D.M. Pore

D.S. Gaikwad

K.A. Undale

T.S. Shaikh

Comptes Rendus Chimie

FulOnatK.ADep1. Iandappabistrutreatenproathtiveneuin tDHthe[5]extsynviatoaC. R. Chimie 14 (2011) 511–515A RArticReceAcceAvaiKeywMulPolyHanOne*163doi:l paper/Mémoiree-pot multi-component synthesis of polyhydroquinolinesambient temperature. Undale, T.S. Shaikh, D.S. Gaikwad, D.M. Pore *artment of Chemistry, Shivaji University, Kolhapur 416 004, IndiantroductionHeterocycles are of immense importance in the designdiscovery of new compounds for pharmaceuticallications [1]. 1,4-dihydropyridine (DHP), which istious nucleus of polyhydroquinolines is an importantctural motif in the preparation of the drugs for thetment of cardiovascular diseases, including hyper-sion [2]. These also exhibit interesting biologicalperties such as vasodilator, bronchodilator, anti-erosclerotic, antitumor, geroprotective, hepatoprotec-and antidiabetic agent [3] and also behave asroprotectants [4a], as well as chemosensitizers [4b]umor therapy. Other than their biological importance,Ps have been meticulously used as reducing agents fordirect reductive amination of aldehydes and ketonesas well as their oxidation to pyridines being alsoensively studied [6].In 1882, Hantzsch and Jusius [7] reported the firstthesis of symmetrical substituted 1,4-dihydropyridinesmulti-component condensation of aldehyde, ethylace-cetate and ammonia in acetic acid or refluxing alcohol,while polyhydroquinolines can be synthesized by theutilization of cyclic 1,3-diketone by replacing one moleculeof ethylacetoacetate in the Hantzsch reaction [8–18]. Adetailed literature survey about the synthesis of polyhy-droquinoline compounds revealed that most of theprotocols employed for this reaction operate under refluxconditions [8] i.e., under high thermal activation or theyneed microwave irradiation [9]. There are a few protocolsoperable at room temperature using triflates of Yb [10] andSc [11], I2 [12], bakers yeast [13], HY-zeolite [14], CAN [15],ionic liquids [16], organo-catalyst [17a], p-TSA [17b], etc.Recently, Mekheimer et al. [18] reported uncatalyzedsynthesis of polyhydroquinolines induced by solar heat.However, many of these methodologies have beenassociated with several shortcomings such as requirementof long reaction times, use of expensive reagents and harshreaction conditions. Similarly, the difficulties such as lowproduct yields, occurrence of several by-products andrecovery of the catalyst are encountered. Therefore, it wasthought that there is room for improvement especiallytowards developing a green protocol for synthesis ofpolyhydroquinolines at ambient temperature. In continu-ation with our research devoted to the development ofgreen organic chemistry [19a–e] we report herein an eco-friendly, one-pot multi-component synthesis of polyhy-droquinolines at ambient temperature (Scheme 1).T I C L E I N F Ole history:ived 13 April 2010pted after revision 24 September 2010lable online 3 December 2010ords:ti-componenthydroquinolinestzsch condensation-pot synthesisA B S T R A C TAn efficient and eco-friendly one-pot synthesis of polyhydroquinolines at ambienttemperature from the multi-component reaction of aldehyde, 1,3-diketone, b-keto esterand ammonium acetate in ethanol medium is described. The probable mechanism oftransformation is suggested. The features of this procedure are mild reaction conditions,no need of external catalyst, good yields and the operational simplicity at ambienttemperature. 