INTRODUCTION:

Azacarbazoles elicit a variety of important beneficial and untoward biological responses. The interest on azacarbazoles has stemmed eversince these nuclei have exhibited promising cytotoxic activity and antioxidant properties¹but their major practical applications have emerged in the medicinal field where their derivatives showed such widely differing activities as bacteriostatic², anti-inflammatory³, anti-viral⁴, anti-cancer⁵, antihistaminic⁶ and psychopharmacological⁷ properties. A recent demonstration that these compounds can be used as potential anti-HIV agents has stimulated further interest in these molecules from yet another perspective⁸.

Indophenazine also exhibit impressive biological activity and has been used as antimicrobial and against herpus virus along with potent anticancer effects⁹. It has been known that incorporation of certain bioactive pharmacophores in the existing drug molecules sometimes exert a profound influence on the biological profiles of that molecule.

Greatly encouraged by this trend and by the biological profiles of indophenazines and azacarbazoles, we thought that it was worthwhile to apply this concept to synthesize scaffolds incorporating in them the structural features of indophenazine and azacarbazole in the same molecule on the premise that their presence in tandem in single molecular framework could contribute significantly to the biological activity in the resulting molecules.

Synthetic approaches¹⁰ to azacarbazoles have been of special interest and of contemporary importance on accont of a large variety of azacarbazole derivatives showing cytotoxic properties^{11, 12}. For many azacarbazoles, the cytotoxicity can be related to DNA dependent topoisomerase and telomerase enzyme inhibitions. Due to their polycyclic and planar structure for intercalation with DNA, the azacarbazoles remain one of the main targets in synthesis for the study of their cytotoxic properties. The utilization of this property of azacarbazole derivatives has triggered the development of a variety of methods for their synthesis. We report in this communication an elegant procedure, involving the application of Japp-Klingemann reaction followed by Fischer indolization to the synthesis of N-substituted-2, 3-dihydro-14H-azacarbazolo(2, 3-b)-pyrazino(2, 3-b)-11H-indole-1-ones 8(a-d) (Scheme-2) and N-substituted-2, 3-dihydro-14H-azacarbazolo(2, 3-b)-6H-pyrrolo (2, 3-b)-quinoxaline-1-ones 11(a-d) (Scheme-3) from 4 and 5- indophenazine amines 4(a-b) and N-substituted-4-piperidones 6(a-d) respectively. Indophenazine amines 4(a, b) required in the synthesis were readily available from the reduction of the corresponding nitro compounds 3(a, b) with Fe+HCl. ^{13, 14} The reaction of isatin with o-phenylenediamine has been reported to provide a very convenient synthetic entry to the indophenazine nucleus. ¹³ This methodology was applied to obtain 4-nitro-indophenazine (3a), from the reaction of isatin (1b) with 4-nitro-o-phenylenediamine (2a). A similar reaction of 5-nitro-isatin (1c) with o-phenylenediamine (2b) afforded the required 5-nitro-indophenzine (3b) respectively (Scheme-1).

Diazotized aryl amines have been known to undergo Japp-Klingemann reaction^15-17with 2-hydroxymethylidine cyclohexanones (which can be generated on treatment of cyclohexanone with ethylformate in presence of EtONa) and give the carbazole derivative on subsequent Fischer indolization. The application of this strategy on N-substituted-3-hydroxymethylidine-4-piperidones 6(a-d) with 4-indophenazine diazonium chloride (5) (Scheme-2) and corresponding 5-indophenazine diazonium chloride (9) (Scheme-3) afforded the azacarbazole derivatives 8(a-d) and 11(a-d) respectively in moderate to good yield.

EXPERIMENTAL:

General method for the preparation of 4-nitro-6H-indolo [2, 3-b] quinoxaline (3a):

A mixture of isatin (1a) (1.4g, 0.01mole) and 4-nitro-o-phenylenediamine (2a) (1.5g, 0.01moles) was refluxed in acetic acid (10ml) for three hours. The residue obtained on dilution of the mixture with water was recrystallized from acetic acid to give yellow leaflets 3a. A similar reaction yielded 3b from 1b and 2b.

