Revision of the Structure and Total Synthesis of Topsentin C

An efficient synthetic approach to access (indol-3-yl)ethane-1,2-diamines with a protecting group at the indole N atom from readily available 3-(2-nitrovinyl)indoles is reported. This approach includes solvent-free conjugate addition of O -pivaloylhydroxylamines to 1-Boc-3-(2-nitrovinyl)indoles followed by mild reduction of the adducts. The obtained (indol-3-yl)ethane-1,2-diamines are convenient synthetic precursors for several classes of marine alkaloids. The first total synthesis of racemic topsentin C, a secondary metabolite from Hexadella sp., based on this approach is reported. The initially proposed structure for topsentin C has been revised.

Secondary metabolites from marine invertebrates continue to be an attractive research topic because new structures and compounds with useful biological activity can be discovered. 1 A whole series of alkaloids containing the (indol-3-yl)ethane-1,2-diamine moiety in their structures and their aromatized derivatives were isolated from deep-water sponges in the last 30 years. 2 In particular, spongotines (1) and topsentins (2) contain two indoles connected through imidazoline or imidazole linker. Two indole substituents in the structures of hamacanthins (3) and dragmacidins (4) are bonded to dihydropyrazinone and piperazine rings, respectively ( Figure 1). The (indol-3-yl)ethane-1,2-diamine moiety in several alkaloids of the examined group contains one or two methyl groups; for example, dragmacidins A and B (4) and topsentin C. The latter compound was isolated from Hexadella sp., and its structure was assigned to imidazoline derivative 5a (Figure 2). 3 Furthermore, the alkaloids could contain one or more Br atoms, which in general is characteristic of marine secondary metabolites. 4 Notably, the 1,2-diaminoethyl group in the indole 3-position, in contrast to 2-aminoethyl, is uncharacteristic for terrestrial indole alkaloids. Total syntheses of many of the natural products from this group have been reported; 2,5,6 however, no synthesis of topsentin C has been reported. Compounds exhibiting anti-

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bacterial, cytotoxic, antiviral, and fungicidal properties were discovered among these alkaloids and their synthetic analogues. 2,7 Figure 2 Proposed structure of topsentin C (Indol-3-yl)ethane-1,2-diamines could be convenient synthetic precursors of topsentins, spongotines, and hamacanthins. 5a-c Furthermore, these diamines are of independent interest because their simple derivatives were recently shown to be capable of preventing the development of resistance to fluoroquinolone antibiotics in Staphylococcus aureus. 8 Only two synthetic approaches to (indol-3-yl)ethane-1,2-diamines have been published and neither of them allows the corresponding N 1 -methyl derivatives to be produced. 5a-c It was also reported that diamines of this type with an unsubstituted indole N atom are relatively stable only as the salts. 5a We propose a convenient preparative synthetic approach to (indol-3-yl)ethane-1,2-diamines (6) with a protected indole N atom that is based on mild reduction of 7, the addition product of O-pivaloylhydroxylamines (8) and 3-nitrovinylindoles (9) (Scheme 1). The proposed method has been used for the total synthesis of topsentin C, the previously proposed structure of which has been revised by us.
Scheme 1 Our Synthetic approach to (indol-3-yl)ethanediamine 6 Starting nitrovinylindoles 9, with tert-butoxycarbonylprotected indole N atoms, were synthesized from the corresponding indoles by formylation using N,N-dimethylformamide (DMF) and SOCl 2 followed by condensation of the obtained aldehydes with nitromethane and addition of the protecting group in the presence of 4-(N,N-dimethylamino)pyridine (DMAP) (Scheme 2). We also prepared 1-ace-tyl-3-[(E)-2-nitrovinyl]-1H-indole (9f) and 1-methyl-3-[(E)-2-nitrovinyl]-1H-indole (9g) according to described procedures. 9,10 Reduction of the adducts of α,β-unsaturated nitrocompounds with amines, O-alkylhydroxylamines or azide anion was proposed earlier for the synthesis of vicinal diamines. [11][12][13] The first type of adduct was unstable, although they could be isolated as the more stable salts. 11 Adducts with O-alkylhydroxylamines or azide anion were more stable. 12,13 However, catalytic hydrogenation or heating with Zn dust in HOAc was required to cleave the hydroxylamine N-O bond. 12 Catalytic hydrogenation was also used to reduce the azido group. 13 The proposed methods turned out to be ineffective for the synthesis of Br-containing (indol-3yl)ethane-1,2-diamines 6. Thus, the products 10 from the reaction of methylamine or benzylamine with nitrovinylindole 9b could not be isolated; starting 9b was recovered and the reaction mixture formed a resin. Stable adduct 11

