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The Chemistry of Heterocycles

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The Chemistry of Heterocycles The Chemistry of Heterocycles Mohammad Jafarzadeh Faculty of Chemistry, Razi University The Chemistry of Heterocycles, (Second Edition). By Theophil Eicher and Siegfried Hauptmann, Wiley-VCH Veriag GmbH, 2003 1 110 5 Five-Membered Heterocycles The indole ring system forms the basis of several pharmaceuticals, for instance the anti-inflammatory indomethacin 54 and the antidepressant iprindole 55: MeO, I CH2— C H 2— C H 2— N M e 2 55 Indigo and other dyes (see p 81) which contain the chromophore 56 O x - ,X = N H o r S , ( E ) - o r ( Z ) - , 56 are known as indigoid dyes, and are vat dyes. These water-insoluble compounds are reduced by sodium dithionite and sodium hydroxide and applied as vat dyes, whereby they become soluble as disodium dihydro compounds, e.g.: ^2H,2NaOH -2H20 Before the availability of sodium dithionite (1871), the reduction of indigo was brought about by bacte- ria with reducing properties (fermentation vat). The vat of indigo has a brown-yellow colour. The cloth is dipped in the vat and then exposed to air to allow reoxidation to indigo which is precipitated and finely distributed onto the fibre. A consequence of this process is the low rubbing fastness of the dyes. It causes the faded appearance of indigo-dyed jeans and makes possible the manufacture of 'faded jeans'. Since the seventies, indole compound5.14s Isoindolehave lost their importance as textile dyes. They have, how- ever, found application in other fields, e.g. in polaroid photography. [A-C] The name isoindole is admissible for benzo[c]pyrrole5.15. AlthoughCarbazolisoindolee has an ortho-quinonoid 111 structure, it can be isolated; it is characterized spectroscopically and chemically: 5.14 Isoindole CO2 + CH3OH 2-(Methoxycarbonyloxy)-l,3-dihydroisoindole (obtained from 2-hydroxy-l,3-dihydroisoindole and 2-(Methoxycarbonyloxy)-1,3-dihydroisoindole (obtained from 2-hydroxy-1,3-dihydroisoindole and The name isoindole is admissible for benzo[c]pyrrolemethyl . (4-nitrophenyl)carbonateAlthough isoindole has a)n wa ortho-qui-s pyrolysed in vacuo at 500°C and isoindole condensed on a cold- nonoid structure, it can bemethyl isolated;( 4it- initrophenyl)carbonate)s characterized spectroscopicallwasy pyrolyzedand chemicallinyvacuo [56]: at 500 °C and isoindole condensed on a cold-finger condenserfingefilledr condensewith liquidr fillenitrogend with .liquid nitrogen. Isoindole crystallizes in colourless needles, darkens at room temperature and polymerizes. Solutions in dichloromethane kept under nitrogen are more stable. Isoindole crystallizesTheiny colorlessgive a redneedles, colour witdarkensh EHRLICH'at roomS reagentemperaturet and formand thpolymerizese correspondin. Solutionsg DIELS-ALDEin R adducts dichloromethane kept(endo:exounder nitrogen= 2:3; seeare p 56more) witstableh Af-phenylmaleinimide. Isoindoles substituted on the pyrrole ring are thermally more stable and can, for instance, be prepared from 2-substituted 1,3-dihydroisoindol-l-ones They give a red colorby thwithe followinEHRLICH'Sg route: reagent and form the corresponding DIELS-ALDER adducts (endo:exo = 2:3) with N-phenylmaleinimide. 2 N_R1 R*M9Br> r^yVRl ^H^ ^X^/ -MgBrCI -H20 A tautomeric equilibrium exists between isoindoles unsubstituted in the 2-position and the correspond- ing 1//-isoindoles: Isomerizations in which an H-atom changes its position in a heterocyclic system are known as annular tautomerisms. This is a special case of prototropy (prototropic rearrangement) [57]. For isoindole, the position of the equilibrium depends to a large extent on the type of the substituent in the 1- and/or 3- position [58]. The 1-phenyl compound exists in the 2//-form and couples with benzenediazonium chlo- ride in the 3-position. 5.15 Carbazole The numbering of carbazole (benzo[6]indole) predates the introduction of systematic no- A,B menclature and is retained for historical reasons. Carbazoles behave like 0,0 -disubstituted diphenylamines. However, the basicity of carbazole, pKa = -4.94, is much lower than that of diphenylamine (pKa = 0.78), and also lower than that of indole and pyrrole. As a consequence, carbazole is insoluble in dilute acids but only soluble in coned H2SÜ4 with protonation of the N-atom. On pouring the solution into water, carbazole precipitates without poly- merization. 