Synthesis and functionalization of some new pyridazino[4,5-b ]indole derivatives

Starting from the indole-fused pyridazinone 5 , a series of new pyridazino[4,5-b ]indoles of potential pharmaceutical interest ( 9 - 18 ), was prepared. Compounds 20 and 21 were obtained by nucleophilic displacement of the chlorine atom of 19. Thionation of the chloro derivative 19 gave the thione compound 23, while its reaction with sodium azide gave a tetracyclic system, namely the tetrazolopyridazinoindole 22 . Dehalogenation of the chloro compound 19 gave a 3-aza analogue of the natural product, harman.


Results and Discussion
In the present work, the cyclisation behaviour of 1-methyl-N'-acetylindole-3-carbohydrazide 3 has been studied.The preparation of this intermediate 3 is straightforward: it has been reported that reaction of methyl indole-3-carboxylate A with methyl iodide in dry DMF in the presence of sodium hydride affords the N-methylated ester 1 in 81% yield. 18We have carried out a slight modification of this procedure, using potassium carbonate as a base, instead of sodium hydride, where the yield was improved to 85%.Heating of the ester 1 in neat hydrazine hydrate under reflux gave the corresponding hydrazide 2. Acetylation of 2 by treatment with acetic anhydride at 100 °C for 1 hour did not give the monoacetyl derivative 3, but afforded the diacetyl derivative 4.However, the monoacetyl compound 3 was obtained by reaction of the hydrazide 2 with acetic anhydride at room temperature for 2-3 hours, giving 3 in a good yield (72%).This latter compound has been prepared previously by treatment of the hydrazide 2 with dimethyl acetamide. 19The 1 H-NMR spectrum of compound 4 shows a singlet at  2.34 ppm with the relative intensity of six protons (two CH3CO groups), and NOE difference experiments revealed positive NOE's at  10.41 ppm (NH),  8.13 ppm (H-2) and  8.06 ppm (H-4) on irradiation of the acetyl singlet at 2.34 ppm.
Treatment of 3 with POCl3 under various conditions (at room temperature or at 100 °C for 1-4 hours) did not give the corresponding pyridazinone 5 but afforded the oxadiazole derivative 6 19 in 77% yield. 1 H-NMR NOE difference experiments provide evidence for the oxadiazole structure 6 and permit exclusion of the pyridazinone structure 5: saturation of the N-CH3 resonance at 3.88 ppm leads to positive NOE's at 8.13 ppm (H-2) and at 7.59-7.56ppm (H-7), thus proving that position 2 of the indole skeleton is unsubstituted (Scheme 1).
However, the key intermediate 4,5-dimethyl-2,5-dihydro-1H-pyridazino [4,5-b]indol-1-one (5) was easily prepared via a different route (Scheme 2).Thus, reaction of the N-unsubstituted pyridazinoindole 8 15 with an equimolar amount of methyl iodide in dry DMF in the presence of potassium carbonate afforded only the corresponding pyridazino[4,5-b]indole 5, and no 2substitution was observed.The structure of compound 5 was confirmed by 1 H-NMR NOE difference experiments which proved the vicinity of the N-CH3 and the C-CH3 groups, thus the CH3 moiety must be attached to N-5 and not to N-2.Furthermore, the structure of 5 was also proven chemically by an unequivocal synthesis: refluxing of the N-methylated keto ester 7 with hydrazine hydrate in ethanol gave a product which is identical with 5 in all respects.Compound 7, in turn, was obtained according to a known procedure 18 by N-methylation and simultaneous esterification of 2-acetylindole-3-carboxylic acid B 20 in a one-pot synthesis using methyl iodide and potassium carbonate in DMF.The literature procedure was slightly modified by using potassium carbonate instead of sodium hydride as a base (Scheme 2).Scheme 1. Attempts to synthesize the key intermediate pyridazinoindole 5.
MeI,1 equiv.The introduction of a variety of different alkyl groups into position 2 of 4,5-dimethyl-2,5dihydro-1H-pyridazino [4,5-b]indol-1-one (5) appeared interesting from a pharmaceutical point of view.Thus, 5 was allowed to react with some alkylating agents such as benzyl chloride, 2diethylaminoethyl chloride, or 4-(2-chloroethyl)morpholine.The reactions were performed in dry DMF in the presence of potassium carbonate to afford the corresponding pyridazinoindole derivatives 9, 10, and 12 as colorless solids in yields of 49-55%.The hydrochloride 11 was obtained in 60% yield by dissolving the free base 10 (which is difficult to recrystallize) in methanolic hydrogen chloride (freshly prepared from acetyl chloride and methanol).The structures of the new compounds (Scheme 3) were established by elemental analyses, IR, MS, HRMS, and 1 H-NMR spectra.Also, the high reactivity of epoxides and their usefulness for further functionalisation reactions prompted us to synthesize and investigate the epoxide 13, which could be prepared by treatment of the pyridazinone 5 with an excess of epichlorohydrin in the presence of sodium hydride in dry DMF solution at 60 °C (Scheme 4).Under these conditions, the epoxide 13 is formed in high yield and in sufficient purity for further transformations, i.e. ring-opening reactions with nitrogen nucleophiles.The 1 H-NMR signal pattern of the newly introduced side chain is in agreement with the spectral data reported for similar structures. 21During work-up of 13, contact with water must be reduced to a minimum, otherwise the sensitive epoxide ring undergoes hydrolysis.In this manner, the dihydroxypropyl derivative 14 was obtained in 25% yield and its structure was spectroscopically confirmed.Regioselective ring opening of 13 with various nitrogen nucleophiles was anticipated to afford a series of new amino alcohols that might exhibit interesting pharmacological properties (Scheme 5).Thus, opening of the oxirane ring of 13 with primary or secondary amines was performed in tetrahydrofuran solution at reflux temperature to afford the corresponding amino alcohols 15-17 in 49-80% yield.The reactions were carried out using three molar equivalents of the nucleophilic reagent (N-phenylpiperazine, piperidine, or 3-diethylamino-1-propylamine, respectively).For the synthesis of the azido alcohol 18, the epoxide was opened regioselectively with sodium azide in aqueous dioxane analogously to a known procedure. 21The structure of the alcohols 15-18 was confirmed by elemental analyses, IR, MS and 1 H-NMR spectra, the latter spectra showed a characteristic signal of the CH(OH) moiety.The appearance of an absorption band at  2104 cm -1 in the IR spectrum of compound 18 is due to the azido group.
