INTRODUCTION:

Leishmaniasis is one of the most devastating complex of diseases and a major health problem of tropical, subtropical and Mediterranean region. It occurs in all continents except Australia. It is a vector borne disease resulting from infection with one of 20 species of the protozoan parasite leishmania; transmitted by 30 phlebotomine sandfly species. Transmitted by the bite of a sandfly, the clinical spectrum of leishmaniasis ranges from a self-resolving cutaneous ulcer, to a mutilating mucocutaneous disease, to a fatal systemic illness. Represented by a spectrum of clinical presentations and prognoses, an annual incidence of 1.5-2.0 million reported new cases in 88 countries with 12 million presently infected¹. Leishmaniasis is a major public health in developing countries, an emerging problem amongst the immuno-compromised in developed countries.Worldwide, there are 2 million new cases each year and 1/10 of the world’s population is at risk of infection². Leishmaniasis recognized as one of the most dreaded parasitic disease ranking second only to malaria, cause considerable morbidity and mortality in tropical and subtropical region of the world.

Recent estimates suggest that it is endemic in 88 countries and nearly 350 million people are at risk of acquiring this infection and approximately 400,000 new cases are reported annually with 12 million currently infected³.

The disease is caused by various strains of haemoflagellated protozoan parasite of genus Leishmania. The genera leishmania belong to the family – Trypanosomatidae of the order – kinetoplastida. All the members of family – Trypanosomatidae, possess a single locomotory flagellum. The species of the order kinetoplastida are heterotrophic and feed as saprozoic or parasitic organism⁴. The leishmania are divided into three general clinical patterns according to the form of the disease - Visceral Leishmaniasis, Cutaneous Leishmaniasis and Mucocutaneous Leishmaniasis. Visceral leishmaniasis is also known as ‘Kala-Azar’ (Black fever, Burdan fever, Sarkari Bimari etc.) and is caused by Leishmania donovani complex. Leishman and Donovan first described VL in 1903. It is most severe form (nearly always fatal if left untreated), characterized by: undulating fever, loss of weight, splenomegaly, hepatomegaly and/or lymphadenopathies and anaemia. Visceral leishmaniasis is endemic in 62 countries, with a total of 200 million people at risk, an estimated 500,000 new cases each year worldwide⁵, and 41,000 recorded deaths in the year 2000⁶. Over 90% of cases of visceral leishmaniasis occur in five countries : India, Bangladesh, Nepal, Sudan and Northeastern Brazil⁷. In 2000, the disease burden associated with visceral leishmaniasis measured in disability – adjusted life years⁸ was estimated to be 1,980,000 (1,067,000 for male and 744,000 for female populations)⁶. Cutaneous Leishmaniasis commonly known as oriental sore organism of the leishmania tropica, leishmania major and leishmania aethiopica complexes causes the disease. Mucocutaneous Leishmaniasis commonly known as American leishmaniasis, espundia etc. The causative parasite for this form of disease belongs to leishmania maxicana, Leishmania braziliensis panamensis and Leishmania braziliensis guayanensis.

Life cycle of L. donovani was worked out by. The parasite Leishmania donovani has two stages in its life cycle⁹ (Figure 1).

(A) Extra cellular, motile flagellated promastigote form occurring in sand fly.

(B) Intracellular, non-motile, a flagellated amastigote form within the phagolysosomes of the macrophages occurring in mammalian hosts.

The response of the immune system to leishmania infection are highly complex. In Leishmania donovani infection, owing to lack of host response due to immunosuppressive effect, the cell-mediated immunity and delayed hypersensitivity reaction of the skin to leishmania antigen do not develop until the visceral system of the affected organ proliferate and become heavily parasitized, accompanied by an increase in the IgG fraction of the serum gamma globulin ratio (Normally globulin 2g and albumin 4.5 g/100 ml in Kala-azar globulin become 4g and albumin 2.8 g/100 ml). Specific antibodies, such as complement fixing haemagglutinating and fluorescent antibodies, which develop in Kala-azar, can be used for diagnostic purposes¹⁰. A number of natural products used as antileishmanial agents i.e. Quinones, Alkaloids, Terpenes, Phenolic derivatives (Chalcones).