2010 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.Corresponding author.E-mail address: p_dattaprasad@rediffmail.com (D.M. Pore).Contents lists available at ScienceDirectComptes Rendus Chimiewww.sciencedi rec t .com1-0748/$ – see front matter  2010 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.10.1016/j.crci.2010.09.009[()TD$FIG] K.A. Undale et al. / C. R. Chimie 14 (2011) 511–5155122. Results and discussionConsidering the view points of green chemistry,initially the reaction of anisaldehyde, dimedone, ethyla-cetoacetate and ammonium acetate was carried outwithout using any external catalyst in aqueous mediumat room temperature. However, the reaction was sluggishand corresponding polyhydroqinoline 5 was obtained in15% (5 h). Since solvent properties play a crucial role inorganic synthesis, the effect of solvent was studied forsynthesis of polyhydroquinolines. The comparativeresults for polyhydroquinolines obtained using differentsolvents are summarized in Table 1 and it was found thatethanol was a more economical and efficient solvent forthe present transformation. The % yield as a function of e(dielectric constant) of solvent is exhibited in graphicalform in Fig. 1.The observed yield of polyhydroquinoline 5 ismaximum in ethanol as a solvent having dielectricconstant (e) of 24.3 (Fig. 1). This is an importantobservation indicating that this transformation requiresthe protic polar solvents of medium dielectric constantfor the enolization of active methylene compounds. In asolvent with high dielectric constant viz H2O (e = 78), theenol form of b-ketoester may be strongly H-bonded inaqueous medium becoming comparatively less reactivefor Knoevenagel condensation. A literature surveyrevealed that there was no formation of Knoevenagelproduct in water when dimedone and aldehyde weretaken in 1:1 proportion [20]. Hence, there is nolikelihood of formation of product 5 in water. In thecase of a low dielectric constant medium, the enolizationis energetically unfavoured, leading to the low yield ofthe product 5. However, these difficulties are easilysurmounted by using EtOH as a reaction medium due toits medium dielectric constant as well as the compara-tively less ability to form H-bonding with the enol formof the b-keto ester. The requirement of protic solventsfor this reaction is also supported by observing very lowyield of the product in aprotic solvents where the smallincrease in % yield with dielectric constant of themedium is observed in the studied solvents (Fig. 1).The formation of polyhydroquinolines without use ofany external catalyst in ethanol medium at ambienttemperature is likely due to in situ formation of acetic acidby hydrolysis of ammonium acetate, which acts as acatalyst for the present transformation. This clearly impliesthat there is no need of any external catalyst for thesynthesis of polyhydroquinolines. To improve the yield ofthe product 5, an excess amount of ammonium acetatecould be used. The reason is twofold; firstly excess use ofammonium acetate results in a large amount of in situR HOR'R'OONRR'R'HOOC2H5OOC2H5OONH4OAcC2H5OHRT1+2345Scheme 1.