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Note Facile Synthesis of Acyclic Analogues of Carbocyclic Indian Journal of Chemistry Vol. 49B, April 2010, pp. 512-520 Note Facile synthesis of acyclic analogues of counterparts, which are useful in biological systems. carbocyclic nucleoside as potential These include increased stability toward loss of base anti-HIV pro-drug in acidic media, a slightly higher lipophilicity and better metabolic stability toward the enzymes which Ibadur R Siddiqui*, Pravin K Singh, Vishal Srivastava & cleave the glycosidic linkage of natural nucleosides. J Singh The compounds incorporating oxazolone ring as a part of the condensed heterocyclic framework have Laboratory of Green Technology-Synthesis Division 8,9 10,11 Department of Chemistry, University of Allahabad, been reported as potential fungicidal , bactericidal , Allahabad 211 002, India herbicidal, insecticidal12, etc. Further, 1,3-thiazine E-mail: [email protected] nucleus has been used for designing various pharmacological agents13,14 owing to its presence in Received 16 January 2009; accepted (revised) 1 December 2009 cephalosporins, which are the most common antibiotics Microwave induced montmorillonite K 10 clay catalyzed in clinical use. Penciclovir (Figure 1) is an acyclic Michael addition of sulphur nucleophile, (4-oxo-butyl)-dithio- carba analogue of guanosine, and has been approved as carbamic acid 1 to 4-arylidene-5(4H)-oxazolones 2a-j followed an antiviral drug for treating diseases caused by HSV by ring transformation of the resultant Michael adducts 3a-j in and VZV (ref.15). Since the designed title compound solvent-free conditions gives 4a-j in excellent yield. Coexistence 6a-j (Scheme I) is an acyclic carba analogue of of acidic and basic sites on surface of montmorillonite K 10 accelerates the organic reactions synergistically. The control Aldol nucleosides of 1,3-thiazine nucleobase, so apparently condensation of compound 4a-j with HCHO gives compound 5a-j, it may act as a potentially anti HIV active agent. which upon chemoselective reduction with NaBH4 gives the title The present day industrialization has led to compound 6a-j. This process minimizes the mechanical loss of immense environmental deterioration. One of the the intermediate during the process of isolation, and thus increases the yield and decreases the cost and time. chief sources of environmental pollution from the chemical industry are the organic solvents. The Keywords: Michael addition, montmorillonite K 10, control aldol solvent vapours contaminate the environment/ condensation, chemoselective reduction, microwave irradiation atmosphere. The increasing awareness throughout the world has brought in a pressing need to develop an Nucleosides have been playing a major role in alternative synthetic approach for biologically and combating tumor and viral diseases. A variety of synthetically important compounds. This requires a modifications of natural nucleosides both with respect new approach which will reduce the material and 1 to base and sugar have been made to discover novel energy consumption, and minimize or eliminate the 2 antitumor and/ or antiviral agents . Among dispersion of harmful chemicals in the environment. nucleosides, carbocyclic analogues have been one of The key element in the current approach is the novel the interesting class of compounds, in which the utilization of oxazolone ring as a building block16-18, a furanose oxygen of the sugar moiety is replaced by a well documented application for the construction of methylene (-CH2-) unit, and are stable towards various oxygen heterocycles of chemical and hydrolysis by phosphorylase. This is due to the biological interest. nonglycosidic nature of the bond between the In view of the above facts and in continuation of carbocyclic moiety and the heterocyclic base, which the work19-24 to develop new synthetic methodologies results in metabolic stability to phosphorylase. This especially for bioactive molecules and finding simple, enzyme cleaves the glycosidic bond in conventional useful and convenient routes in heterocyclic synthesis, nucleoside. Consequently, carbocyclic nucleosides new facile green protocol for synthesis of potential 3-5 display an enhance biostability . Further, carbocyclic antiviral title compounds has been developed. nucleosides like Carbovir6 and Abacavir7 (Figure 1) are of considerable interest due to their anti-tumour Results and Discussion and anti HIV activities. Moreover, acyclic analogs of It is noteworthy that all the acyclic analogues of nucleosides exhibit properties, relative to their natural nucleosides are new. After some preliminary experi- NOTES 513 O NH O N N N NH NH N N HO N HO N N NH2 N NH2 N NH2 HO Carbovir Abacavir HO Penciclovir Figure 1 Ar H Ar Ar H H H N SH