Curcumin is a potent antioxidant and known for its plethora of biological activities is under investigation for its antileishmanial activity profile. There are several reports on its potent in vitro activity profile. Synthetic analoging of curcumin indicates that protection of phenol resulted in the better antileishmanial profile indicating that 1,3-diketone is essential for the antileishmanial activity profile phenolic glycosides have also been synthesized but the activity profile is not reported¹¹. In recent reports, it reveals that there are in vivo activities found in the curcumin for anti-leishmanial activity. There has been research carried out for in vivo potent drugs of curcumin¹².

RESULT AND DISCUSSION:

Synthesis of Aryl Substituted Ketene Dithioacetals:

Ketene dithioacetals have a tremendous synthetic potential. The sulfur atom exercises a stabilizing effect on the neighbouring positive as well as negative charge. The b-alkylthio group in these intermediates are activated by the presence of polar substituents at the a-position and can therefore, be displaced sequentially either one or both by various carbon, nitrogen and oxygen nucleophiles thus creating further scope for introducing new functionality at the b-position, which finds application in many synthetic transformations, a-oxoketene dithioacetals have proved useful in the synthesis of variety of compounds¹³. The presence of carbonyl functionality and its position in conjugation with double bond carrying aryl thio-groups at the b-position places them among the versatile 1,3-electrophilic 3-carbon equivalents. The carbonyl and the b-carbon atoms in these system can also be regarded as hard and soft electrophilic centres, since carbonyl is adjacent to the hard base oxygen while the b-carbon is flanked by the soft base thiomethyl group.

The curcumin is a versatile molecule for its various biological profiles. It posses two functional sites i.e. phenolic hydroxyl / methoxy function which is not only responsible for its antioxidant profile but it is also responsible for the metal chelations. The second important functionality i.e. 1,3-diketone function again contributes towards metal chelation.

Synthesis of a-Oxo ketene Dithioacetals:

The preparation and properties of a-oxo-ketene dithioacetals have been extensively studied. The sulfur atom exercises a stabilizing effect on the neighbouring positive as well as negative charge. This makes the double bond in ketene dithioacetal, responsive towards both nucleophilic as well as electrophilic addition reactions which is an extremely useful feature for synthetic transformation. The reaction of enolate anions derived from the active methylene carbonyl compounds in the presence of a suitable base/solvent combination (coupled with temperature manipulation) with carbon disulphide followed by alkylation continues to be the preferred method for the preparation of these compounds. The reaction of ketones and active methylene compounds with carbon disulfide in the presence of hydroxide and alkoxide bases has been known, the first synthesis of an a-oxo ketene dithioacetal was reported¹⁴. The formation of bis-ketene dithioacetal was favoured in less polar solvents, better yields of mono ketone dithioacetals were obtained when THF was used as the solvent¹⁵. The literature method mostly used and developed for the synthesis of aryl substituted ketene dithioacetals proved least useful for the synthesis of acetone ketene dithioacetals (A) as shown in Figure 2.

(A)

Fig. 2: Synthesis Of Acetone Ketene Dithioacetals

The method developed for the terpenyl substrates was found useful for the synthesis of acetone based ketene dithioacetals (A). When acetone based ketene dithioacetals (A) is condensed with aromatic aldehyde we get the a-oxoketene dithioacetals of aromatic substrates. The aromatic a-oxoketenedithio acetals are very useful synthons in the synthesis of variety of heterocyclic and carbocyclic compounds. In the synthesis of a-oxo ketene dithioacetals, b-ionones and other aromatic and aliphatic ketone; the chemistry of reaction is based on aldol condensation.

The reaction of a series of aromatic aldehyde with specific reagents in each reaction with optimal conditions as outlined in Scheme I, II, III and IV. The results are summarized in Table 1. Firstly, synthesis of acetone ketene thioacetals (2) is done by raction of acetone with potassium hydroxide, carbon disulfide, methyl iodide and dihydropyrene (Scheme I). Further, acetone ketene thioacetals is used in reaction with substituted benzaldehyde. When reaction between acetone ketene thioacetals with p- hydroxybenzaldehyde and 3,4-dihydro- 2H- pyran then product formation is para substituted hydroxy-dihydropyranbenzaldehyde. Again, synthesis of 5-[(4-Tetrahydropyran-2-yloxy)-phenyl]-1,1-bis(methylthio)-penta-1,4-dien-3-one is done by reaction of 4-Tetrahydropyranyl-1-benzaldehyde with methanolic potassium hydroxide. There is 44.48% yield with yellow crystalline solids. Afterwards formation of hydroxyl-compound of 5-[(4-Tetrahydropyran-2-yloxy)-phenyl]-1,1-bis(methylthio)-penta-1,4-dien-3-one is done with the yield 65.4% (6) (Scheme II). Again, further proceed the same methods with suitable reagents for the synthesis of 3-Methoxy-4-tetrahydropyranyl-1-benzaldehyde and 5-[(3-Methoxy-4-tetrahydropyran-2-yloxy)phenyl]-1,1-bis(methylthio)penta-1,4-dien-3-one. Methoxy substituted p-hydroxy benzaldehyde (7) is taken as reactant for synthesis. (7) is reacted with pyran, PPTS and dry dichloromethane to formation of 3-Methoxy-4-tetrahydropyranyl-1-benzaldehyde takes place. When (8) is reacted with methanolic KOH and acetone ketene thioacetals then synthesis of (9) occurred with yield (10) (Scheme III). In Scheme IV, disubstituted methoxy reacted with methanolic potassium hydroxyl and acetone ketene thioacetals formation of corresponding 5-(2,4-Dimethoxyphenyl)-1,1-bis(methylthio)-penta-1,4-dien-3-one.

