Some novel isoxazole and pyrazole fused pyrano[2,3-b]quinolines were synthesized from simple acetanilides via intramolecular

1,3-dipolar cycloaddition reactions involving nitrones, nitrile oxides and nitrile imines as 1,3-dipoles, in a regioselective

manner

BARUAH , B

BHUYAN , P J

KALITA , P K

IR@NEIST - North East Institute of Science and Technology (CSIR)

This article was originally published in a journal published byElsevier, and the attached copy is provided by Elsevier for theauthor’s benefit and for the benefit of the author’s institution, fornon-commercial research and educational use including withoutlimitation use in instruction at your institution, sending it to specificcolleagues that you know, and providing a copy to your institution’sadministrator.All other uses, reproduction and distribution, including withoutlimitation commercial reprints, selling or licensing copies or access,or posting on open internet sites, your personal or institution’swebsite or repository, are prohibited. For exceptions, permissionmay be sought for such use through Elsevier’s permissions site at:http://www.elsevier.com/locate/permissionusematerialAuthor's   personal   copySynthesis of novel pyrano[2,3-b]quinolines from simpleacetanilides via intramolecular 1,3-dipolar cycloadditionPabitra K. Kalita, Biswajita Baruah and Pulak J. Bhuyan*Medicinal Chemistry Division, Regional Research Laboratory, Jorhat 785 006, Assam, IndiaReceived 3 July 2006; revised 17 August 2006; accepted 23 August 2006Available online 15 September 2006Abstract—Some novel isoxazole and pyrazole fused pyrano[2,3-b]quinolines were synthesized from simple acetanilides via intra-molecular 1,3-dipolar cycloaddition reactions involving nitrones, nitrile oxides and nitrile imines as 1,3-dipoles, in a regioselectivemanner. 2006 Elsevier Ltd. All rights reserved.The importance of quinoline and its annelated deriva-tives is well recognized by synthetic and biological chem-ists.1 Compounds possessing this ring system have wideapplications as drugs and pharmaceuticals.2 Pyrano-quinolines are an important class of compounds thatconstitute the basic frameworks of a number of alka-loids of biological signiﬁcance, for example, geibalasine,ribalinine, ﬂindersine, etc. (Fig. 1).3 Therefore, consider-able eﬀorts have been directed towards the preparationand synthetic manipulation of these molecules.4 As aresult, a number of compounds have been obtained withdiverse biological activities.Cycloaddition reactions are among the most useful reac-tions in synthetic and mechanistic organic chemistry.5They allow the direct construction of a new ring witha wide variety of substituents by simple addition oftwo or more reagents. Within this class, inter- and intra-molecular 1,3-dipolar cycloaddition reactions havefound extensive use as eﬃcient regio- and stereoselectivemethods for the synthesis of a variety of natural prod-ucts6 and other heterocyclic compounds of biologicalsigniﬁcance.7 Cycloaddition reactions, particularlyhetero Diels–Alder reactions, are eﬀective proceduresemployed for the preparation of pyranoquinolines.8Isoxazoles and pyrazoles are important classes of bio-logically active compounds. They have a rich chemistrybecause of their easy reductive cleavage and susceptibil-ity