5.15 Carbazole 111 CO2 + CH3OH 5.15 Carbazole 111 2-(Methoxycarbonyloxy)-l,3-dihydroisoindole (obtained from 2-hydroxy-l,3-dihydroisoindole and methyl (4-nitrophenyl)carbonate) was pyrolysed in vacuo at 500°C and isoindole condensed on a cold- finger condenser filled with liquid nitrogen. Isoindole crystallizes in colourless needles, darkens at CO2 + CH3OH room temperature and polymerizes. Solutions in dichloromethane kept under nitrogen are more stable. They give a red colour with EHRLICH'S reagent and form the corresponding DIELS-ALDER adducts (endo:exo = 2:32-(Methoxycarbonyloxy)-l,3-dihydroisoindol; see p 56) with Af-phenylmaleinimidee . (obtaineIsoindoled fros substitutem 2-hydroxy-l,3-dihydroisoindold on the pyrrole rine g anarde methyl (4-nitrophenyl)carbonate) was pyrolysed in vacuo at 500°C and isoindole condensed on a cold- thermally more stable and can, for instance, be prepared from 2-substituted 1,3-dihydroisoindol-l-ones finger condenser filled with liquid nitrogen. Isoindole crystallizes in colourless needles, darkens at by the following route: Isoindoles substituted rooonm thetemperaturpyrrolee and ringpolymerizesare .thermally Solutions in moredichloromethanstablee kepandt undecan,r nitrogeforn instance,are more stablebe. prepared from 2-substitutedThey giv1,e3 -adihydroisoindol red colour with EHRLICH'-1-onesS reagenby thet anfollowingd form the routecorrespondin. g DIELS-ALDER adducts (endo:exo = 2:3; see p 56) with Af-phenylmaleinimide. Isoindoles substituted on the pyrrole ring are thermally more stable and can, for instance, be prepared from 2-substituted 1,3-dihydroisoindol-l-ones A tautomeric equilibrium exists between isoindoles unsubstituted in the 2-position and the by the Nfollowin_R1 g routeR*M: 9Br> r^yVRl ^H^ corresponding 1H-isoindoles: ^X^/ -MgBrCI -H20 N_R1 R*M9Br> r^yVRl ^H^ A tautomeric equilibrium exists between isoindoles unsubstitute^X^/ d in th-MgBrCe 2-positioI n and the correspond- ing 1//-isoindoles: -H20 A tautomeric equilibrium exists between isoindoles unsubstituted in the 2-position and the correspond- ing 1//-isoindoles: IsomerizationsIsomerizationin whichs in whican Hh- atoman H-atochangesm changeitss itsposition position inin aa heterocycliheterocyclicc systesystemm are knoware nknown as annularas annular tautomerismstautomerisms.. ThiThiss isis aa speciaspeciall cascasee of prototropof prototropyy (prototropi(prototropicc rearrangementrearrangement)) [57]. Fo. r isoindole, the position of the Isomerizationequilibriums dependin whichs atno H-atoa largm echange extenst itos npositio the typn ine ao heterocyclif the substituenc systemt arine thknowe 1n- aand/os annularr 3- position [58]. Thtautomerisms.e 1-phenyl Thicompouns is a speciad existl cass ei no fth prototrope 2//-fory m(prototropi and couplec rearrangements with benzenediazoniu) [57]. For isoindolem chlo, the- For isoindole, the positionpositioofn theof thequilibriume equilibrium dependdependss to a tolargae largeextent oextentn the typone ofthe the typesubstituenof thet in thsubstituente 1- and/or 3- ride in the 3-position. in the 1- and/or 3-positionposition. [58]The. Th1e -1-phenyphenyll compouncompoundd exists iexistsn the 2//-forin mthe and couple2H-forms with andbenzenediazoniucouplesmwith chlo- benzenediazonium chlorideride in inthethe 3-position3-position. 3 5.15 Carbazole 5.15 Carbazole The numberinA,B Thg eo numberinf carbazolg oef (benzo[6]indolecarbazole (benzo[6]indole) predate) predates the s introductiothe introduction onf osystematif systematicc nono-- A,B menclature anmenclaturd is retainee andd fois rretaine historicad forl historica reasonsl .reasons. Carbazoles behave like 0,0 -disubstituted diphenylamines. However, the basicity of carbazole, pKa = Carbazoles behave like 0,0 -disubstituted diphenylamines. However, the basicity of carbazole, pKa = -4.94, is much -4.94lowe, ri stha mucn htha lowet orf thadiphenylaminn that of diphenylamine (pK =e 0.78)(pKa =, 0.78)and ,als anod lowealso lower thar ntha than that ot fo findol indolee anand pyrrole. As a consequence, carbazole is insolubla e in dilute acids but only soluble in coned H2SÜ4 with pyrrole. As a consequenceprotonation o, fcarbazol the N-atome i.s Oinsolubln pourineg ithn edilut solutioe acidn intso buwatert onl, carbazoly soluble eprecipitate in coneds H2SÜwithou4t polywith- protonation of merizationthe N-atom. On pouring the solution into water, carbazole precipitates without poly- merization. 5.15 Carbazole 111 CO2 + CH3OH 2-(Methoxycarbonyloxy)-l,3-dihydroisoindole (obtained from 2-hydroxy-l,3-dihydroisoindole and methyl (4-nitrophenyl)carbonate) was pyrolysed in vacuo at 500°C and isoindole condensed on a cold- finger condenser filled with liquid nitrogen. Isoindole crystallizes in colourless needles, darkens at room temperature and polymerizes. Solutions in dichloromethane kept under nitrogen are more stable. They give a red colour with EHRLICH'S reagent and form the corresponding DIELS-ALDER adducts (endo:exo = 2:3; see p 56) with Af-phenylmaleinimide. Isoindoles substituted on the pyrrole ring are thermally more stable and can, for instance, be prepared from 2-substituted 1,3-dihydroisoindol-l-ones by the following route: N_R1 R*M9Br> r^yVRl ^H^ ^X^/ -MgBrCI -H20 A tautomeric equilibrium exists between isoindoles
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