Synthesis of Pyrroloquinones via a CAN Mediated Oxidative Free Radical Reaction of 1,3-Dicarbonyl Compounds with Aminoquinones

Pyrroloquinone ring systems are important structural units present in many biologically active molecules including a number of marine alkaloids. For example, they are found in a series of marine metabolites, such as tsitsikammamines, zyzzyanones, wakayin, and terreusinone. Several of these alkaloids have exhibited antimicrobial, antimalarial, antifungal, antitumor, and photoprotecting activities. Synthesis of pyrroloquinone unit is the key step in the synthesis of many of these important organic molecules. Here, we present a ceric (IV) ammonium nitrate (CAN) mediated oxidative free radical cyclization reaction of 1,3-dicarbonyl compounds with aminoquinones as a facile methodology for making various substituted pyrroloquinones. 1,3-dicarbonyl compounds used in this study are ethyl acetoacetate, acetylacetone, benzoyl acetone, and N,N-dimethyl acetoacetamide. The aminoquinones used in this study are 2-(benzylamino)naphthalene-1,4-dione and 6-(benzylamino)-1-tosyl-1H-indole-4,7-dione. The yields of the synthesized pyrroloquinones ranged from 23–91%.

derivatives, for example, inhibit indoleamine-2,3-dioxygenase, an important enzyme contributing to tumor immune invasion [7]. Tsitsikammamines A-B also exhibit antimicrobial, antimalarial, and antifungal properties, cytotoxicity, and topoisomerase inhibition [8]. Additionally, marine alkaloid wakayin inhibits topoisomerase II, damages DNA, exhibits strong antimicrobial property against Bacillus subtilis, and is potent toward human colon cancer cell lines [1]. Zyzzyanones A-D contain a bispyrroloquinone ring system and have exhibited cytotoxicity against Ehrlich carcinoma cells at micromolar range [2,3]. Moreover, bispyrroloquinone is also the core structure of the marine fungus metabolite terreusinone (1), which is a potent UV-A protectant [9]. The photoprotecting activity of terreusinone is stronger than that of the commercial sunscreen ingredient oxybenzone [10]. For these reasons, pyrroloquinone alkaloids are regarded as a source of new antitumor and dermatological drugs [11][12][13][14][15][16][17]. In addition to this, 3-methyl-1H-benz[f] indole-4,9-dione (3) is a natural product isolated from the barks of G. tapis and G. uvaroides containing pyrrolonaphthaquinone ring system. This compound shows strong inhibitory effects on platelet-activating factor (PAF) receptor [18]. Selected few examples of the natural products containing pyrroloquinone units are given in Figure 1.
Unfortunately, these natural products are isolated from natural sources only in minute quantities, which impose a limitation on their thorough biological evaluation. Additionally, the unique fused-ring aromatic structure poses a challenge in the total synthesis of these natural products. Several efforts have been made towards achieving the synthesis of these natural products [8,10,17,19]. Our group has recently used an oxidative free radical cyclization reaction as a key step in the synthesis of zyzzyanones and the intermediates [19,20].
Oxidative free radical reactions facilitated by transition metals have been known to promote carbon-carbon bond formation. In these reactions, the electron transfer between the radical precursor and metal complex generates electrophilic radicals, which ultimately react with alkenes, alkynes, or quinones to form carbon-carbon bonds [21][22][23][24][25][26]. Among the metal salts that have been investigated in the past two decades for facilitating oxidative free radical cyclization, Mn-(OAc) 3 and ceric (IV) ammonium nitrate (CAN) were proven to be the most efficient catalysts. A proposed mechanism of action for these reagents for effecting oxidative free radical cyclization has also been reported [21,27,28]. Mn-(OAc) 3 and CAN have been extensively used in the synthesis of naphthoquinone, which is an important skeleton of natural products, such as mitosenes, kinamycins, and murrayaquinones [29,30]. The synthesis of bispyrroloquinone by a CAN mediated oxidative free radical cyclization has been reported from our lab [20]. An Mn (OAc) 3 mediated oxidative free radical cyclization leading to the total synthesis of zyzzyanones A-D has also been reported from our lab [19]. As an extension of these studies, herein, we demonstrate the general synthetic utility of CAN mediated oxidative free radical cyclization of various aminoquinones with 1,3-dicarbonyl compounds to form 24 new substituted pyrroloquinones.