to ring transformations.9In continuation of our interest10 in the development ofhighly expedient methods and syntheses of heterocycliccompounds of biological importance, we report herethe synthesis of novel tetrahydroisoxazolo-, dihydro-isoxazolo- and dihydropyrazolo-fused pyrano[2,3-b]-quinolines from simple acetanilides and via intramole-cular 1,3-dipolar cycloaddition reactions involvingnitrones, nitrile oxides and nitrile imines as 1,3-dipoles,in a regioselective manner.Acetanilides 1 were chosen as the parent molecules(Scheme 1). The 2-chloro-3-formyl quinolines 2 wereprepared from 1 by modifying the existing method.11Thus acetanilide 1a (R1 = H) on treatment with the Vils-meier reagent (DMF + POCl3) gave 2-chloro-3-formylquinoline 2a in excellent yield.12 The ether derivative3a with an isolated dipolarophile site was prepared from2a by treatment with allyl alcohol in the presence ofsodium hydroxide (50% aqueous solution) under phasetransfer catalytic conditions.13 Allyl ether 3a, on treat-ment with N-methylhydroxylamine hydrochloride in0040-4039/$ - see front matter  2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.tetlet.2006.08.086Keywords: Quinolines; Pyrano[2,3-b]quinolines; 1,3-Dipolar cycloaddi-tion reaction; Nitrones; Nitrile oxides; Nitrile imines.* Corresponding author. E-mail: pulak_jyoti@yahoo.comN OOHOMeNOOflindersineHgeibalansineOOHNMeOribalinineFigure 1.Tetrahedron Letters 47 (2006) 7779–7782Author's   personal   copythe presence of triethylamine aﬀorded the nitrone 4a as awhite solid (mp 102 C).14 The structure of 4a was ascer-tained from the spectroscopic data. The 1H NMR spec-tra showed the presence of the N-Me protons of thenitrone at d 3.96 as a singlet. The allylic protonsappeared at d 6.21 (m, 1H), 5.29 (dd, 2H), 5.47 (d, 2H).On reﬂuxing 4a in toluene at 110 C, two isomers, 5a(cis) and 6a (trans) of tetrahydroisoxazolo[3 04 0:4,5]pyr-ano[2,3-b]quinolines were obtained in 70% and 7%yields, respectively. The structures of 5a and 6a weredetermined from spectroscopic data and elemental anal-ysis.15 The stereochemistries were determined from thecoupling constant of the H-6b and H-9a protons (forthe cis isomer J = 3.0 Hz and for the trans isomerJ = 9.0 Hz). The chemical shifts of the N-Me protonsat d 2.90 and d 2.85, respectively, as singlets further con-ﬁrmed the involvement of the nitrone in the cycloaddi-tion process. Both stereoisomers 5a and 6a exhibitedstrong molecular ion peaks (M+H)+ at 243 (using posi-tive ionization technique). Similarly, tetrahydroisox-azolo[3 0,4 0:4,5]pyrano[2,3-b]quinoline derivatives 5a–eand 6a–e were synthesized by N-methyl- and N-phenyl-hydroxylamine with compound 4 (Table 1). It is note-worthy that although the nitrones form easily, thecycloaddition required forcing conditions (110 C,reﬂuxing toluene). The reaction is totally regioselectiveand there was no evidence of the formation of any1,5-electrocyclized product [X].In order to prepare the dihydroisoxazolo[3 0,4 0:4,5]pyr-ano[2,3-b]quinolines, we ﬁrst prepared oximes 7 from 3by treatment with hydroxylamine hydrochloride in thepresence of aqueous sodium hydroxide (Scheme 2).16The oximes on treatment with NaOCl in the presenceof Et3N at 0–20 C aﬀorded the desired dihydroisox-azolo[3 0,4 0:4,5]pyrano[2,3-b]quinolines 