Beckmann rearrangement for the synthesis of derivatives of β - and γ -carbolinones , dihydropyrrolopyridinone and tetrahydroisoquinolinone

Beckmann rearrangement was used as a key step to synthesize six-membered lactams fused to a heteroaromatic or aromatic system. Derivatives of biologically active scaffolds like β - and γ - carbolinone, dihydropyrrolopyridinone and tetrahydroisoquinolinone were synthesized using this strategy.


Introduction
Since the discovery of this Beckmann rearrangement (BR) in 1896, successive investigations have been largely carried out and applied in many ways.BR is a skeletal rearrangement, which accomplishes both, the cleavage of C-C bond and formation of C-N bond.It has become a very useful and efficient method for incorporation of nitrogen atom in both cyclic and acyclic systems and also for the synthesis of various alkaloids.][10] It was thus envisaged to use BR for the synthesis of cyclic lactams fused to indole, pyrrole and benzene ring.
Dihydropyrrolopyridinones are compounds in which, a dihydropyridinone ring is fused to the pyrrole ring.It is reported 11 that substituted pyrrolopyridinones act as Cdc7 kinase inhibitors and are also known to be potential antitumor agents.][14] They have been isolated from various natural sources and show variety of activities like antileukemic, 15 serotonin regulating activity, 16,17 etc.
Tetrahydroisoquinolinones are part of biologically important molecules and have been used as intermediates in the synthesis of various scaffolds.Grunewald et al 18 have demonstrated the use of tetrahydroisoquinolinones as substrates exhibiting PNMT inhibitory activity and these derivatives have been synthesized [19][20][21][22] in the past using strategies like Bischler-Napieralski cyclization and amide formation reactions.
We are herewith reporting the successful use of BR for the synthesis of derivatives of β-and γ-carbolinones, dihydropyrrolopyridinone and tetrahydroisoquinolinone scaffolds.

