton of β-carbolin-1-one is proposed. The structures of these compounds were established by IR, 1H-NMR, 13C-NMR, mass spectrometry and elemental analysis, as well as X-ray crystallographic analysis of 4-2 and 6-2.


Introduction
Natural products have a profound impact on both chemical biology and drug discovery, and the substituted β-carboline moiety is an example. β-Carboline is a key pharmacophore present in a large number of natural tricyclic alkaloids, which can be found in numerous plants and animals, exhibiting potent biological activities [1][2][3][4][5][6][7][8][9][10]. As a key member of the β-carboline family, its structural variant tricyclic β-carbolinone (9H-pyrido [3,4-b]indol-1(2H)-one derivative or β-carbolin-1-one, Figure 1) has served as an important intermediate for the preparation of complex alkaloids [11][12][13][14][15][16][17][18][19] and has been found to possess potent bioactivities. The natural and synthetic β-carbolinones are reported to have OPEN ACCESS pharmacological effects in several aspects, such as the anticancer activity against colon and lung cancers, central nervous system activity in mammals, and also as the biological control agent for receptor research on bio-enzyme inhibitors, such as the inhibition of HLE (Human leukocyte elastase) [20][21][22][23]. In continuation of our work on biologically active β-carbolinone alkaloids [24], we focused our interest on the synthesis of 3-disubstituted β-carbolinones. The total synthesis of these substituted tricyclic β-carbolinones has attracted great attention, however, few facile synthetic approaches to β-carbolinones have been published over the years. A majority of alkaloids are substituted at the 3-position of β-carbolinones, and two major synthetic strategies have been adopted for this purpose: one approach starting from tryptamine which offers an easy access to β-carbolinone (9H-pyrido [3,4-b]indol-1(2H)-one, Scheme 1) [25], and another three step route to synthesize 3-aryl-β-carbolinone by reaction of chalcone derivatives with N-acetyl-2cyanoglycine ethyl ester (Scheme 2) [26]. Nevertheless, it is still difficult to introduce other groups directly at the 3-position of the β-carbolinone by these methodologies, especially for the synthesis of electron-withdrawing substituents at the 3-position of β-carbolinone. So it is urgent and significant to find a new and effective way to synthesize a large number of 3-substituted (electron-withdrawing) βcarbolinones. Herein, we report a novel synthetic route for the preparation of 3-substituted (electronwithdrawing substituents) β-carbolinones.

Results and Discussion
In this paper, we describe a two-step preparation of 3-substituted β-carbolinones using 3-substituted β-carbolines as the starting materials (Scheme 3).

Scheme 3.
A route to synthesize 3-substituted β-carbolinones. For the first step, in a mixed and refluxing 1:1 chloroform/ethanol solution, β-carboline derivatives X were treated with 3-chloro-peroxybenzoic acid (m-CPBA) yielding the corresponding N-oxides X-1 in excellent yields, which then were purified by flash column chromatography; in the second step, the β-carbolinones were obtained through the regioselective rearrangement of the β-carboline-N-oxides in acetic anhydride at refluxing and subsequently hydrolysis in a solution of EtOH/aqueous 2 M NaOH (1:1) at room temperature. The 3-substituted β-carboline substrates were synthesized according to the literature procedure [27][28][29][30][31], which is summarized in Scheme 4. Based on our experimental results and other similar reactions [32][33][34][35][36][37], the reaction mechanism of βcarboline N-oxides with glacial acetic acid and sodium hydroxide was presumed to be as shown in Scheme 5. The overall process may be summarized as follows: i) β-carboline-N-oxide X-1 was refluxed with excess acetic anhydride to yield light brown syrup from which intermediate I (2-acetoxyβ-carboline derivative) was formed; ii) another acetyl oxygen anion (CH 3 COO -) attacks the carbon atom at 1-position of 2-acetoxy-β-carboline, and meanwhile the (C 1 = N 2 ) double bond was broken to give ntermediate II (1,2-diacetoxy-1,2-dihydro-β-carboline); iii) elimination of one molecule of acetic acid, and regeneration of the (C 1 = N 2 ) double bond to give intermediate III (1-acetoxy-β-carboline derivative)' iv) by the hydrolysis of 1-acetoxy-β-carbolines III in sodium hydroxide solution, the target compounds were obtained via intermediate IV.
Scheme 5. The possible mechanism for the synthesis of 3-substituted 9H-pyrido [3,4- The various β-carbolines were used as substrates, and the results are summarized in Table 1. Under identical reaction conditions, using 3-ethoxycarbonyl-β-carboline, 3-hydroxymethyl-β-carboline, 3-cyano-β-carboline, β-carboline-3-carbohydrazide, or 3-(N-methylcarbamoyl)-β-carboline as starting materials, we obtained good yields (about 67-85%) of corresponding products (entries 2, 3, 5, 6 and 10 in Table 1). However, other β-carboline derivatives gave moderate yields (about 30-50%) of the corresponding β-carbolinones (entries 4, 7, 8 and 9). This discrepancy may be attributed to the different groups at the 3-position, because electron withdrawing substituents at the 3-position can enhance the electronic stability of intermediate I, and therefore, a relatively higher yield of product could be obtained. However, an electron-rich substituent is not beneficial for the electronic stability of intermediate I, so poor yield were observed in our experiments. All products were fully characterized by spectroscopic means.

Conclusions
We have developed a simple and efficient two-step synthetic route for the synthesis in moderate to good yields of 3-substituted β-carbolinone derivatives from 3-substituted β-carbolines, and the reagents used are not hazardous and are easy to handle. This method offered a facile way to introduce various electron-withdrawing or electron-rich substituents into β-carbolinone derivatives at 3-position, which should broaden the application scope of the β-carbolinone skeleton. This type of reaction has been widely applied in our laboratory for the preparation of 3-substituted β-carbolinone derivatives. In addition, a plausible reaction mechanism has been proposed.

General
Unless otherwise specified, reagents were purchased from commercial suppliers and used without further purification. Reaction progress was monitored using analytical thin layer chromatography (TLC) on percolated Merck silica gel Kiesegel 60 F254 plates, and the spots were detected under UV light (254 nm). Melting points were determined with a digital melting point apparatus and are reported uncorrected. 1 H-NMR and 13 C-NMR spectra were recorded at 300 MHz ( 1 H) or 75 MHz ( 13 C) on a Bruker ARX 300 spectrometer. IR spectra were measured on a Jasco FT/IR-430 spectrophotometer. Mass spectra were recorded on an a Quattro microMS Micromass UK mass spectrometer, and was recorded on an electrospray ionization mass spectrometer as the value m/z. The X-ray measurements were made on a Rigaku RAXIS RAPID diffractometer with a graphite monochromatised Mo Kα radiation (λ= 0.71069 Å) using ω scan mode.

General Procedure for the synthesis of 9H-pyrido[3,4-b]indol-1(2H)-one derivatives
A β-carboline N-oxide derivative (1 mmol) were dissolved in acetic anhydride (10 mL), and the mixture was heated under reflux for 6 hours until there were no starting materials left (TLC control). After being cooled to ambient temperature, the reaction mixture was concentrated under reduced pressure. The intermediate 2-acetoxy-β-carboline derivative was dissolved in EtOH/aqueous 2 M NaOH (1:1), and stirred at room temperature for 2 hours. The solvent was removed in vacuo, and the residue was purified by column chromatography using MeOH/CHCl 3 as an eluent to give the title pyridoindolone.