8 in excellentyields via the formation of the nitrile oxides [A].17 Thestructures of 8a–c were determined from spectroscopicdata and elemental analysis (Table 1). The 1,5-electro-cyclization product [Y] was not formed.For the preparation of pyrazolo[3 0,4 0:4,5]pyrano[2,3-b]-quinolines, we ﬁrst synthesized hydrazone 9a via reac-tion of aldehyde 3a with phenylhydrazine (Scheme 3).18Chlorination of 9a with N-chlorosuccinimide in carbontetrachloride at 50 C aﬀorded chloride 10a in very goodyield.19 This intermediate could not be puriﬁed due todecomposition. Hence, the nitrile imine was generatedin situ from the reaction of 10a with triethylamine at80 C which underwent intramolecular cyclization togive exclusively, the desired dihydropyrazolo-[3 0,4 0:4,5]-pyrano[2,3-b]quinoline 11a without the formation ofany of the 1,5-electrocyclization product [Z].20 The struc-ture of 11a was ascertained from the spectroscopic dataand elemental analysis. Similarly, compounds 11b–cwere synthesized and characterized (Table 1). Unlikethe nitrones, the nitrile oxides and nitrile imines werefound to be highly reactive and gave the desired com-pounds in high yields.In conclusion, we have reported the synthesis of severalnovel tetrahydroisoxazolo-, dihydroisoxazolo- anddihydropyrazolo-fused pyrano[2,3-b]quinolines fromTable 1. Synthesis of novel pyrano[2,3-b]quinoline derivatives 5, 6, 8and 11 via intramolecular 1,3-dipolar cycloadditionEntry Product R1 R2 Mp (C) Yield (%)1 5a H CH3 153 752 6a H CH3 109 73 5b H C6H5 167 674 6b H C6H5 162 55 5c CH3 CH3 142 736 6c CH3 CH3 104 767 5d CH3 C6H5 139 758 6d CH3 C6H5 127 69 5e OCH3 CH3 149 7810 6e OCH3 CH3 106 711 8a H — 203 8712 8b CH3 — 216 8513 8c OCH3 — 224 7814 11a H — 232 8515 11b CH3 — 214 8316 11c OCH3 — 197 80NH2OHN ORN OHN OCHORNaOClEt3N NRON O7a-c3a-c1 11NROON8a-c1+ -N ORN OH71NRONO[Y]1NaOClEt3N[A]Scheme 2.R2NHOHN ORNNR CHOClNCH3ROHDMFPOCl3 NaOH,TBABAllyl alcohol,N OCHOR+O1a-c 2a-c4a-f6a-e  N ORHHON3a-cR2165437 8911101a5a29a6b6a11aR2N OR HONH165437 8911101a5a29a6b6a11aR25a-e1 11 111N ORN O4a-fR21N ORNO[X]1R2Scheme 1.7780 P. K. Kalita et al. / Tetrahedron Letters 47 (2006) 7779–7782Author's   personal   copysimple acetanilides via intramolecular 1,3-dipolar cyclo-addition reactions involving nitrones, nitrile oxides andnitrile imines as 1,3-dipoles, in a regioselective manner.The present intramolecular 1,3-dipolar cycloadditionreaction strategy, which is the ﬁrst example in quinolinechemistry can be further explored for the synthesis ofvarious heterocycle-fused quinoline derivatives.AcknowledgementThe authors thank the Department of Science andTechnology, India, for ﬁnancial support.References and notes1. (a) Elderﬁeld, R. C. In Heterocyclic Compounds; Elder-ﬁeld, R. C., Ed.; John Wiley: New York, London, 1960;Vol. 4, Chapter 1, p 1; (b) Wright, C. W.; Addac-Kyereme,J.; Breen, A. G.; Brown, J. E.; Cox, M. F.; Croft, S. L.;Gokcek, Y.; Kendrick, H.; Phillips, R. M.; Pollet, P. L. J.Med. Chem. 2001, 44, 3187; (c) Sahu, N. S.; Pal, C.;Mandal, N. B.; Banerjee, S.; Raha, M.; Kundu, A. P.;Basu, A.; Ghosh, M.; Roy, K.; Bandyopadhyay, S. Bioorg.Med. Chem. 2002, 10, 1687; (d) Bringmann, G.; Reichert,Y.; Kane, V. Tetrahedron 2004, 60, 3539; (e) Kournetsov,V. V.; Mendez, L. Y. V.; Gomez, C. M. M. Curr. Org.Chem. 