Convenient synthesis of new 3-aminocarbazole and pyrimido [5,4-b ]carbazole derivatives

Regioselective hydrolysis of dimethyl 1-methyl-9 H -carbazole-2,3-dicarboxylate permits functionalisation of the carbazole skeleton in position 3 by conversion of the carboxylic acid 2 thus obtained into the azide 3 . Curtius degradation of the latter gives the amine 5 , various urethane derivatives thereof ( 4 ), and the urea 6 . Base-induced cyclization of 6 leads to the pyrimido[5,4-b ]carbazole 7a , whereas an analogous compound bearing a basic side chain ( 7b ) is formed by spontaneous cyclization of an intermediate urea.


Introduction
The discovery of the pronounced antitumor activity of the alkaloid ellipticine (5,11-dimethyl-6Hpyrido [4,3-b]carbazole) about fourty years ago has stimulated considerable efforts to modify this natural lead compound in order to find congeners with a superior pharmacological profile. 1,2One of the possible approaches to new ellipticine analogs is the modification of the pyridine part (ring D) of the tetracyclic skeleton, which seems to be a sensitive substructure in terms of a modulation of the molecule's antineoplastic properties.Thus, the position of the pyridine nitrogen atom has been systematically varied. 3Another strategy consists in replacement of the pyridine ring by other heterocyclic substructures: the synthesis of several new pyridazino [4,3b]carbazoles ("3azaellipticines"), some of them exhibiting significant antitumor activity, has been described by us recently. 4,5In continuation of these investigations, we became interested in new 2,3-bifunctional 1-methylcarbazoles which should serve as intermediates in the synthesis of so far unknown types of heterocycle-annelated carbazoles.In particular, derivatives of 1methylcarbazole-2-carboxylic acid with a (free or protected) amino function in position 3 were regarded as useful synthons for the construction of such tetracyclic systems.Here, we report on the synthesis and transformations of these compounds, starting from the known diester 1 which can be easily prepared by Diels-Alder reaction of 4-methylpyrano [3,4-b]indol-3(9H)-one with dimethyl acetylene dicarboxylate. 6,7

Results and Discussion
Taking into account the different steric environments of the two ester groups in compound 1, it should be possible to selectively hydrolyze the COOCH 3 function in position 3. Indeed, this proved to be the case and the monoacid 2 could be obtained in good yield by refluxing 1 in aqueous-methanolic sodium hydroxide for 5 hours.In order to further convert the COOH group into nitrogen functionalities, Curtius degradation of an appropriate carboxylic acid azide was chosen as the key step.

Scheme 1
In a first approach, we found that the isopropyl urethane 4a can be prepared in >70% yield without isolation of the intermediate azide 3 by heating the acid 2 with diphenylphosphoryl azide (DPPA) in the presence of triethylamine, using 2-propanol as the solvent.However, this "onepot" procedure could not be successfully applied to primary alcohols like 1-propanol or methanol.Obviously, the higher nucleophilicity of the latter solvents/reagents results in substantial consumption of DPPA which is converted into the corresponding diphenyl monoalkyl phosphate (identified by GC-MS analyses).

Scheme 2
This problem could be solved by separating the two reaction steps.Thus, the acid 2 was first transformed into the azide 3 by treatment with DPPA-triethylamine in DMF solution (yield: 87%) (The azide 3 is thermally labile and could not be recrystallized; it was used for the subsequent transformations without further purification).In a second step, the urethanes 4b-e then were obtained by refluxing this key intermediate either in the neat alcohol component or in a toluene-ROH mixture (10:1).The latter variant was chosen for employment of the high-boiling alcohols, 2,2,2-trichloroethanol and benzyl alcohol, which gave rise to the formation of the Cbzand Troc-protected aminocarbazole derivatives 4d and 4e, respectively.
Moreover, the preparation of the unprotected primary amine 5 was easily accomplished by acidic hydrolysis (6N HCl, reflux) of the intermediate isocyanate which is formed on heating of a solution of the azide 3 in dry toluene.

Scheme 3
When the Curtius degradation of the azide 3 was performed in the presence of benzylamine, the unsymmetrically N,N′-disubstituted urea 6 was obtained in high yield (On attempted purification of the urea 6 by recrystallization, partial transformation into the pyrimidine derivative 7a was observed).Refluxing of this compound in methanolic sodium methoxide effected ring closure into the fused pyrimidinedione 7a almost quantitatively.On the other hand, treatment of the (in situ generated) isocyanate with N,N-diethylpropane-1,3-diamine did not afford the expected urea derivative but, instead, the tetracyclic compound 7b was obtained in 83% yield.Obviously, in this case the basicity of the employed amine component is sufficient to promote spontaneous cyclization, leading to the pyrimidocarbazole.

Scheme 4
Compound 7b, as a b-fused carbazole bearing a basic diethylaminopropyl side chain which is known to enhance the affinity of ellipticine-type drug molecules (e.g.retelliptine, 8 pazelliptine 9,10 ) to the phosphate backbone of DNA, is of particular interest as a novel analog of this class of antitumor agents and is currently undergoing in-vitro screening in several human tumor cell lines.

Experimental Section
General Procedures.Melting points were determined on a Kofler hot-stage microscope.IR spectra (KBr pellets) were recorded on a Perkin-Elmer Spectrum 1000 FT-IR instrument. 1 H-NMR spectra were recorded on a Varian Unityplus 300 (300 MHz) and on a Bruker Avance 200 (200 MHz) spectrometer (DMSO-d 6 as solvent, TMS as internal reference, δ values in ppm).Mass spectra were obtained on a Shimadzu QP 5050A DI 50 instrument, GC-MS analyses were done on a Hewlett-Packard 5890A/5970B GC/MSD instrument.For column chromatography Merck Kieselgel 60, 0.063-0.200mm was used, the medium pressure liquid chromatography (MPLC) was carried out on Merck LiChroprep Si 60, 0.040-0.063mm (detection at 280 nm).Light petroleum refers to the fraction of bp 50-70 °C.Microanalyses were performed at the Institute of Physical Chemistry (Microanalytical Laboratory), University of Vienna.

Methyl 3-azidocarbonyl-1-methyl-9H-carbazole-2-carboxylate (3).
To an ice-cooled solution of the acid 2 (849 mg, 3 mmol) and triethylamine (303 mg, 3 mmol) in dry DMF (6 mL) was added dropwise a solution of diphenylphosphoryl azide (825 mg, 3 mmol) in dry DMF (3 mL) over a period of 2 h.After removal of the cooling bath, the mixture was stirred at room temperature for 3 h.Then, the mixture was poured into a mixture of ether and ice.The phases were separated and the aqueous layer was exhaustively extracted with ether.The combined extracts were washed with aq.NaHCO 3 and water, dried, and evaporated under reduced pressure (bath temperature below 30 °C).The residue was washed several times with small amounts of diisopropyl ether and dried in vacuo.The azide 3 (835 mg, 87%) was obtained as tiny, paleyellow crystals, mp >110 °C (dec);