GENERAL SCHEME

SCHEME – I

Reagents: (a) 3, 4-dihydro-2H-pyran, PPTS, dry CH₂Cl₂

(b) Methanolic KOH, 2, 44.48% yield

Reagents: (a) 3, 4-dihydro-2H-pyran, PPTS, dry CH₂Cl₂

(b) Methanolic KOH, 2, 10.1% yield

Reagents: Methanolic KOH, 2, 34.9% yield

EXPERIMENTAL:

Infrared spectra are recorded in KBr and Neat, on a Perkin Elmer Model-881, NMR spectra were obtained in CDCl₃ (with Me₄Si internal standard, Aldrich) and reported in ppm downfield from Me₄Si. Proton, carbon NMR spectra were recorded on Bruker Advance DRX-2000 instrument. Electron impact (EI) mass spectra were recorded on a Jeol-D-300 spectrometer with the ionization potential of 70 eV.

1,1-Dithiomethyl-but-1-ene-3-one (2)

To a magnetically stirred mixture of sodium hydroxide powder (20.0 g, 0.5 mol) in dry, THF (200 ml), carbodisulfide (19.0 g, 15 ml, 0.25 mol) was added dropwise at room temperature and stirred for 15 minutes. Acetone (20.0 g, 36.25 ml, 0.5 mol) was added dropwise in a reaction mixture and stirred the reaction mixture for 1 hour methyliodide (71.0 g, 36.0 ml, 0.5 mol) was added in it and stirred the reaction mixture for 5 hours at room temperature. The reaction mixture was poured in brine solution (2 × 100 ml) and extracted with ethyl acetate (3 × 125 ml). The combined organic layer washed with brine solution (3 × 50 ml), dried on sodium sulphate and solvent removed in vacuo. The yellow crude product thus obtained was chromatographed (SiO₂, 60-120 mesh) to furnish (2) as brown colored solid (16.98 g, 60%), m. p. 54^oC.

IR (KBr, cm^-1) : 3260, 2990, 2920, 1636, 1496, 1204, 950;

¹H NMR (CDCl₃, 200 MHz) : d 2.2 (s, 2H), 2.35 (s, 3H), 2.40 (s, 3H), 6.0 (s, 1H);

¹³C NMR (CDCl₃, 200 MHz), 15.1 (q), 17.47 (q), 30.69 (q), 113.38 (d), 163.56 (s), 193.01 (s);

MS (m/z): 162 (M⁺).

4-Tetrahydropyranyl-1-benzaldehyde (4)

To a solution of p-hydroxybenzaldehyde (6.10 g, 50 mmol) in dry dichloromethane (200 ml) was added PPTS (0.40 g) and dihydropyran (6.12 gm, 72 mmol) and stirred at room temperature for 6 hours. The reaction mixture was taken up in separating funnel and washed with aqueous sodium carbonate solution (5 × 100 ml). Solvent was removed in vacuo to furnish (4) as a pale brown thick liquid (8.48 g, 85%).

IR (KBr, cm^-1) 2947, 1694, 1581;

MS (m/z): 206 (M⁺).