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with O-benzylhydroxylamine was reduced as expected by H 2 over Pd/C with hydrogenolysis of the C-Br bond to form diamine 6a (Scheme 3).
Indolic adduct 11 decomposed upon heating with Zn in HOAc, and reduction with Zn under milder conditions was not complete. The yield of diamine 6b was <15% even when the amount of Zn and the reaction time were increased; the main product was 12 (Scheme 3).
O-Acylhydroxylamines have highly labile N-O bonds and have recently been used in synthetic procedures based on sigmatropic shifts with cleavage of N-O bonds 14 in addition to amination reaction. 15 Conjugate addition of O-acylhydroxylamines to electron-deficient alkenes has not yet been described, in contrast to O-alkylhydroxylamines. We decided to study the possibility of adding O-pivaloylhydroxylamine and its N-methyl derivative to nitrovinylindoles 9 followed by reduction of the resulting adducts. Derivatives of sterically hindered pivalic acid were chosen because they isomerize rather slowly into the corresponding hydroxamic acids, in contrast to the simpler O-acylhydroxylamines. 16 Hydrochlorides of O-pivaloylhydroxylamine and Nmethyl-O-pivaloylhydroxylamine were synthesized by using the previously reported methods. 14a,16,17 The corresponding free bases were isolated immediately before performing the next step.
As it turned out, the reaction of nitrovinylindoles 9a with O-pivaloylhydroxylamine (8a, 1.5 equiv) in CH 2 Cl 2 was complete in 96 hours and gave target adduct 7a. We also found that the solvent-free reaction was much faster. The reagents could be mixed and left overnight in a closed vessel. The nitrovinylindoles dissolved gradually, then crystals of the product formed. The solvent-free reaction was clearly advantageous from a green chemistry point of view. In this manner, we obtained the series of adducts 7a-h in high yields (Table 1). The next step was the reduction of adducts 7. We decided to use Zn and acid, anticipating that their hydroxylamine N-O bond would undergo reductive cleavage at room or reduced temperature. Thus, the reduction of 7a using Zn (10 equiv) and HOAc in MeOH afforded target diamine 6a in 34% yield ( Table 2, entry 1).

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The reaction proceeded rather quickly. However, it was accompanied by the formation of several unidentified side products. Increasing the reaction time did not lead to an increase in the yield of 6a. We found that the yield could be increased by adding H 2 O and EtOAc to the reaction mixture (keeping the solution homogeneous) and by doubling the amount of Zn (cf. Table 2, entries 3 and 4). Replacing HOAc with concentrated HCl at reduced temperature led to a further increase in the yield (entry 5). Finally, the use of HBr (40%) allowed target diamine 6a to be obtained with a very good yield (entry 6). It was also possible to conduct the reduction of adduct 7a in the presence of NH 4 Br (entry 7). The reduction of adduct 7c, with a methyl group on the hydroxylamine N atom, was more difficult. However, increasing the reaction time and amount of Zn provided a high yield of diamine 6c (entry 9). The developed method was extended to adducts 7b and 7d-f (Table 3). Boc derivatives gave the corresponding diamines 6b and 6d-g in good and high yields, whereas diamine 6h, with an acetyl on the indole N atom, was unstable and decomposed even in solution. Having established a convenient preparative method for N 1 -methyl(indol-3-yl)ethane-1,2-diamines, we focused on the total synthesis of the proposed structure for topsentin C (5a), which is related to spongotines 1 and other previously isolated topsentines. The imidazoline fragment of 5a and its analogue 5b, without a Br atom, was constructed by using the previously reported synthetic method for imidazolines that involved condensation of the vicinal diamines with aldehydes (including α-keto aldehydes) followed by oxidation of the resulting cyclic aminal. 5c, 18 The required indolylglyox-als 14a and 14b were prepared from corresponding 3acetylindoles 15a and 15b through iodination followed by Kornblum oxidation (Scheme 4). 19