2005, 9, 141.2. Antimalarial Drugs II; Peters, W., Richards, W. H. G.,Eds.; Springer Verlag: Berlin Heidelberg, New York,Tokyo, 1984.3. (a) Corral, R. A.; Orazi, O. O. Tetrahedron Lett. 1967, 7,583; (b) Sekar, M.; Rajendra Prasad, K. J. J. Nat. Prod.1998, 61, 294; (c) Puricelli, L.; Innocenti, G.; DelleMonache, G.; Caniato, R.; Filippini, R.; Cappelletti, E.M. Nat. Prod. Lett. 2002, 16, 95; (d) Marco, J. L.;Carreiras, M. C. J. Med. Chem. 2003, 6, 518.4. (a) Pozzo, J. L.; Samat, A.; Guglielmeth, R.; Dubest, R.;Aubard, J.Helv. Chim. Acta 2004, 80, 725; (b) Ghorab, M.M.; Abdel-Hamide, S. G.; Farrag, H. A. Acta Pol. Pharm.2001, 58, 175; (c) Wang, Y. G.; Lin, X.-F.; Cui, S.-U.Synlett 2004, 1175; (d) Bowman, R. M.; Grundon, M. F.;James, K. J. J. Chem. Soc., Perkin Trans.1 1973, 10, 1055;(e) Morel, A. F.; Larghi, E. L. Tetrahedron: Asymmetry2004, 15, 9.5. (a) Carruthers, W. In Cycloaddition Reactions in OrganicSynthesis. Tetrahedron Organic Chemistry Series; Perg-amon Press: Oxford, UK, 1990; Vol. 8, (b) Oppolzer, W.In Comprehensive Organic Synthesis; Trost, B. M., Flem-ing, I., Paquette, L. A., Eds.; Pergamon Press: Oxford,UK, 1991; Vol. 5, Chapter 4:1, (c) Tietze, L. F. Chem. Rev.1996, 96, 115; (d) Broggini, G.; Chiesa, K.; Marchi, I. De.;Martinelli, M.; Pilati, T.; Zecchi, G. Tetrahedron 2005, 61,3525; (e) Coldham, I.; Hufton, R. Chem. Rev. 2005, 105,2765.6. (a) Kitahara, Y.; Kato, T.; Funanizu, M.; Otatani, N.;Izuumi, H. Chem. Commun. 1968, 1632; (b) Crooaert, W.N.; Clercq, E. De. Tetrahedron Lett. 1982, 23, 3291, andreferences cited therein; (c) Benwell, M.; Flyan, B.;Hockless, D. Chem. Commun. 1997, 2259.7. (a) Le Bel, N. A.; Whang, J. J. J. Am. Chem. Soc. 1959, 81,6334; (b) Jung, M. E.; Yuk-Sun, L. P.; Mansuri, M. M.;Speltz, L. M. J. Org. Chem. 1985, 50, 1087; (c) Coldham,L.; Hufton, R. Chem. Rev. 2005, 105, 2765.8. (a) Magesh, C. J.; Makesh, S. V.; Perumal, P. T. Bioorg.Med. Chem. Lett. 2004, 14, 2035; (b) Sabitha, G.; Reddy,M. S. K.; Arundhathi, K.; Yadav, J. S. ARKIVOC 2000,153; (c) Yadav, J. S.; Reddy, B. V. S.; Rao, R. S.; Kumar,S. K.; Kunwar, A. C. Tetrahedron 2002, 39, 7891.9. (a) Jaeger, V.; Buss, V.; Schwab, W. Tetrahedron Lett.1978, 3133; (b) Kotera, K.; Takano, Y.; Matsuura, A.;Kitahonoki, K. Tetrahedron 1970, 26, 539; (c) Lang, S. A.,Jr.; Lin, Y. I. In Comprehensive Heterocyclic Chemistry;Katritzky, A. R., Rees, C. W., Eds.; Pergamon Press:Oxford, 1984; Vol. 6.10. (a) Devi, I.; Bhuyan, P. J. Synlett 2004, 283; (b) Devi, I.;Bhuyan, P. J. Tetrahedron Lett. 2004, 45, 8625; (c) Devi, I.;Bhuyan, P. J. Tetrahedron Lett. 2004, 45, 7727; (d) Deb,M. L.; Bhuyan, P. J. Tetrahedron 2005, 46, 6453.11. Meth-Cohn, O.; Narine, B.; Tarnowski, B. TetrahedronLett. 1979, 33, 3111.12. POCl3 (9 ml, 98.28 mmol) was added dropwise to DMF(2.7 ml, 34.65 mmol) whilst maintaining the temperatureat 0–5 C. The mixture was allowed to stir for about5 min. Acetanilide 1a (10.37 mmol) was then added andthe resulting solution heated for 8 h at 75–80 C. Thereaction mixture was cooled to room temperature andthen poured into crushed ice with stirring. A pale yellowprecipitate appeared immediately and was ﬁltered andwashed with water and then dried. The crude compoundwas recrystallized from ethyl acetate. Aldehydes 2b (74%)and 