5-[(4-Tetrahydropyran-2-yloxy)-phenyl]-1,1-bis(methylthio)-penta-1,4-dien-3-one (5)

To a solution of 1,1-bis(methylthio)-pent-1-en-3-one (6.48 g, 40 mmol) in methanol (100 ml) was added 10 (9.44 gm, 40 mmol) followed by aqueous KOH solution (4.48 g, 80 mmol in 40 ml water) and the resulting reaction mixture was stirred at toom temperature for 18 hours. It was concentrated in vacuo, poured into water and extracted with dichloromethane (100 ml × 3). The combined extract was washed with water (3 × 100 ml), brine solution (2 × 100 ml), dried (Na₂SO₄) and the solvent was removed in vacuo. The crude product was purified by column chromatography (SiO₂, 60-120 mesh). Elution with 20% ethylacetate in hexane furnished (5) as a yellow crystalline solid (6.30 g, 44.48%) m.p. 148-150^oC;

IR (KBr, cm^-1): 2938, 1640, 1587, 1438, 1239, 1119;

¹H NMR (CDCl₃, 200 MHz): d 1.90 (m, 6H), 2.50 (s, 6H), 3.70 (m, 1H), 3.90 (m, 1H), 5.45 (m, 1H), 6.20 (s, 1H), 6.70 (d, J=16.00Hz, 1H), 7.05 (d, J=8.00, 2H), 7.60 (d, J=16.00 Hz, 1H);

¹³C NMR (CDCl₃, 200 MHz), 15.49 (q), 17.65 (q), 19.01 (t), 25.52 (t), 30.61 (t), 62.42 (t), 96.61 (d), 114.03 (d), 130.00 (d), 141.39 (d);

MS (m/z): 351 (M⁺ + 1).

Table 1: Data record Table

Entry	Substrate	Product	Reaction condition	m.p. (^oC)	Yield (%)
1.			R.T., 6 hrs	54	60
2.			R.T., 6 hrs	-	85
3.			R.T., 18 hrs	150	44.48
4.			R.T., 6 hrs	170	65.4
5.			R.T., 6 hrs	-	73.6
6.			R.T., 28 hrs	83	10.1
7.			R.T., 10 hrs	140	34.9

5, 4(Hydroxyphenyl)-1, 1-bis(methylthio)penta-1,4-dien-3-one (6)

To a solution of (5) (0.35 gm, 1 mmol) in methanol (25 ml) was added PPTS (0.029 m, 0.1 mmol) and stirred it for 3 hours at room temperature. Water (3-4 drops) was added to the above reaction mixture and further stirred for 3 hours. Solvent was removed in vacuo, poured into water and extracted with dichloromethane (50 ml × 3). The combined extract was washed with aqueous soldium bicarbonate solution (5 × 50 ml). It was dried (Na₂SO₄) solvent was removed in vacuo. The crude product was chromatographed (SiO₂, 60-120 mesh). Elution with 20% ethylacetate in hexane furnished (6) as a yellow crystalline solid (2.02 g, 65.4%); m.p. 169-170^oC;

IR (KBr cm^-1) : 3447, 1600, 1559, 1465, 1237, 1140;

¹H NMR (CDCl₃, 200 MHz) : d 2.51 (s, 6H), 5.80 (s, 1H), 6.20 (s, 1H), 6.70 (d, J=16.00 Hz, 1H), 6.90 (d, J=8.00 Hz, 2H), 7.50 (d, J=8.00 Hz, 2H), 7.60 (d, J=16.00 Hz, 1H);

¹³C NMR (200 MHz, DMSO) d 14.67 (q), 17.10 (q), 114.12 (d), 2x116.18 (d), 125.12 (d), 126.39 (s), 2x130.40 (d), 140.82 (d), 159.83 (s), 163.07 (s), 183.76 (s);

MS (m/z): 267 (M⁺+1).

3-Methoxy-4-tetrahydropyranyl-1-benzaldehyde (8)

To a solution of 3-methoxy-4-hydroxy benzaldehyde (9.12 gm, 60 mmol) in dry dichloromethane (200 ml) was added PPTS (0.40 g) and dihydropyran (6.12 gm, 72 mmol) and stirred at room temperature for 6 hours. The reaction mixture was taken up in separating funnel and washed with aqueous sodium carbonate solution (5 × 100 ml). Solvent was removed in vacuo. The obtained product furnished (8) as pale brown thick liquid (10.43 g, 73.6%).

IR (KBr cm^-1) : 2947, 1688, 1591, 1269;

MS (m/z): 236 (M⁺+1).