Results and Discussion
The work started with the synthesis of β-and γ-carbolinones.Thus, indole was formylated (Scheme 1) to get 3-formyl indole followed by Wittig reaction to furnish E isomer of 3-(βcarboethoxyvinyl)indole, 1.Further reduction and hydrolysis furnished acid 3. PPA (polyphosphoric acid) cyclization of acid 3 resulted in formation of cyclic ketone 4, which was converted to oxime 6. 23,24 Ketone 4 was also N-methylated using dimsyl anion and methyl iodide to get ketone 5, which was converted to oxime 7. 24,25 Oxime 6 was obtained as a mixture of geometrical isomers in the ratio 4:1 and oxime 7 as a single isomer as seen from the spectral data.However, geometry could not be assigned to the oximes at this stage.
Oximes 6 and 7 were further separately treated with POCl 3 to furnish selectively only one BR product in each reaction as β-carboline-1-one (8) and N-methyl-β-carboline-1-one (9)  respectively.Both the structures were confirmed by comparing the NMR values with the reported [26][27][28] data.Though the starting oxime 6 was a mixture of isomers, selectively a single product was obtained.This could be explained by the conversion of anti isomer of oxime to its syn isomer during the reaction course followed by BR to furnish the product 8.The conversion of one isomer to the other was also observed on keeping the mixture of oximes at room temperature for a long time.Thus, use of BR was demonstrated for the synthesis of βcarbolinones 8 and 9 (Scheme 1).Subsequently, BR was planned using two more oximes 18 and 22, having oximino group at C 1 .A new method was attempted for the synthesis of ketone 11 29 which was the starting material for above oximes.Thus, Michael addition of indole (Scheme 2) was carried out on methylmethacrylate at 100 ºC in PPA.Unexpectedly, a dimer 30 10 was obtained as a major compound in 40% yield along with two ketones 11 and 12 in 7% and 3% yield respectively.Scheme 2. One pot reaction for synthesis of cyclic ketone 11.
The formation of two ketones can be explained by migration of two bonds, one at a time from the possible spiro intermediate 13 as shown in Scheme 3. The structure of compound 11 was assigned and confirmed from the downfield position of the doublet at δ 7.9 for C 8 H due to its peri position with respect to carbonyl group.Formation of unwanted dimer 10 in major amount can be attributed to the presence of free NH group.To get the required ketone 11 as a major compound, N-benzenesulphonyl indole (14)  was used for the reaction.It was expected that the -SO 2 Ph group would decrease the reactivity of indole ring and would give selectively the required product.The reaction of N-benzenesulphonyl indole with methylmethacrylate at 150 °C using PPA (Scheme 2) furnished two products, 11 (15%) and 15 (70%).Ketone 15 was then deprotected using sodium ethoxide to get desired ketone 11 in 40% yield.In this reaction, a new compound was isolated in 50% yield along with 11, which was shown to be N-ethylketone 16 (Scheme 4).The ketone 11 was N-methylated using dimsyl anion and methyliodide to get the N-methyl ketone 17.
Having the ketones 11, 16 and 17 in hand, they were further converted to single isomer of oximes 18, 20 and 22 respectively.BR of oximes 18, 20 and 22 using POCl 3 furnished selectively unreported γ-carboline-4-ones 19, 21 and 23 respectively (Scheme 4).The structures of these compounds were confirmed using spectral and analytical data.With this successful synthesis of β-and γ-carbolinones using BR, pyrrolopyridinone was the next synthetic target.The synthesis towards dihydropyrrolopyridinone started with protection of pyrrole with benzenesulphonyl chloride to get the N-protected pyrrole 24 (Scheme 5).The protected pyrrole was further treated with methyl methacrylate in PPA at 140 °C to get the cyclic ketone 25.Subsequently, one pot deprotection and oxime formation was attempted by treatment of ketone 25 with hydroxyl amine hydrochloride and sodium hydroxide in methanol.However, the spectral analysis of the isolated product indicated that only deprotection had occurred to give compound 26.Even after continuation of reaction for longer time and refluxing the reaction mixture did not furnish the required oxime.Further, reaction of deprotected ketone 26 with hydroxylamine hydrochloride in presence of pyridine as a base, furnished the oxime 27 in 95% yield after refluxing the contents at 80 °C for 5 hours.The Beckmann reaction of this oxime using POCl 3 at lower temperature, furnished the desired unreported dihydropyrrolopyridinone 28 in good yield.In order to check the BR strategy for non-heterocyclic oxime, tetrahydroisoquinolinone 31, was identified as the target.Thus, as shown in Scheme 6, anisole was treated with methyl methacrylate in PPA at 140 °C for 10 hours to afford the ketone 29.This ketone was subsequently treated with hydroxyl amine hydrochloride and sodium hydroxide in methanol giving the required oxime 30 after one hour of stirring at room temperature.Oxime 30 was obtained as an inseparable mixture of syn and anti products (1:1 ratio) which was clear from the NMR spectra.The mixture of oximes was treated as such with POCl 3 in ethyl acetate to afford the BR product 31 31 in 87% yield.In this case also, though the starting oxime was a mixture of isomers, selectively a single product was obtained, indicating the possibility of conversion of one isomer of oxime to the other during the reaction course followed by BR.This method thus gives a better result than a similar report in literature which uses PPA/BIT as a reagent for the rearrangement.

Conclusions
In this work, Beckmann rearrangement was successfully used to synthesize derivatives of biologically active scaffolds.The synthesis of two β-carbolinone, three new γ-carbolinone, new pyrrolopyridinone and tetrahydroisoquinolinone derivatives was achieved in good to moderate yields.