General method for the preparation of 6H-indolo [2, 3-b] quinoxaline-4-amine (4a):

Compound 2a (2.6g, 0.01mole) in methanol (25ml, 0.056mole) and conc. HCl (9ml) were taken in 250ml round bottom flask and the mixture was heated on the water bath. To it iron powder (1.12g, 0.02mole) was added pinch by pinch for an hour with continous stirring. The reaction mixture was further refluxed for 1 hour and then filtered hot. The filterate was neutralized with aqueous ammonia (50%) and the mixture it was extracted with ethylacetate. Drying and evaporation of the solvent gave the crude mass which was recrystallized from ethanol to give as fine dark brown powder 4a. A similar reaction yielded 4b from 3b.

General method for the preparation of 8(a-d):

Preparation of hydrazone (7a):

A solution of indophenazine-4-amine (4a) (1.6g, 0. 005mol) in aqueous hydrochloric acid (0.2ml conc. HCl in 0.5 ml water) was treated with a cold saturated solution of sodium nitrite (0.1g in 0.20 ml water) while the temperature was maintained at 0 to 5ºC. The solution was kept aside for 10 minute. It was then added portion wise to an ice cooled mixture containing 1-benzyl-3-(hydroxymethylidine)-piperidin-4-one 6(a) (0.05mol) (prepared from the reaction of 1-benzyl-piperidin-4-one with ethyl formate in presence of sodium ethoxide), sodium acetate trihydrate (0.25g), methanol (1.5 ml) and water (10 ml) over a period of 0.5 hr with stirring. The contents were allowed to stand for further 0. 5 hrs and the resulting solid mass 7(a) was filtered, washed with water, dried and crystallized from ethanol. The compounds 7(b-d) and 10(a-d) were prepared following the same procedure.

Cyclisation of hydrazone (7a):

A solution of hydrazone 7(a) (1. 7g, 0. 005 mol) in a mixture of acetic acid (10 ml) and hydrochloric acid (0. 5ml) was refluxed on an oil bath preheated to 125-130⁰C for 0. 5 hrs. The content were then cooled and poured in ice cold water with continuous stirring. The separated brown solid 1. 24 g 8(a) was purified by passing through a column of silica gel using 50% benzene in pet. ether as eluant (yield 72%), m. p. , 299-302⁰C. Compounds 8(b-d) and 11(a-d) were prepared in a similar way.

Scheme 1

Scheme:2

RESULTS AND DISCUSSION:

The procedure developed by Japp-Klingemann for the preparation of the hydrazones from the carbonyl compounds containing an adjacent methylene group was applied in the present work in the preparation of 7(a-d) from 5 (Scheme:2) and 10(a-d) from 9 (Scheme:3).

The mechanism for this reaction is outlined in Scheme-4. The intermediates 6(a-d) were obtained by the base catalyzed condensation of N-substituted-4-piperidones with ethyl formate. Compounds 7(a-d) and 10(a-d) underwent a facile acid catalyzed cyclocondensation with Kent’s reagent (a 4:1 mixture of acetic acid and hydrochloric acid) under the condition of Fischer indolization to furnish the desired azacarbazole derivatives 8(a-d) and 11(a-d) respectively in good yield.

Table 1: Physical and analytical data of compounds 4(a-d) and 7(a-d):

Comp ound	Molecular Formula	M. P(⁰C)	yeild(%)	N Analysis(%) (Cald. /Found)
8a	C₂₆H₁₉N₅O	299-302	72	16. 78/16. 64
8b	C₂₂H₁₉N₅O	260-265	71	18. 96/18. 85
8c	C₂₁H₁₅N₅O₂	209-301	60	18. 96/18. 84
8d	C₂₁H₁₅N₅O₃	288-291	65	18. 17/18. 09
11a	C₂₆H₁₉N₅O	270-275	65	16. 78/16. 70
11b	C₂₂H₁₉N₅O	275-278	72	18. 96/18. 85
11c	C₂₁H₁₅N₅O₂	260-265	75	18. 96/18. 84
11d	C₂₁H₁₅N₅O₃	267-272	65	18. 17/18. 09

Table:2 Spectral data of compounds 4(a-d) and 7(a-d)

Comp

IR (KBr) cm-1

1HNMR (δppm)

MS: m/z

3110. 1710,

1612, 1080

10. 1(2H, s, NH);

8. 07(2H, d, CH);

7. 0-7. 55 (8H, m, Ar-H)