On the other hand, heating compound 5 in boiling POCl3 gave the corresponding chloro derivative 19.The chlorine atom of the latter compound is labile and could be easily substituted by nitrogen nucleophiles such as 3-diethylaminopropylamine and benzylamine in the absence of a solvent, giving the corresponding pyridazinoindoles 20 and 21, respectively.Analytical data showed that the diethylamino compound 20 was obtained in its hydrochloride form.The IR spectrum showed two bands at  3457 and 3387 cm -1 , attributable to the two NH groups.The 1 H-NMR spectrum of compound 21 shows a singlet at  4.89 ppm (2H, NCH2) and a triplet at 7.03 ppm (NH) which gives a positive NOE on irradiation at  8.52 (H-9) and at 4.89 (NCH2) (Scheme 6).Moreover, reaction of the chloro derivative 19 with sodium azide in dry DMF gave the tetrazolo compound 22 in 82% yield via nucleophilic displacement of chlorine by azide, followed by intramolecular tetrazole ring closure.On the other hand, reaction of 19 with thiourea, followed by saponification of an intermediate isothiourea derivative with sodium hydroxide and subsequent acidification afforded the corresponding thione 23. 1 H-NMR data (absence of a noticeable NOE for the exchangeable signal at 13.95 ppm on saturation of the H-9 resonance at  9.11 ppm) as well as IR data (appearance of a new absorption band at  3280 cm -1 due to NH, in addition to C=S absorption bands at  1550 and 1269 cm -1 ) support the thione structure rather than its thiol tautomer (Scheme 7).Finally, hydrazinolysis of 19 resulted in dechlorination giving 24 rather than the corresponding hydrazino derivative 25 (Scheme 8).The formation of 24 can be attributed most probably to an oxidative dehydrazination reaction of the unstable hydrazino compound 25 in the presence of air oxygen.This behaviour is not surprising, as we had previously observed an analogous transformation with a similar pyridazinoindole. 15,16The 1-unsubstituted pyridazine 24 could be prepared alternatively by catalytic transfer hydrogenation of 19 using ammonium formate as the hydrogen source and Pd/C as a catalyst in refluxing methanol, affording analytically pure 24 in 68% yield.methyl 1-methyl-1H-indole-3-carboxylate (1), 18,22 1-methyl-1H-indole-3-carbohydrazide (2), 23 N'-acetyl-1-methyl-1H-indole-3-carbohydrazide (3), 19 2-methyl-5-(1-methyl-1H-indol-3-yl)-1,3,4-oxadiazole (6) 19 and 2,5dihydro-4-methyl-1H-pyridazino [4,5-b]indol-1-one (8) 15 have been reported before.
Ring opening of 13 with various nitrogen nucleophiles.
Melting points (uncorrected) were determined on a Kofler hot-stage microscope (Reichert) and are uncorrected.IR spectra were recorded for KBr pellets on a Perkin-Elmer 1605 FTIR instrument, 1 H NMR spectra were recorded on a Varian Unity-Plus 300 (300 MHz) and on a Bruker Avance DPX 200 (200 MHz) spectrometer (TMS as internal reference,  values in ppm)Mass spectra were obtained with a Hewlett-Packard 5890A/5970B-GC/MSD or with a Shimadzu QP5050 DI 50 spectrometer.HRMS spectra were taken on a Finnigan MAT 8230 instrument at the Institute of Organic Chemistry, University of Vienna.For thin layer chromatography, Merck aluminium sheets pre-coated with Kieselgel 60 F254 were used (detection using UV254 and UV366 light).Column chromatography was done on Merck Kieselgel 60, 0.063-0.200mm.Elemental analyses were carried out at the Microanalytical Laboratory, Department of Chemistry, University of Vienna and Assiut University.Methyl indole-3-carboxylate (A) is commercially available, 2-acetyl-1H-indole-3-carboxylic acid (B), The mixture was heated under reflux for 21 h.After cooling, the solvent was removed under reduced pressure and the residue was triturated with water, then the product was extracted with dichloromethane (3 x 50 ml).The combined extracts were washed with water (3 x 50 ml), dried over Na2SO4, and concentrated under reduced pressure to give a buff solid product which was recrystallized from ethyl acetate/light petroleum to afford 147 mg (49%) To a stirred mixture of the pyridazinone 5 (213 mg, 1 mmol) and K2CO3 (276 mg, 2 mmol) in dry DMF (15 ml) was added diethylaminoethyl chloride hydrochloride (200 mg, 1.16 mmol).The mixture was heated to 140 °C for 30 h, and the reaction was monitored by TLC.The solvent was removed under reduced pressure and the solid residue was purified by column chromatography (dichloromethane/methanol, 9:1).The first fraction was discarded and the second fraction afforded 157 mg (50%) of 10 as yellow crystals mp 110-112 °C.IR:  3048, 2966, 2925, 2815, 2797, 1645, 1554, 1464, 1430, 1382, 1205, 1099, 920, 780, 750 cm -1 .