An Unprecedented Straightforward Synthesis of Chiral Pyrrolo[3,4-b]quinolone and Pyrrolo[3,2-b]quinolone Backbones Starting from trans-4-Hydroxy-L-proline

The straightforward synthesis of pyrrolo[3,4-b]quinolone andpyrrolo[3,2-b]quinolone backbones, which can be found inmolecules exhibiting anticancer activities, is presented.The key step of the process is an efficient and unprecedented Friedländer condensation between an oxoproline carbamate, obtained in 3 steps and good yield starting from commercially available and relatively cheap trans-4-hydroxy-L-proline, and various 2-amino-substituted carbonyl derivatives. It was demonstrated that the formation of the two possible regioisomers was fully triggered by both the R substituent onto the 2-amino-substituted carbonyl compounds and the ester function onto the oxoproline carbamate. Thus, in some cases, a complete regiocontrol for the Friedländer reaction could be attained.


Introduction
Natural or synthetic N-heterocyclic compounds containing a condensed quinoline ring system continue to attract the interest of both organic and medicinal chemists mostly due to their antitumor activities [1,2].Among them, pyrrolo [3,4b]quinolone I and pyrrolo [3,2-b]quinolone II are two backbones of important value which can be found in molecules of high biological interest (Figure 1).Indeed, pyrrolo [3,4b]quinolone I is a key constituent of the alkaloid camptothecin (CPT, 1), selective poison of DNA topoisomerase 1, and thus well known for its anticancer activities.Over the years, the synthesis or isolation of many analogues of CPT possessing a pyrrolo [3,4-b]quinolone backbone, together with their biological evaluation, has been published [3].For example, the alkaloids luotonin A (2a) and rosettacin (2b), belonging to the aromathecin family, were proved to exhibit important cytotoxicity demonstrating once again the great interest of this skeleton [4].Pyrrolo [3,2-b]quinolone moiety II is the central core of the alkaloid cryptolepine (3) traditionally used as an antimalarial drug [5,6].It was shown that this alkaloid was able to intercalate with DNA, prompting it as a potential chemotherapeutic agent [7].Compounds 4 have been published recently and were tested successfully over different cancer types, highlighting their biological potential [8].
Interested for many years in the synthesis of aromathecins, we have devoted a lot of efforts for the development of short and efficient routes for the access to these skeletons [9,10].Recently, we have shown that the total synthesis of rosettacin (2b) could be achieved in few steps using an aryl radical cyclization onto an enamide [11].
In this paper, the unprecedented access to both pyrrolo [3,4-b]quinolone and pyrrolo [3,2-b]quinolone backbones 5 and 6 starting from oxoproline carbamate 8 using Friedländer reaction for the formation of the C ring is discussed (see lower part of Scheme 1).It is important to notice that very interesting works were published employing the Friedländer reaction for the synthesis of pyrrolo [3,4b]quinolone and/or pyrrolo [3,2-b]quinolone backbones [12,13].Nevertheless, to the best of our knowledge, this is the first time that this reaction is used with oxoproline carbamate Cryptolepine 3 Ref [12] Ref [13] Friedländer reaction

Friedländer reaction
Friedländer reaction 8 in order to study the influence of the ester function onto the regioselectivity of the process.In fact, we have shown that the formation of regioisomers 5 and/or 6 was triggered by both the R substituent onto 7 and the ester function onto 8. Indeed, when R group is not a hydrogen atom, only regioisomers 6 were obtained in good yield, demonstrating the dramatic role of these two groups.Moreover, besides the biological interest of the scaffolds 5 and 6, the access to these new proline derivatives could be of great importance in organocatalysis chemistry.The assignments for compounds 8 and 10 matched those previously published [14].

General
Procedure for the Synthesis of Compounds 5 and 6.The required aniline derivative 7 (0.55 mmol, 1.1 eq) and oxoproline carbamate 8 (160 mg, 0.5 mmol, 1 eq) were dissolved in freshly distilled toluene (5 mL).p-Toluenesulfonic acid (PTSA, 9.5 mg, 0.05 mmol, 0.1 eq) was then added.The mixture was stirred under reflux for 14 hours and the solvent was removed under vacuum.The crude mixture was dissolved in EtOAc (20 mL) and was then washed successively with a saturated aqueous solution of NaHCO 3 (10 mL) and brine (10 mL).The organic layer was dried over MgSO 4 and concentrated under vacuum.The crude mixture was directly used in the next step without further purification.
The crude of the previous step was dissolved in freshly distilled CH 2 Cl 2 (5 mL) and trifluoroacetic acid (TFA, 0.37 mL, 10 eq) was then added.After one night of stirring at room temperature, the solvent was removed under vacuum.The crude mixture was dissolved in EtOAc (20 mL) and was then washed successively with a saturated aqueous solution of NaHCO 3 (2 * 10 mL) and brine (10 mL).The organic layer was dried over MgSO 4 and concentrated under vacuum.The residue was then chromatographed on silica gel to provide the desired compound (typically, AcOEt/cyclohexane, 20 : 80).