4. 32 (2H, s, benzyl-CH₂)

3. 39(2H, t, CH₂)

2. 63(2H, t, CH₂)

410. 10(100%)

417. 16(30%)

3150. 1750,

1612, 1050

10. 1(2H, s, NH);

8. 07(2H, d, CH);

7. 6(1H, s, CH)

7. 68-8. 07 (4H, m,

Ar-H)

3. 39 (2H, s, benzyl-CH₂)

2. 97(1H, m,

N-substituted-H)

2. 63(2H, t, CH₂)

1. 05(6H, d, CH₃)

330. 15(100%)

369. 42(20%)

3209. 1702,

1585, 1060

10. 1(2H, s, NH)

8. 07(2H, d, CH);

7. 00-7. 55 (4H, m,

Ar-H)

3. 71 (2H, t, CH₂),

2. 55(2H, t, CH₂)

2. 02(3H, s, CH₃)

350. 20(100%)

369. 38(40%)

3300. 1700,

1560, 1090

10. 1(2H, s, NH);

8. 07(2H, d, CH);

7. 00-7. 55 (4H, m,

Ar-H)

3. 67 (3H, s, CH₃),

3. 29(2H, t, CH₂)

2. 66(2H, t, CH₂)

360. 10(100%)

385. 38(40%)

11a

3100. 1715,

1610, 1050

10. 1(2H, s, NH);

8. 07(2H, d, CH);

7. 0-7. 55 (8H, m, Ar-H)

4. 32 (2H, s, CH₂)

3. 39(2H, t, CH₂)

2. 63(2H, t, CH₂)

410. 10(100%)

417. 16(30%)

11b

3130. 1710,

1600, 1070

10. 1(2H, s, NH);

8. 07(2H, d, CH);

7. 6(1H, s, CH)

7. 68-8. 07 (4H, m,

Ar-CH)

3. 39 (2H, s, CH₂),

2. 97(1H, m,

N-Substituted-H)

2. 63(2H, t, CH₂)

1. 05(6H, d, CH₃)

330. 15(100%)

369. 42(20%)

11c

3230. 1710,

1580, 1040

10. 1(2H, s, NH);

8. 07(2H, d, CH);

7. 00-7. 55 (4H, m,

Ar-H)

3. 71 (2H, t, CH₂),

2. 55(2H, t, CH₂)

2. 02(3H, s, CH₃)

350. 20(100%)

369. 38(40%)

11d

3290. 1710,

1540, 1070

10. 1(2H, s, NH);

8. 07(2H, d, CH);

7. 00-7. 55 (4H, m,

Ar-H)

3. 67 (3H, s, CH₃),

3. 29(2H, t, CH₂)

2. 66(2H, t, CH₂)

360. 10(100%)

385. 38(40%)

The nitrogen analysis and spectral data of all the compounds (Table 1 and 2) were found to be in good agreement to the assigned structures. The most diagnostic evidence for the incorporation of N-substituted-4-piperidone skeleton in the indole framework was provided by the appearance of NH proton of indole in the ¹H-NMR spectrum in the region of δ10. 0 in all the compounds.

CONCLUSION:

Two noteworthy features of the reactions employed in the preparation of indolopyrazino condensed azacarbazoles 8(a-d) and pyrroloquinoxalino condensed azacarbazoles 11(a-d) derivatives are apparent from our study. Firstly, it established that the Fischer indolization of the 3-arylhydrazones of pyrazino(2, 3-b)-indole-1-ones 7(a-b) and pyrrolo (3, 2-b)-indole-1-ones 10(a-b) provided a very convenient synthetic entry to the difficultly accessible fused azacarbazole derivatives 8(a-b) (Scheme 2) and 11(a-b) (Scheme 3) respectively. Secondly, it established further the versatility of Japp-Klingemann reaction to provide a one pot synthetic approach to the preparation of the aryl hydrazones on the adjacent methylene carbon, in a cyclic nitrogen containing carbonyl species.

ACKNOWLEDGEMENT:

Authors are thankful to CDRI, Lucknow and SAIF Department Chandigarh for providing the spectral data of the compounds.

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Received on 07.12.2010 Modified on 02.01.2011

Asian J. Research Chem. 4(3): March 2011; Page 441-444