S N S N MWI NHCOR O 3 + R 2-4 min. R S O O S NH O O S N O 1 2a-j O O 3a-j 4a-j Ar H Ar H H S H NHCOR S HCHO / BF3.OEt2 NaBH4 NHCOR MWI S N O S N O O OH OH OH 5a-j 6a-j 2/3/4/5/6 R Ar 2/3/4/5/6 R Ar a -CH3 C6H5- f -CH3 p-CH3.C6H4- b -CH3 p-CH3O.C6H4- g -CH3 o-CH3O.C6H4- c -CH3 p-HO.C6H4- h -CH3 o-HO.C6H4- d -CH3 p-Cl.C6H4- i -CH3 o-Cl.C6H4- e -CH3 p-NO2.C6H4- j -CH3 o-NO2.C6H4- Scheme I mentation, it was found that the synthesis of 4a-j can present approach is that the intermediate 3a-j were be effected starting from oxazolones 2 and (4-oxo- transformed into compound 4a-j in-situ via cyclo- butyl)-dithiocarbamic acid 1. Michael addition of condensation without isolation and exposure of 3a-j 4-oxo-butyl)-dithiocarbamic acid 1 on oxazolones 2, to the environment. Although the resulting product gave intermediate 3a-j. The key element in the 4a-j could have been formed as diastereomeric pairs, 514 INDIAN J. CHEM., SEC B, APRIL 2010 the products could not be separated into diastereo- structures of synthesized compound were confirmed mers. It seems that cis-isomers, if formed, probably by spectral and elemental analysis. isomerised into the more stable trans-products. The (4-oxo-butyl)-dithiocarbamic acid 1. Following 1H NMR spectra of the products show distinct the standard procedure27, concentrated ammonia doublets at δ 4.66 (d, 1H, J = 7.0, S-CH-Ar) and 5.19 solution (20 mL) was added to CS2 (3 mL, 50 mmole). (d, 1H, J = 7.0, -N-CH) of 1,3-thiazene ring. So the Then 4-aminobutyraldehyde (3.48 g, 40 mmole) was diastereomers obtained were assigned the trans added in small portions over a period of 10 min. The configuration25,26. This process, thus, minimizes the dithiocarbamate was allowed to stand for 3 hr and mechanical loss of the intermediate during the process then acidified with conc HCl. The dithiocarbamic acid of isolation. Coexistence of acidic and basic sites on precipitate was filtered, washed with dry ether and the surface of montmorillonite K10 accelerated the dried under suction. organic reactions synergistically. However, the use of other mineral supports viz. silica gel, neutral or basic N-[4-Oxo-3-(4-oxo-butyl)-6-phenyl-2-thioxo-[1,3]- alumina was far less effective, resulting in either no thiazinan-5-yl]-acetamide 4a-j reaction (in the case of basic alumina) or relatively Method A (Thermal). A mixture of 1 (0.25 mole) very low yields (20-35%) of 4a-j (in the case of silica and 4-arylidene-5(4H)-oxazolones28 2a-j (0.25 mole) gel and neutral alumina). Moreover, the reactions did were dissolved in dioxane (30 mL) and refluxed on not take place if they were performed using thermostated water-bath for the specified time microwave without the montmorillonite K10, either (Table I). The completion of the reaction was checked neat or in an organic solvent. Compound 4a-j on by TLC using benzene:MeOH (7:3 v/v). The reaction- control aldol condensation with formaldehyde yielded mixture was concentrated to half of its volume, cooled 5a-j, which on subsequent chemoselective reduction to RT and poured into water. The product 4a-j thus with NaBH4 gave title compounds 6a-j. obtained was separated by flash chromatography and purified by recrystallization from ethanol to obtain The reactions were also carried out using a pure 4a-j. thermostated oil-bath at the same temperature (90°C) Method B (Microwave Irradiation). A mixture of as for the MW-activated method for a longer 1 (2.5 mmole) and 4-arylidene-5(4H)-oxazolones28 (optimized) period of time to ascertain whether the 2a-j (2.5 mmole) were adsorbed on montmorillonite MW method improved the yield or increased K-10 clay (0.5 mmole) in a 100 mL pyrex conical conversion rates. It was found that significantly lower flask capped with a funnel and subjected to micro- yields (34-40%) were obtained using oil-bath heating wave irradiation for the specified time (Table I). The rather than the MW-activated method. completion of the reaction was checked by TLC using benzene:MeOH (7:3 v/v). The reaction-mixture was Experimental Section cooled to RT and eluted with acetone (3 × 10 mL). Solvents were of reagent grade and dried using The elute was evaporated to dryness and washed with standard procedures. Melting points were determined NaHCO3 (3.0% w/v) and finally with cold H2O and by open glass capillary method and are uncorrected. dried over anhydrous MgSO4. The residue on puri- All chemicals used were of reagent grade and were fication by silica gel column chromatography used as received without further purification. 1H NMR (benzene:MeOH; 8:2 v/v) furnished analytically pure spectra were recorded at 400 MHz on a Bruker 4a-j. AVANCE DPX (400 MHz) FT spectrometer in Similar procedures were used with other solid DMSO-d6 using TMS as an internal reference.
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