Experimental Section
General.All reactions were carried out under an inert atmosphere with dry solvents, unless otherwise stated.Reactions were monitored by thin layer chromatography (TLC) on silica gel plates (Kieselgel 60 F254, Merck).Visualization of the spots on TLC plates was achieved either by UV light or by staining the plates in 2, 4-dinitrophenylhydrazine/anisaldehyde and charring on hot plate.All products were characterized by 1 H NMR and 13 C NMR, IR and HRMS/elemental analysis. 1H NMR and 13 C NMR were recorded on Varian Mercury 300 MHz and 75 MHz instrument respectively.Chemical shifts are expressed in parts per million values and 1 H NMR spectra are referenced to 0.00 ppm for Me 4 Si (TMS) and 13 C NMR spectra are referenced to 77.00 ppm for CDCl 3 .Peak multiplicities are designated by the following abbreviations: s, singlet; brs, broad singlet; d, doublet; t, triplet; q, quartet; quint, quintet; m, multiplet; exch, D 2 O exchangeable; J, coupling constant in Hertz.IR spectra were recorded on Bruker instrument as ATR and PerkineElmer 1600 instrument as KBr.Mass spectra were recorded on a Shimadzu QP 5050.HRMS spectra were obtained on a Micromass Q-TOF apparatus.Elemental analyses were recorded on Flash EA1112 Thermo instrument.Melting points recorded are uncorrected.Column chromatography on silica gel (100-200 mesh) was performed with reagent grade ethyl acetate and hexane as an eluent.
1,2-Dihydrocyclopenta[b]indol-3(4H)-one 36 (4).3-(1H-Indol-3-yl)propanoic acid (3, 2.08 g, 0.011 mol) was added to a solution of polyphosphoric acid (PPA, 10 g) in toluene (30 mL).The mixture was stirred at 110 °C for 4h, and then ice water (100 mL) was added to it.The aqueous layer was extracted three times with heated ethyl acetate.The combined organic phases were dried with sodium sulphate, and the solvent was evaporated.(7).To a solution of 4-methyl-1,2-dihydrocyclopenta[b]indol-3(4H)-one (0.64 g, 0.0032 mol) in methanol (20 mL), hydroxylamine hydrochloride (0.9 g, 0.013 mol) and (1.0 mL, 0.013 mol) of pyridine were added, the reaction mixture was heated on water bath for 10 hours.The reaction was monitored by tlc.After confirming the absence of starting compound, the reaction mixture was kept to cool when pale yellow crystals separated out.Methanol was removed on rota evaporator, crushed ice was added, and the solid was filtered off as a pale yellow crystalline compound 7 in 95% yield.mp 194-196 °C; FTIR (KBr cm -1 ) 3263, 1659; 1  To a solution of 1,2dihydrocyclopenta[b]indol-3(4H)-one oxime, 6 (0.47 g, 0.0025 mol) in dry THF, phosphorous oxychloride (0.5 mL, 0.005 mol) was added and the reaction mixture was heated on water bath at 60 °C for two hours.The reaction was monitored by tlc.During the course of the reaction, it was observed that the reaction was not going to completion.The reaction mixture was cooled and cold water was added drop wise.It was extracted with ethyl acetate, dried over sodium sulphate and concentrated by vacuum distillation.The crude product obtained was column chromatographed on alumina using hexane and ethyl acetate (50%) as an eluent yielding pure compound as a pale yellow crystalline compound in 45% yield; mp 183-185 °C.Reaction of indole and methyl methacrylate.Polyphosphoric acid (5 g) was added to a mixture of indole (0.5 g, 0.0043 mol) and methyl methacrylate (15 mL).The reaction mixture was heated on oil bath at 100 °C for 5 min.Crushed ice was poured into the cooled reaction mixture with stirring and the reaction mixture was extracted with hot ethyl acetate.The organic layer was treated with saturated sodium bicarbonate, dried over sodium sulphate and concentrated by vacuum distillation.Crude product was thick brown liquid which was column chromatographed on silica using hexane and ethyl acetate as an eluent yielding dimer 10 of indole 30 as a white powder (40%), a pale yellow crystalline compound 11 (7%) and a white crystalline compound 12 (3%) which decomposed at 220 °C before melting.

Scheme 3 .
Scheme 3. Mechanism for migration of bond.
1 H NMR showed the down field position of proton adjacent to nitrogen [for 19 at δ 3.94-4.1 (m), for 21 at δ 3.93-4.07(m) and for 23 at δ 3.99 (m)] as compared to its position in the corresponding ketones [for 11 at δ 3.1 (m), for 16 at δ 3.08 (m) and for 17 at δ 3.0-3.12(m)], which confirmed the structures as γcarbolinones and not δ-carbolinones.