[()TD$FIG]Table 1Effect of solvent on the yield of polyhydroquinolines.Entry Solvent Yield (%)a1 H2O 152 CH3OH 543 CH3CH2OH 784 CH3CH2CH2OH 545 CH3CH2CH2CH2OH 426 CH3CN 327 THF 158 CH3COCH3 26a Reaction conditions: anisaldehyde (1 mmol), ethyl acetoacetate(1 mmol), dimedone (1 mmol), ammonium acetate (1 mmol), solventFig. 1. Effect of dielectric constant on the yield of 5.(5 mL), time = 8 h, room temp.gensecremsligusiecocarreaexcandlinedimwesolcywiddivas wethsumin adonmoyieeffi5k)reaaldTabSyntEnabcdefghijklmnopqrstuvabK.A. Undale et al. / C. R. Chimie 14 (2011) 511–515 513eration of acetic acid that acts as a catalyst andondly unreacted ammonium acetate can be easilyoved from the reaction. This prompted us to use ahtly excess amount of ammonium acetate rather thanng any external catalyst and led us to explore thisnomical protocol. The present transformation was alsoried out in 100, 95, 90, 85 and 80% ethanol usingction conditions as specified in Table 1 with theeption that amount of ammonium acetate (1.5 mmol)obtained 76, 91, 73, 68, 63% yield of polyhydroquino-, respectively.Gratifyingly, polyhydroquinoline 5 of anisaldehyde,edone, ethylacetoacetate and ammonium acetatere obtained in 91% yield using 95% ethanol as avent at ambient temperature. The scope and efficien-of this approach was explored for the synthesis of ae variety of polyhydroquinolines by treating aerse range of aldehydes possessing electron-donatingell as electron-withdrawing groups, with dimedone,yl acetoacetate, ammonium acetate and results aremarized in Table 2. Therefore, it is amply clear thatll the cases irrespective of the presence of electron-ating or electron-withdrawing groups in an aldehydeiety, the desired products were obtained in goodlds. These reaction conditions were almost equallycient for heterocyclic aldehydes (Table 2, entries 5j,. Although the heterocyclic and aromatic aldehydescted efficiently under the similar conditions, aliphatic(Table 2, entries 5m and 5t). Furthermore, the variationof 1,3-diketone compound from dimedone to cyclohex-ane-1,3-dione, which has been rarely used for presenttransformation, worked equally well, proving the broadscope of the present protocol.Prediction of the mechanism of multi-componentreactions is a skillful task. In the mechanism of polyhy-droquinoline there is ambiguity amongst the reactivity ofactive methylene compounds viz 1,3-diketones and b-ketoester towards Knoevenagel condensation. To clarify this(Table 3), Knoevenagel condensation was carried out using1+2, 1+3 without any catalyst and yielded 6, 8 in 13 and 0%,respectively. The influence of ammonium acetate on thesame reactions i.e. 1+2+4 yielded 6 in 82% while for 1+3+4yielded mixture of 7 and 10 in 28 and 17%, respectively.le 2hesis of polyhydroquinolines in ethanol medium at ambient temperature.try Product (5) Time (h) Yield (%)a,b MP Obs. (lit. 8C)[TD$INLINE]NHOC2H5OO RR'R'R = C6H5; R’ = CH3 6.5 89 199-201(202-204) [8a]R = 4-CH3-C6H4; R’ = CH3 7 87 258-260(260-261) [8a]R = 4-CH3 O - C6H4; R’ = CH3 8 91 256-258(257-259) [8a]R = 4-Cl- C6H4; R’ = CH3 6 84 244-246(245-246) [8a]R = 2-NO2 - C6H4; R’ = CH3 7 87 205-207(206-208) [8a]R = 4-isopropyl-C6H4; R’ = CH3 6 88 180-182 (–)R = 2,5-(CH3)2- C6H3; R’ = CH3 6 89 244-246 (–)R = 4-OH- C6H4; R’ = CH3 