Yields and the specific reaction conditions used for the oxidative free radical cyclization of the aminoquinones (8a-c and 10a-c) with four 1,3-dicarbonyl compounds are summarized in Figure 4. 1,3-dicarbonyl compounds used in this study are ethyl acetoacetate, acetylacetone, benzoyl acetone and N,N-dimethyl acetoacetamide. The reactions resulted in the formation of 24 new substituted pyrroloquinones with yields ranging from 23% to 91%. Most of these reactions were carried out in CH 2 Cl 2 and MeOH, except for entries 2 and 3 where a combination of CH 2 Cl 2 and EtOH was used as solvent. Initially, the reaction of 8a with 4 equivalents of ethyl acetoacetate yielded the expected product 4a in good yield (80%, entry 1). However, latter reactions employed fewer equivalents of β-carbonyl compounds, which tended to result in cleaner reactions and made purification easier. Unfortunately, when compound 8b reacted with ethyl acetoacetate in CH 2 Cl 2 and MeOH, the reaction proceeded well, but a mixture of methyl and ethyl esters was obtained due to transesterification. So, a combination of CH 2 Cl 2 and EtOH was used in these cases to obtain the products, 4b and 4c in 35% and 82%, respectively (entries 2 and 3).
The majority of pyrroloquinones 4a-l were obtained in good yields regardless of the types of β-carbonyl starting materials used. The products 4a-l were usually yellow compared to the red orange starting materials 8a-c. Additionally, we experimented with 1, 1.8, and 4 equivalents of β-carbonyl compounds and found that only 1 equivalent of β-carbonyl compounds was sufficient to bring the reaction to the completion. Highest yields were obtained when the electron donating methoxy group was present on the benzyl substituent (entries 3, 6, 9, and 12, 82-91% yield). In contrast, the presence of nitro group on the benzyl group resulted in significantly lower yields (entries 2, 5, 8, and 11, 31-38% yield). Entries with unsubstituted benzyl substituents resulted in moderate yields (entries 1, 4, 7, and 10, 51-80%). The reactions of 8b with all four 1,3-dicarbonyl compounds proved to be difficult. This is due to several reasons, firstly, poor solubility of 8b in CH 2 Cl 2 and MeOH which required the usage of triple amount of solvent volume and heating to dissolve the starting material. Secondly, the reaction did not proceed at room temperature as in the other cases and needed refluxing to force the reactions to go to completion. In addition, more CAN (1.5-2.5 equiv) and longer reaction times were required for the reaction to go to completion.
Finally, the reactions always resulted in a significant amount of side products, which ultimately led to low product yields. When 1,3-diketones such as acetylacetone and benzoylacetone are used in these experiments, theoretically, two regioisomeric products are possible. The two possible products (A and B) for the reaction between benzoylacetone and 2-benzylamino naphthalene-1,4-dione mediated by CAN are illustrated in Figure 2. However, only one product (4g) was formed in this reaction, and it was proved to be the isomer A by 1 H-NMR, 13 C-NMR, and NOESY experiments. In the NOESY NMR experiment of compound 4g as indicated in Figure 3, the methyl group (CH 3 ; singlet; 2.18ppm) clearly had a NOESY correlation with the benzyl methylene group (CH 2 ; singlet; 5.76 ppm). The experiment clearly establishes that the product formed is regioisomer A. The absence of the regioisomer B is perhaps due to the steric hindrance between the two bulky phenyl groups, which makes the structure significantly more unstable.
Compounds 10a-c were reacted with the 1,3-dicarbonyl reagents, including ethyl acetoacetate, benzoyl acetone, N,N-dimethyl acetoacetamide, and acetylacetone. The reactions were carried out in the 1 : 5 ratio mixtures of CH 2 Cl 2 and MeOH. The yields and reaction condition to obtain the final products 5a-l are given in Figure 4. In this study, four equivalents of CAN were necessary for the reactions to complete while less equivalents or absence of CAN resulted in the incomplete or no reactions. The types of 1,3-dicarbonyl reagents did not affect the outcome of the reaction as there were no trends affecting percent yields when different β-carbonyl reagents were used. Interestingly, the reaction of 6-(benzylamino)-1-tosyl-1H-indole-4,7-dione with 1,3-diketones is expected to yield two regioisomeric products, but only one product was formed as in the case of 2-(benzylamino)naphthalene-1,4-dione system.

Conclusions
Synthesis of pyrroloquinone unit is the key step in the synthesis of several biologically important organic molecules. A CAN mediated oxidative free radical cyclization reaction of 1,3-dicarbonyl compounds with aminoquinones leading to the formation of various substituted pyrroloquinones is presented. 1,3-dicarbonyl compounds used in this study are acetylacetone, benzoyl acetone, ethyl acetoacetate, and N,N-dimethyl acetoacetamide. The aminoquinones used in this study are 2-(benzylamino)naphthalene-1,4-dione and 6-(benzylamino)-1-tosyl-1H-indole-4,7-dione. The yields of the synthesized pyrroloquinones ranged from 23 to 91%. Interestingly, we found that only one regioisomer was formed even when 1,3-diketones like benzoyl acetone were used. Finally, the majority of the oxidative free radical cyclized products were isolated as yellow solids in good yields.