2c (67%) were prepared similarly.PhNHNH2N ORN NHPhN OCHOR NCSCCl4 NRONNPh9a-c11a-c3a-c1 11N ORN NHPhCl10a-c1Et3NN ORNNPh110 [Z]Et3NN ORN NHPhClScheme 3.P. K. Kalita et al. / Tetrahedron Letters 47 (2006) 7779–7782 7781Author's   personal   copy13. To a solution of 2-chloro-3-formyl quinoline 2a (8 mmol)in dichloromethane (10 ml) were added allyl alcohol(0.4 ml, 10 mmol) and a catalytic amount of tetrabutyl-ammonium bromide. To this was added, 10 ml of 50%aqueous KOH solution and the mixture was allowed to stirfor 8 h. The organic layer was separated and washed2–3 times with water. The solvent was evaporated and thecrude product was puriﬁed by column chromatographyusing 5% ethyl acetate in hexane as eluent. Compound 3a,yield 78%, mp 56 C. 1H NMR (300 MHz, CDCl3) d 5.30(dd, J = 9.7, 6.8 Hz, 2H), 5.40 (d, J = 6.00 Hz, 2H), 6.29(m, 1H), 6.95–7.80 (m, 4H), 8.20 (s, 1H), 9.80 (s, 1H).Compound 3b (75%) and 3c (70%) were prepared as above.14. To a solution of 2-allyloxy 3-formylquinoline 3a (2 mmol)in 5 ml toluene was added MeNHOHÆHCl (2 mmol), andthe reaction stirred at room temperature. NaHCO3(168 mg, 2 mmol) was added portionwise over a periodof 5 min (NaHCO3 was not required in the case ofPhNHOH) and stirring was continued for 2 h. The solventwas removed under reduced pressure and the product 4awas puriﬁed by preparative TLC using CHCl3 as eluent.Compound 4a, yield 90%, mp 102 C. 1H NMR(300 MHz, CDCl3) d 3.96 (s, 3H), 5.29 (dd, J = 9.8,6.2 Hz, 2H), 5.47 (d, J = 6.0 Hz, 2H), 6.21 (m, 1H), 7.20–7.90 (m, 4H), 8.34 (s, 1H), 8.60 (s, 1H).15. Compound 4a (1 mmol) was reﬂuxed in toluene (5 ml) for10 h. The solvent was removed under reduced pressure.The residue was puriﬁed by preparative TLC using ethylacetate/hexane (4:6) as eluent to give 5a and 6a. Com-pound 5a: yield 75%, mp 153–154 C. 1H NMR(300 MHz, CDCl3): d 2.90 (s, 3H), 3.22 (m, 1H), 4.30(m, 2H), 4.43 (m, 2H), 3.86 (d, J = 3.0 Hz, 1H), 7.20–7.90(m, 4H), 8.10 (s, 1H). 13C NMR (75 MHz, CDCl3): d152.96 (C-1a), 149.1 (C-11a), 147.89 (C-5a), 130.45 (C-6a),127.36 (C-6), 127.22 (C-3), 123.92 (C-2), 123.84 (C-4),112.51 (C-5), 66.8 (C-10), 65.5 (C-9), 30.5 (N-CH3), 29.5(C-9a), 28.3 (C-6b). m/z [M+H]+ 243. CHN analysis(calcd%) C, 69.43; H, 5.78; N, 11.57; C14H14N2O2(found%) C, 69.40; H, 5.72; N, 11.52. Compound 6a:yield 7%, mp 109 C. 1H NMR (300 MHz, CDCl3): d 2.85(s, 3H), 3.22 (m, 1H), 4.30 (m, 2H), 4.45 (m, 2H), 3.90 (d,J = 9.0 Hz, 1H), 7.2–8.0 (m, 4H), 8.15 (s, 1H). 13C NMR(75 MHz, CDCl3): d 152.80 (C-1a), 149.01 (C-11a), 147.00(C-5a), 131.15 (C-6a), 126.86 (C-6), 126.20 (C-3), 124.00(C-2), 123.70 (C-4), 113.00 (C-5), 67.5 (C-10), 65.5 (C-9),31.0 (N-CH3), 29.5 (C-9a), 28.3 (C-6b). m/z [M+H]+ 243.CHN analysis (calcd%) C, 69.43; H, 5.78; N, 11.57;C14H14N2O2 (found%) C, 69.50; H, 5.91; N, 11.48.16. Compound 3a (2 mmol) in 6 ml of EtOH/H2O mixture(1:1) was reacted with an aqueous solution of hydroxyl-amine prepared by adding NaOH (175 mg in 4 ml H2O) toa solution of NH2OHÆHCl (166.7 mg, 2 mmol in 3 mlwater), with stirring at room temperature. After 10 min,the solution was clear. The reaction mixture was allowedto stir at room temperature for 1 h after which the EtOHwas evaporated and the compound was separated byextraction with dichloromethane. The organic extract wasdried over