6.5 87 233-234(232-234) [8a]R = 4-OH-3-CH3O- C6H3; R’ = CH3 7 84 210-212(210-212) [8a]R = 2-furyl; R’ = CH3 6 77 242-245(246-248) [8a]R = 2-thienyl; R’ = CH3 6 78 236-239(238-240) [8a]R = 4-N(CH3)2- C6H4; R’ = CH3 7 87 261-263(263-264) [8a]R = CH3-CH2 ; R’ = CH3 7 44 145-146(145-146) [8a]R = 4-CH3 O – C6H4; R’ = H 7 82 193-195(194-195) [9a]R = 4-CH3- C6H4; R’ = H 7 87 241-242(241-242) [9a]R = 4-OH - C6H4; R’ = H 7 82 233-235(234-235) [9a]R = Piperinyl - C6H3; R’ = H 6 85 242-244(—)R = C6H5; R’ = H 6.5 86 239-241(240-241) [12]R = 3-NO2 - C6H4; R’ = H 6.5 87 196-199(198-200) [12]R = (CH3)2- CH ; R’ = H 7 46 179-181(180-182) [12]R = 3,4-(CH3 O)2- C6H3; R’ = H 11 85 192-194(—)R = 3,4,5-(CH3 O)3- C6H2; R’ = H 11 85 180-182(—)All products showed satisfactory spectroscopic data (IR, 1H and 13C NMR, MS, elemental analysis).Yields refer to pure, isolated products.Table 3Results of Knoevenagel condensation.Entry Reactants Catalyst Product Yield (%)a1 1+2 —— 6 132 1+3 —— 8 ——3 1+2+4 —— 6 824 1+3+4 —— 71028175 1+3 CH3COOH 8 ——6 3+4 —— 7 96a Reaction conditions: anisaldehyde(1 mmol), ethyl acetoacetate/dimedone (1 mmol), ethanol (5 mL), ammonium acetate (1.5 mmol),ic acid (1 mmol), time = 6 h, room temp.ehydes resulted the lower yield of the products. acet[()TD$FIG] K.A. Undale et al. / C. R. Chimie 14 (2011) 511–515514Even though, addition of acetic acid for reaction of 1+3failed to form 8. The reaction between 3+4 resulted inexclusive formation of product 7.The above observations allow us to conclude thatKnoevenagel product 6 is formed by reaction of aldehydewith 1,3-diketone and subsequent Michael addition of 7yielding corresponding polyhydroquinoline 5 (Scheme 2).The formation of acridinedione [21] 11 is possible when 9reacts with 6, since this possibility is discarded due to non-formation of intermediates 8 and 9. A literature survey alsocomplies with non-formation of acridinediones [8–18] as aby-product during present transformation justifying theclear mechanism for synthesis of polyhydroquinolines 5.3. ConclusionThe developed one-pot procedure for multi-compo-nent synthesis of polyhydroquinolines was found to bequite general for a variety of aryl and heterocyclicaldehydes as well as for 1,3-diketones viz dimedone andcyclohexane-1,3-dione. The attractive features of thismethod are simple experimental procedure worked wellat ambient temperature and easy isolation of product. Thebeauty of protocol lies in mechanistic and green approachin terms of absence of external catalyst, which makes theprocess economical.4. Experimental: Typical procedureOne-pot synthesis of polyhydroquinolines: A mixture ofan aldehyde (1 mmol), ethyl acetoacetate (1 mmol),dimedone (1 mmol) and ammonium acetate (1.5 mmol)in ethanol (5 mL) was stirred at room temperature, for anappropriate time (Table 2). On completion of reaction(TLC), the reaction mixture was poured into crushed iceand the solid product separated was filtered and recrys-tallized from ethanol to afford pure polyhydroquinoline,which was characterized by physical constant andspectroscopic methods.OOOORRCHOEtOHONH2 CH3OEtOOEtO CH3NH4OAcH2O + AcOHONH2NRHO OOEtOOEtO CH3RNH4OAcH2O + AcOHEtOHRCHOEtOHRCHOON CH3OEtRNRHO O++Step 1Step 21234Step 356 78149110611Scheme 2. Proposed mechanism for the formation of 