5-[(3-Methoxy-4-tetrahydropyran-2-yloxy)phenyl]-1,1-bis(methylthio)penta-1,4-dien-3-one (9)

To a solution of 1,1-bis(methylthio)-pent-1-en-3-one (6.48 gm, 40 mmol) in methanol (150 ml) was added (8) (9.44 gm, 40 mmol) followed by aqueous KOH solution (4.48 g, 80 mmol in 40 ml water) and the resulting reaction mixture was stirred at room temperature 28 hours. It was concentrated in vacuo poured into water and extracted with dichloromethane (100 ml × 3). The combined extract was washed with water (3 × 100 ml), brine solution (2 × 100 ml); dried (Na₂SO₄) and the solvent was removed in vacuo. The crude product was purified by column chromatography (SiO₂, 60-120 mesh). Elution with methanol furnished (9) as a yellow crystalline solid (1.4 g, 10.1%); m.p. 80-85^oC;

IR (KBr cm^-1) : 2941, 1590, 1484, 1255, 1121, 956;

¹H NMR (CDCl₃, 200 MHz) : d 1.6-1.94 (m, 6H), 2.51 (s, 6H), 7.11 (s, 1H), 6.23 (s, 1H), 5.46 (s, 1H), 3.90 (s, 3H), 7.55 (d, J=16.00 Hz, 1H), 6.70 (d, J=16.00 Hz, 1H),

¹³C NMR (200 MHz, DMSO) d 15.49 (q), 17.68 (q), 19.11 (t), 25.56 (t), 30.61 (t), 56.54 (q), 62.56 (t), 64.21 (t), 97.61 (d), 110.05 (d), 113.83 (d), 115.17 (d), 123.31 (d), 125.52 (d), 128.21 (s), 141.82 (d), 147.20 (s), 148.15 (s), 165.18 (s), 184.40 (s);

MS (m/z): 381 (M⁺+1).

5-(2,4-Dimethoxyphenyl)-1,1-bis(methylthio)-penta-1,4-dien-3-one (11)

To a solution of 1,1-bis(methylthio)-pent-1-en-3-one (6.48 g, 40 mmol) in methanol (100 ml) was added 2,4-dimethoxybenzaldehyde (6.64 gm, 40 mmol) followed by aqueous KOH solution (4.48 g, 80 mmol in 40 ml water) and the resulting reaction mixture was stirred at room temperature for 10 hours. The reaction mixture was concentrated in vacuo; poured into water and extracted it with dichloromethane (100 ml × 3). The combined extract was washed with water (2 × 100 ml), brine (2 × 100 ml) dried (Na₂SO₄) and solvent was removed in vacuo. The crude product was crystallized by methanol and ether to furnish (11) as a yellow crystalline solid (4.2 g, 34.9% ); m.p. 140-145^oC;

IR (KBr cm^-1) 2997, 1636, 1575, 1494, 1328;

¹HNMR (200 MHz, CDCl₃) : d 2.51 (s, 6H), 6.24 (s, 1H), 6.52 (d, 1H), 6.47 (d, 1H), 3.86 (s, 3H), 3.83 (s, 3H), 7.87 (d, J=16.00 Hz, 1H), 7.48 (d, J=10.00 Hz, 1H), 6.80 (d, J=16.00 Hz, 1H);

¹³C NMR (200 MHz, DMSO) 15.49 (q), 17.66 (q), 53.90 (q), 55.91 (q), 98.78 (d), 105.71 (d), 114.21 (d), 117.75 (d), 126.08 (d), 130.60 (s), 137.13 9s), 160.32 (s), 162.89 (s), 163.90 (s), 185.24 (s);

MS (m/z): 311 (M⁺+1).

CONCLUSION:

In the present investigation we thus achieved the synthesis of a-oxo ketene dithioacetals from their corresponding ketones using potassium hydroxide as an efficient base. The utility of potassium hydroxide as a base has been generalized on a series of ketones as shown in Table 1.

We have also synthesized a-oxo ketene dithioacetals from their corresponding aldehydes using acetone ketene acetal. We have also found that use of sodium hydroxide as a base in the preparation of a-oxo ketene dithioacetals gives good yield.

ACKNOWLEDGMENT:

Author, Rachana Rani, is thankful to the Division of Medicinal Chemistry, Central Drug Research Institute, Lucknow for providing laboratory facilities.

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Received on 25.05.2011 Modified on 03.06.2011

Asian J. Research Chem. 4(7): July, 2011; Page 1188-1193

Synthesis of Diaryl Heptanoid Based Ketene Dithioacetals as Novel Antileishmanial Agents

Table 1: Data record Table