General Considerations
The NMR spectra were recorded on a Bruker DPX 300, DRX 400, or AVANCE 700 spectrometers. Chemical shifts are reported in ppm relative to TMS or CDCl 3 as internal standard. The values of chemical shift (δ) and coupling constants J were given in parts per million and in Hz, respectively. Mass spectra were recorded using an Applied Biosystems 4000Q Trap and Micromass Platform LCC instruments. Thin-layer chromatography was performed with silica gel plates with fluorescent indicator (Whatman, silica gel, UV254, and 25 μm plates) and visualized by UV (wavelengths 254 and 365 nm). The reaction mixture was purified by column chromatography using silica gel (32-63 μm) from Dynamic Absorbent Inc. Melting points were uncorrected and obtained from Mel-Temp II apparatus. Solvents were removed in vacuo by using rotatory evaporator. The recrystallization was assisted by Fisher Scientific FS30 sonicator. Anhydrous solvents were purchased in Sure-Seal bottles from Aldrich chemical company. Other reagents were obtained from Aldrich and Acros chemical companies.

2-(Benzylamino)naphthalene-1,4-dione (8a)-
To a solution of 1,4naphthoquinone 7 (5.0 g, 31.62mmol) in THF (50mL), a solution of benzylamine 6a (6.91 mL, 63mmol) in MeOH (50 mL) was added. The reaction was refluxed under N 2 atm for 36 h. Upon the completion of the reaction as indicated by TLC (100% CH 2 Cl 2 ), the solvents were removed in vacuo. The residue obtained was dissolved in EtOAc (700 mL) and washed with water (2 × 200 mL), brine (200 mL) and dried over Na 2 SO 4 . The drying agent was filtered off, and the solution was concentrated under reduced pressure to obtain the crude product which was then purified by chromatography over silica gel using 100% CH 2 Cl 2 as eluent to afford compound 8a as a red solid (6.5 g, 80%); Mp: 137-141°C; 1

Ethyl 1-benzyl-2-methyl-4,9-dioxo-4,9-dihydro-1H-benzo[f]indole-3carboxylate (4a)-
To a solution of compound 8a (0.080 g, 0.30mmol) and ethyl acetoacetate (0.158 g, 1.21mmol) in MeOH and CH 2 Cl 2 (7 : 3, 10 mL), CAN (0.584 g, 1.06mmol) was added in four portions at 10min intervals. After another 10min of stirring at room temperature, TLC analysis (100% CH 2 Cl 2 ) revealed the completion of the reaction. The solvents were removed in vacuo. Water (50 mL) was added to the residue and extracted with CH 2 Cl 2 (3 × 30mL). The combined CH 2 Cl 2 layer was washed with water (2×30 mL), brine (20 mL) and dried over Na 2 SO 4 . The drying agent was filtered off, and the filtrate was concentrated under reduced pressure to obtain the crude product which was purified by column chromatography over silica gel using EtOAc/hexanes (1 : 3) as eluent to furnish the product 4a as a yellow solid (0.091 g, 80%); Mp: 157-160°C; 1 HNMR(CDCl 3 , 300MHz)  Compound 8b (0.050 g, 0.16mmol) and ethyl acetoacetate (0.021 g, 0.16mmol) were dissolved in a mixture of EtOH and CH 2 Cl 2 (7 : 3, 33 mL) by heating the solution for 15min. After the removal of heating, CAN (0.388 g, 0.71mmol) was added in four installments at 10min intervals. After 16 h of stirring at room temperature, TLC analysis (100% CH 2 Cl 2 ) showed the completion of reaction. After the solvents were removed in vacuo, water (50 mL) was added to the residue and extracted with CH 2 Cl 2 (3 × 30mL). The combined CH 2 Cl 2 layer was washed with water (2×30 mL), brine (20 mL) and dried over Na 2 SO 4 . The drying agent was filtered off, and the crude product obtained was purified by column chromatography over silica gel using 100% CH 2 Cl 2 and recrystallized with CH 2 Cl 2 /hexanes to isolate compound 4b as a yellow solid (0.023 g, 35%); Mp: 96-98°C; 1 To a solution of compound 8c (0.080 g, 0.27mmol) in a mixture of EtOH and CH 2 Cl 2 (7 : 3, 10 mL), ethyl acetoacetate (0.036 g, 0.27mmol) was added, and the solution was charged with CAN (0.523 g, 0.96mmol) in four portions at 10min intervals and stirred at room temperature for another 10min. TLC analysis (100% CH 2 Cl 2 ) indicated the reaction was complete. After the solvents were removed under reduced pressure, water (50 mL) was added to the residue. It was extracted with CH 2 Cl 2 (3 × 30mL), washed with water (2 × 30 mL), brine (20 mL) and dried over Na 2 SO 4 . Drying agent was filtered off, and the filtrate was concentrated in vacuo to obtain the crude product, which was purified by column chromatography over silica gel (eluted with 100% CH 2 Cl 2 ) and recrystallized with CH 2 Cl 2 /hexanes to isolate compound 4c as a yellow solid (0.090 g, 82%); Mp: 127-129°C; 1