Scheme 4 Synthesis of spongotine analogues 5a and 5b
The aforementioned syntheses of indolylglyoxals 14a and 14b, condensations with diamines 6c and 6e, and subsequent oxidations to imidazolines 16a and 16b were carried out in one pot. This made the developed procedure attractive for preparative reactions. The protecting group could be removed to afford 5a and 5b. As it turned out, the spectral characteristics of 5a synthesized by us and the characteristics of topsentin C that was isolated from the natural source, differed dramatically. Therefore, the initially proposed structure of topsentin C had to be revised. Thus, the synthesized 5a was a methylated spongotine C derivative that has not yet been observed in nature. The developed method enables analogues of spongotines and topsentins to be synthesized to study their biological properties, which are known to change abruptly if even a single methyl is added to the molecule. 17 We assumed that natural topsentin C was structurally related to hamacanthins A (3) but not spongotines (1), and was the 1-methyl derivative of hamacanthin A 17a ( Figure  3), 20 which should have a set of NMR signals similar to that of 5a.

Scheme 5 Synthesis of topsentin C and its analogue
We synthesized bis(indolyl)dihydropyrazinones 17a and 17b through cyclization of diamines 6e and 6c with indoleglyoxylic acid chlorides 18a and 18b to confirm this hypothesis (Scheme 5).
Acid chlorides 18a and 18b were obtained through acylation of the corresponding indoles by using oxalylchloride according to published methods. 6k The reaction first gave a mixture of amides 19 and 20, which were further cyclized without isolation (Scheme 6). As noted earlier during the development of synthetic methods for hamacanthins, these amides can undergo reversible transformations under the cyclization conditions via intermediate 21, and can form a mixture of isomeric bis(indolyl)dihydropyrazinones. 6b In this case, the cyclization was unidirectional because of the methyl group, so that 22a and 22b were isolated only. 21 These compounds were converted into target 17a and 17b by removing the protecting group. The structure of compound 17a was established by X-ray crystallographic analysis ( Figure 4).
The spectral characteristics of bis(indolyl)dihydropyrazinone 17a were consistent with those of natural topsentin C, 3,22 in contrast to imidazoline 5a (Table 4). This data con-

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firmed our hypothesis regarding the structure of the natural topsentin C.
Thus, we have developed a convenient preparative synthetic method to prepare (indol-3-yl)ethane-1,2-diamines and found that the natural topsentin C has the structure 17a. The total synthesis of racemic 17a was carried out in seven steps from 6-bromoindole 9c in 55% overall yield.
Starting reagents were either purchased from commercial sources and used without additional purification or were prepared according to reported procedures. 1 H and 13 C NMR spectra were acquired with 400, 500, or 600 MHz spectrometers at r.t. and referenced to the residual signals of the solvent (for 1 H and 13 C). The solvents for NMR samples were DMSO-d 6 , CDCl 3 , and acetone-d 6 . Chemical shifts are reported in parts per million (δ, ppm). Coupling constants are reported in Hertz (J, Hz). The peak patterns are indicated as: s, singlet; d, doublet; t, triplet; q, quadruplet; m, multiplet; dd, doublet of doublets; br s, broad singlet. Signal assignment was based on COSY, HSQC, HMBC and NOESY experiments. Infrared spectra were measured with an Infralum FT-801 FT/IR instrument. The wavelengths are reported in reciprocal centimeters (ν max , cm -1 ). Mass spectra were recorded with LCMS-8040 triple quadrupole liquid chromatograph mass-spectrometer from Shimadzu (ESI) and Kratos MS-30 mass spectrometer (EI, 70 eV). Elemental analysis was performed with an Euro Vector EA-3000 elemental analyzer. The X-ray data collection of 17a was performed with a Bruker APEX-II CCD diffractometer at 120 K. Details of the X-ray structure determination are given in the Supporting Information. The progress of the reaction was monitored by TLC and the spots were visualized under UV light (254 or 365 nm). Column chromatography was performed using silica gel (230-400 mesh). Melting points were determined with a SMP-10 apparatus and are uncorrected. Solvents were distilled and dried according to standard procedures.