anhydrous sodium sulfate and then evaporatedunder reduced pressure to obtain 7a. Yield 98%, mp 156–158 C. 1H NMR (300 MHz, CDCl3): d 5.12 (dd, J = 9.6,5.8 Hz, 2H), 5.46 (d, J = 6.4 Hz, 2H), 6.10 (m, 1H), 7.20–7.60 (m, 4H), 8.15 (s, 1H), 8.46 (s, 1H). Compounds 7b(94%) and 7c (88%) were prepared similarly.17. To a mixture of oxime 7a (2 mmol) and Et3N (202 mg,2 mmol) in dichloromethane (8 ml), 10% aqueous NaOClsolution (3.5 ml) was added dropwise at 10 C. Thereaction mixture was allowed to stir for 1 h at roomtemperature. The organic phase was separated and thesolvent was removed under reduced pressure. Product 8awas puriﬁed by preparative TLC using dichloromethaneand hexane (7:3) as eluent. Yield 87%, mp 203 C. 1HNMR (300 MHz, CDCl3): d 4.07 (m, 2H), 4.35 (m, 1H),4.86 (m, 2H), 7.20–7.75 (m, 4H), 7.95 (s, 1H). 13C NMR(75 MHz, CDCl3): d 152.2 (C-11a), 153.96 (C-1a), 147.80(C-5a), 132.10 (C-6b), 130.55 (C-6a), 127.00 (C-6), 126.15(C-3), 124.20 (C-2), 123.95 (C-4), 112.00 (C-5), 66.85 (C-10), 65.50 (C-9), 30.5 (C-9a). m/z [M+H]+ 227. CHNanalysis (calcd%) C, 69.02; H, 4.43; N, 12.39; C13H10N2O2(found%) C, 68.98; H, 4.38; N, 12.34. Compounds 8b–cwere prepared similarly.18. Compound 3a (2 mmol) and phenylhydrazine (2.5 mmol)in 15 ml of ethanol were reacted at room temperature forhalf an hour and then warmed for 10 min. The reactionmixture was ﬁltered hot. The solution was evaporated andthe solid compound obtained was recrystallized fromethanol. Compound 9a, yield 87%, mp 161–162 C. 1HNMR (300 MHz, CDCl3): d 5.05 (dd, J = 9.8, 6.7 Hz, 2H),5.32 (d, J = 6.2 Hz, 2H), 5.95 (m, 1H), 6.90–7.40 (m, 9H),8.00 (s, 1H), 8.32 (s, 1H). Compounds 9b (82%) and 9c(77%) were prepared similarly.19. To a suspension of phenylhydrazone 9a (2 mmol) incarbon tetrachloride (10 ml) was added a solution of N-chlorosuccinimide (2.5 mmol) in 10 ml of carbon tetra-chloride. The reaction mixture was heated at 50 C for 1 h,then cooled and ﬁltered. The ﬁltrate was concentrated invacuo to aﬀord the hydrazonyl chloride 10a. Chlorides10b–c were prepared in a similar way and used in the nextstep without further puriﬁcation.20. A mixture of compound 10a (2 mmol) and triethylamine(2 mmol) was reﬂuxed in toluene (15 ml) for 2 h. Thesolvent was removed under reduced pressure and theresidue was puriﬁed by preparative TLC using chloroformand hexane (1:2) as eluent to give 11a, yield 85%, mp189 C. 1H NMR (300 MHz, CDCl3): d 4.12 (m, 1H), 4.21(m, 2H), 4.69 (m, 2H), 6.95–7.72 (m, 9H), 7.82 (s, 1H). 13CNMR (75 MHz, CDCl3): d 154.05 (C-1a), 151.25 (C-11a),148.00 (C-5a), 128.20 (C-6b), 129.55 (C-6a), 127.00 (C-6),126.85, 126.20, 125.10, 124.30, 123.53 (two C), 122.19 (twoC), 121.00 [all Ar], 67.25 (C-10), 65.00 (C-9), 31.15 (C-9a).m/z [M+H]+ 302. CHN analysis (calcd%) C, 75.75; H,4.98; N, 13.95; C19H15N3O (found%) C, 75.65; H, 4.94; N,13.90. Compounds 11b–c were synthesized similarly.7782 P. K. Kalita et al. / Tetrahedron Letters 47 (2006) 7779–7782

Synthesis of novel pyrano[2,3-b]quinolines from simple acetanilides via intramolecular 1,3-dipolar cycloaddition

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Synthesis of novel pyrano[2,3-b]quinolines from simple acetanilides via intramolecular 1,3-dipolar cycloaddition

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