5.5. S1601.02.35.02H)32.143(M+fou1601.02.31H)6.921.11319575.1603H)4.06.6CDC107167C205.91603H)5H)1H)1H)21.119z 373.71603H)3.81H)20.1064013.4AckfinaresK.A. Undale et al. / C. R. Chimie 14 (2011) 511–515 515pectral data of unknown compoundsEntry 5f: mp. 180–182 8C; IR (KBr):3280, 3210,1649,5 cm1; 1H NMR (400 MHz, CDCl3): d 0.96 (s, 3H),8(s, 3H), 1.17-1.2(m, 9H), 1.68-1.70(m, 2H), 2.19-4(m, 5H), 2.79-2.82(m, 1H), 4.07 (q, J = 7.2 Hz, 2H),2(s, 1H), 6.02(s, 1H), 7.03(d, J = 8 Hz, 2H), 7.19(d, J = 8 Hz,; 13C NMR (100 MHz, CDCl3): d 14.1, 19.1, 27.3, 29.3,7, 33.6, 36.1, 41.1, 50.9, 59.6, 106.3, 112.1, 125.9, 127.5,.2, 144.5, 146.2, 148.5, 167.6, 195.5; EIMS: m/z 381); anal. calcd. for C24H31O3N: C, 75.56; H, 8.19; N, 3.67;nd: C, 75.61; H, 8.19; N, 3.69.Entry 5g: mp. 244–246 8C; IR (KBr):3284, 3237, 1699,2 cm1; 1H NMR (400 MHz, CDCl3): d 0.94 (s, 3H),8(s, 3H), 1.19 (t, J = 7.2 Hz, 3H), 1.61-1.64(m, 2H), 2.11-1(m, 5H), 2.38(s, 3H), 2.65(s, 3H), 4.06 (m, 2H), 5.11(s,, 5.67(s, 1H), 6.79(d, J = 8 Hz, 1H), 6.90(d, J = 8 Hz, 1H),6(s, 1H); 13C NMR (100 MHz, CDCl3): d 14.2, 19.1, 19.6,1, 27.2, 28.4, 29.2, 32.7, 33.1, 41.6, 50.9, 59.7, 107.7,.7, 126.8, 129.5, 129.8, 133, 134.7, 142.3, 146.3, 147.1,.1; EIMS: m/z 367 (M+); anal. calcd. for C23H29O3N: C,17; H, 7.95; N, 3.81; found: C, 75.14; H, 7.94; N, 3.79.Entry 5q: mp. 242–244 8C; IR (KBr): 3281, 1692, 1646,7 cm1; 1H NMR (400 MHz, CDCl3): d 1.21 (t, J = 7.2 Hz,, 1.65-1.69(m, 3H), 1.91-2.02(m, 1H), 2.27-2.46(m, 5H),8 (q, J = 7.2 Hz, 2H), 5.02(s, 1H), 5.87(s, 2H), 5.88(s, 1H),5(d, J = 8 Hz, 1H), 6.78-6.80(m,2H); 13C NMR (100 MHz,l3): d 14.2, 19.1, 21.1, 27.4, 36.2, 37.1, 59.8, 100.6, 106.4,.7, 108.7, 113.5, 121, 141.5, 143, 145.7, 147.3, 149.6,.4, 195.6; EIMS: m/z 355 (M+); anal. calcd. forH21O5N: C, 67.59; H, 5.96; N, 3.94; found: C, 67.63; H,7; N, 3.95.Entry 5u: mp. 192–194 8C; IR (KBr):3307, 1697, 1647,7 cm1; 1H NMR (400 MHz, CDCl3): d 1.20 (t, J = 7.2 Hz,, 1.68-1.71(m, 3H), 1.94-2.02 (m, 1H), 2.28-2.45(m,, 3.80(s, 3H), 3.85(s, 3H), 4.07(q, J = 7.2 Hz, 2H), 5.05(s,, 6.07(s, 1H), 6.71(d, J = 8.2 Hz, 1H), 6.76(d, J = 8.2 Hz,, 6.95(s, 1H); 13C NMR (100 MHz, CDCl3): d 14.2, 19,1, 27.3, 36, 37.1, 56, 59.7, 106.2, 111.5, 112.5, 113.3,.8, 140.4, 143.2, 147.5, 148.5, 150, 167.5, 196; EIMS: m/1 (M+); anal. calcd. for C21H25O5N: C, 67.91; H, 6.78; N,7 found: C, 67.86; H, 6.75; N, 3.79.Entry 5v: mp. 180–182 8C; IR (KBr): 3275, 3192, 1695,7 cm1; 1H NMR (400 MHz, CDCl3): d 1.21 (t, J = 7.2 Hz,, 1.95-2.01(m, 2H), 2.25-2.40(m, 7H), 3.78(s, 3H),0(s, 6H), 4.09 (q, J = 7.2 Hz, 2H), 5.07(s, 1H), 6.30(s,, 6.5(s, 2H); 13C NMR (100 MHz, CDCl3): d 14.2, 19.3,3, 21.2, 27.7, 33, 36.5, 37.2, 56.3, 59.7, 60.8, 105.1, 106,.4, 113.5, 142.6, 142.8, 149, 153, 167.5, 196; EIMS: m/z(M+); anal. calcd. for C22H27O6N: C, 65.82; H, 6.78; N,9 found: C, 65.85; H, 6.78; N, 3.52.nowledgmentsAuthors DMP and KAU thank UGC, New Delhi forncial assistance [F.2-3/2007(Policy/SR)] and for theearch fellowship, respectively.References[1] (a) E.A. 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One-pot multi-component synthesis of polyhydroquinolines at ambient temperature

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One-pot multi-component synthesis of polyhydroquinolines at ambient temperature

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