Addition of O-Pivaloylhydroxylamines 8a,b to Nitrovinylindoles 9a-f; General Procedure
The hydrochloride salt of the corresponding O-pivaloylhydroxylamine (6 mmol) was dissolved in CH 2 Cl 2 (20 mL) and carefully shaken with saturated aqueous NaHCO 3 solution, then the organic phase was washed with concd NaCl solution and dried over anhydrous Na 2 SO 4 . The solvent was removed in vacuo at 30 °C and the resulting O-pivaloylhydroxylamine (ca. 5.2 mmol) was carefully mixed with nitrovinylindole (3.5 mmol) in a round-bottom vial and allowed to stand overnight at r.t. The reaction mixture was triturated with hexane (7 mL) and the resulting crystals of adducts 7a-h were filtered, washed with cold hexane (2 × 4 mL), and dried in vacuo.

Catalytic Hydrogenation of Adduct 11
To a solution of adduct 11 (0.54 g, 1.1 mmol) in MeOH (10 mL) was added AcOH (9.5 mL, 1.165 mol) and 5% Pd on charcoal (0.05 g) and the mixture was purged with hydrogen. The mixture was vigorously stirred under a hydrogen atmosphere (1 atm) for 18 h, filtered, and concentrated in vacuo. The residue was dissolved in CH 2 Cl 2 (50 mL), treated with cold NaOH solution (10%, 40 mL) and concd NaCl solutions (20 mL), and dried over anhydrous Na 2 SO 4 . The solvent was removed in vacuo and the residue was purified by chromatography on a column of silica gel (CHCl 3 -MeOH-NH 3(aq) , 100:10:0.2) to afford diamine 7b (0.206 g, 68 %). Its characteristics correspond to the sample obtained from adduct 9a (see below).

Reduction of Adduct 11 with Zn Dust
To a solution of adduct 11 (0.54 g, 1.1 mmol) in MeOH (10 mL) was added AcOH (9.5 mL, 1.165 mol) and Zn dust (1.44 g, 22 mmol). The mixture was vigorously stirred at r.t. for 2 h, and then another portion of Zn dust (1.43 g, 22 mmol) was added. The resulting mixture was vigorously stirred further for 12 h, filtered, and concentrated in vacuo. The residue was dissolved in CH 2 Cl 2 (100 mL), treated with chipped ice (10 g), and carefully shaken with cold NaOH solution (10%, 90 mL). The organic layer was separated and washed with cold NaOH (10%, 30 mL) and concd NaCl solutions (20 mL), and dried over anhydrous Na 2 SO 4 . The solvent was removed in vacuo and the residue was purified by chromatography on a column of silica gel with gradient of MeOH in CHCl 3 to afford compound 12.

Synthesis of (Indol-3-yl)ethane-1,2-diamines 6; General Procedure A
A solution of adduct 7 (2.2 mmol) in EtOAc (8.3 mL) was added to a cooled (-10 °C) mixture of MeOH (16.5 mL) and HBr (40%, 5.5 mL, 37.8 mmol) under vigorous stirring and treated with Zn dust (2.2 g, 33 mmol). The reaction mixture was allowed to warm to 0-5 °C (1 h), stirred at that temperature for 1 h, and filtered. The precipitate was filtered off and rinsed with a small amount of MeOH. The filtrate was diluted with CH 2 Cl 2 (100 mL), treated with crushed ice (10 g), and carefully shaken with cold NaOH solution (10%, 90 mL). The organic layer was separated and the aqueous layer was washed with CH 2 Cl 2 (20 mL). The combined organic extracts were washed with cold NaOH (10%, 30 mL) and concd NaCl solutions (20 mL), and dried over anhydrous Na 2 SO 4 . The solvent was removed in vacuo to afford target diamine 6 as a yellowish oil. The product was pure enough for further syntheses. Chromatographic purification on a column with silica gel (CHCl 3 -MeOH-NH 3(aq) , 100:10:0.2) was performed, if necessary.

Synthesis of (Indol-3-yl)ethane-1,2-diamines 6; General Procedure B
A solution of adduct 7 (2.2 mmol) in EtOAc (13 mL) was added to a cooled (-10 °C) mixture of MeOH (26 mL) and HBr (40%, 13 mL, 89.0 mmol) under vigorous stirring and treated with Zn dust (1.44 g, 22 mmol). The mixture was allowed to warm to 0 °C (1 h), then another portion of Zn dust (1.44 g, 22 mmol) was added. The reaction mixture was stirred further at 0-5 °C for 5 h, and filtered. The precipitate was rinsed with a small amount of MeOH and the filtrate was diluted with CH 2 Cl 2 (100 mL), treated with crushed ice (10 g), and shaken carefully with cold NaOH solution (10%, 120 mL). General Procedure A was then followed.