Results and Discussion
The key oxoproline carbamate 8 was synthesized starting from commercially available trans-4-hydroxy-L-proline (9) using a modified version of the route reported by Qiu and Qing (Scheme 2) [14].Starting material (9) was first converted in two steps, that is, Boc protection of the nitrogen atom and benzylation of the acid function, into the corresponding N-Boc-benzyl ester 10 in 44% overall yield.Oxidation of the alcohol moiety was performed using PCC and led to the desired product 8 in a good 78% yield.
Friedländer condensations between oxoproline carbamate 8 and aniline derivatives 7 were carried out using classical procedure (Scheme 3) [15].In order to facilitate the NMR spectra analysis, the Friedländer reaction was directly followed by Boc deprotection of the nitrogen atom of the pyrrolidine ring using trifluoroacetic acid.In the case of 7a (see lower part of Scheme 3), the two regioisomers 5a and 6a were formed in a good 79% yield and in a 70 : 30 ratio.This regioselectivity is in accordance with the one observed by Scheme 4: Proposed mechanism for the Friedländer reaction leading to 5 and 6.
Akue-Gedu and coworkers for similar substrate [13].Interestingly, when substrates 7b and 7c were employed, only regioisomers 6b and 6c were observed by 1 H NMR of the crude mixture prior to any purification.They were isolated after chromatographic purification in 63% and 58% yield, respectively.
The unprecedented high regioselectivity in favour of products 6 is not fully understood yet.Nevertheless, based on the mechanism commonly accepted for the Friedländer reaction [15], we propose the following mechanism (Scheme 4).In the presence of p-toluenesulfonic acid (PTSA), starting materials 7 and 8 led to the formation of the two possible enamines 11 and 12 in equilibrium with the corresponding tricyclic compounds 13 and 14.The observed regioselectivity could result from the steric hindrance and interactions between the R group and the ester function in intermediate 13 versus the R group and the Boc function in intermediate 14.This hypothesis will have to be confirmed by a computational analysis.Finally, when R ̸ = H, intermediate 14 was favoured and led to 6 by loss of water and Boc deprotection.
The structure of product 5a was deduced from its 1 H NMR and 13 C NMR spectral data and by comparison with similar structures [12,13].For example, the 1 H NMR spectrum of 5a contained two doublets ( = 4.64 and 4.40 ppm,  = 15.7 Hz) for the two CH 2 protons of the pyrrolidine ring confirming the presence of a stereogenic center.
(S)-Benzyl 2,3-Dihydro-1H-pyrrolo [3,4-b]quinoline-1-carboxylate (5a).This product was obtained by reaction between 8 and 7a.54% over 2 steps (AcOEt/cyclohexane, 20 : 80).Moreover, 5a 13 C DEPT-135 NMR spectrum showed both the presence of the carbonyl carbon at  = 171.8ppm and the CH and CH 2 carbons of the pyrrolidine ring at 63.6 and 52.4 ppm, respectively.In the case of the 1 H NMR spectrum for products 6, they all displayed the same characteristic doublet of doublet for the two CH 2 and CH proton of the pyrrolidine ring.For example, compound 6b exhibits a doublet of doublet ( = 9.

Conclusion
The regioselective access to both pyrrolo [3,4-b]quinolone and pyrrolo [3,2-b]quinolone backbones was achieved in 5 steps and good yield starting from commercially available trans-4-hydroxy-L-proline. The Friedländer condensation between 2-amino-substituted carbonyl compounds and an oxoproline carbamate was efficient and allowed the formation of the desired products in good yield.More importantly, and to the best of our knowledge, this is the first time that this reaction between these two partners was studied.We have shown that the access to the two possible regioisomers was controlled by both the R substituent onto the 2-amino-substituted carbonyl derivatives and the ester function of the oxoproline carbamate.This synthetic strategy could be used in the future to regioselectively access either the pyrrolo [3,4-b]quinolone or pyrrolo [3,2-b]quinolone backbones by modulating substituents onto Friedländer reaction's partners.This reactivity is already under investigation and the results will be published in due course.