Synthesis of new Bis- and Tetra-Acridines

A new series of bis- and tetra-acridines has been prepared from 4-(bromo-methyl)acridine; some of them exhibited encouraging in vitro cytotoxic activities against murine cell lines.


Introduction
Acridine derivatives are well known therapeutic agents whose mutagenic properties depend on their ability to interact with nucleic acids. One mechanism of this interaction is the intercalation between the acridine moiety and the base pairs of the DNA helix.
[1] Moreover the pharmacological activity of these intercalating drugs derives from their ability to inhibit the synthesis of nucleic acids by blocking the action of DNA metabolizing proteins. [2] To develop more effective antagonists of DNA metabolism, many chemists have made molecules combining two or more intercalating ligands. For instance, Denny and co-workers prepared tetra-acridinic derivatives [3] and demonstrated that the higher order structure of DNA can be controlled by a complexation using ligands with multiple binding sites. The biological response of such compounds could be of interest because the higher order DNA structure plays an essential part in the regulation of gene expression by the cooperative binding proteins.
We prepared bis-and tetra-acridine compounds linked by a short nitrogen chain, with acridine moities closed enough to avoid the self-stacking of the aromatic planes; consequently all the synthesized compounds include a specific fragment like the N-bisacridine represented in Scheme 1.

Results and Discussion
Our synthetic pathway was based first on the preparation in 5 steps of the 4-(bromomethyl)acridine 5, followed by N-alkylation with different amines. First, the condensation of ortho-toluidine and 2bromobenzoic acid by Ullmann condensation in ethylene glycol dimethyl ether with copper and anhydrous potassium carbonate gave the 2-[(2-methylphenyl)amino]benzoic acid (1) [4]. The cyclization of 1 in refluxing polyphosphoric acid (PPA) led finally to the 4-methyl-9(10H)-acridinone 2 by an intra-molecular Friedel-Crafts acylation [5]. The acridinone can be prepared alternatively with sulfuric acid or phosphorus oxychloride, but PPA is better because it induces less degradation of the anthranilic acid.  was obtained in one pot after direct acidic oxidation of 4-methyl-acridane (3) (62%). Then, benzylic bromination of 4 was performed by a photochemical reaction. The use of Nbromosuccinimide (NBS) was described by Ledochowski [6] (50% yield), but we obtained better results (69% yield) with 1, 0.5 equiv.) [7] in cyclohexane and 6 hours reaction time (Scheme 2). The dimeric and tetrameric polyacridine derivatives were obtained from the reaction of 4-(bromomethyl)acridine (5) in dichloromethane or acetonitrile with different arylamines or aliphatic primary amines. Moreover, dimers and tetramers can be obtained selectively using a specific reactant ratio as described in Table 1. To obtain the dimeric acridine derivatives 6a-g we used aliphatic and aromatic primary amines (1.5 equivalent of amine) in dichloromethane; the yields are 41-97 % after purification, except for the N,Nbis[methyl(4'-acridinyl)]-p-chlorophenyl 6c preparation (26 %) (Scheme 3).

N CH 2 Br
With two cyclic diamines, such as piperazine and homopiperazine, we obtained two dimers 8a-b, in yields of 55 and 41% respectively (Scheme 5).

Biological Activity
The cytotoxicities of ten compounds (6a, 6d-g, 7, 9a, 9c-e) were determined in a panel of cell lines. The murine L1210 leukemia and A549 cell lines were used as a straightforward comparison of antiproliferative properties. Dimers 6e and 6f and tetramer 9a had the best results against DNA reparation, while polyacridine 9c had the least action against the L1210 leukemia cell line.

Conclusions
We propose an easy way to prepare a new class of polyacridinic derivatives using 4-(bromomethyl)acridines. Most of them display some biological activity and we are now expanding the scope of this approach to the synthesis of other polyacridines.