Synthesis of Novel, Potentially Biologically Active Dibenzosuberone Derivatives

Novel representatives of the important group of biologically active dibenzosuberone derivatives were prepared: 3,7-dibromo-5-(dimethylaminoethyl-oxyimino)-10,11-dihydro-5H-dibenzo[a,d]cyclohepta-1,4-diene (1), 3,7-dibromo-5-(3-dimethylaminopropylidene)-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (2) and 1,7-dibromo-5-(3-dimethylaminopropylidene)-10,11-dihydro-5H-dibenzo-[a,d]-cycloheptene (3). These compounds are potential tricyclic antidepressants (TCAs), which are still the most frequently prescribed antidepressants in many countries.


Introduction
A wide range of compounds that contain in their structure the dibenzosuberone unit formed by two aromatic rings fused with a seven-atom cycle have been proven effective for the treatment of depressive disorders, nowadays a highly prevalent illness, as well as for a variety of painful conditions: migraine headache, noncardiac chest pain, functional dyspepsia and irritable bowel syndrome (IBS) [1][2][3][4][5]. Tricyclic antidepressants (TCAs) containing dibenzosuberone moieties mostly affect the autonomic and central nervous systems, and traditional cyclic antidepressants, like amitriptyline (4) [6][7][8], imipramine (5) [9] and noxiptiline (6) [10,11] continue to be used as first-line agents in treating depressive disorders and an expanding list of additional conditions [12][13][14][15], with amitriptyline being the most commonly prescribed TCA. CH  Although TCAs have been in use for decades they are still considered safer and more effective than the newer class of antidepressants, the selective serotonin reuptake inhibitors (SSRIs). The latter have been found to have numerous side effects that are still not well understood. No better antidepressants have been released heretofore and new results on available antidepressants regarding their lack of efficacy, unwanted secondary effects, toxicity and possible association with suicides are being continuously documented and evaluated [12][13][14][15][16][17].
In light of the obvious need to produce agents representing an improvement on both the TCA and the SSRI classes, we decided to revisit the tricyclic antidepressant class and to prepare new dibenzosuberone derivatives [18]. The preparation involved the regioselective bromination of dibenzosuberone and subsequent use of the isomers obtained for the synthesis of new TCAs. Amitriptyline and noxiptiline served as a guide in the design due to their wide efficacy spectrum, success in treating severe psychiatric disorders and their lower costs. The halogen group in the structure of some known antidepressants showed positive effect in treatment of depressive disorders [19][20][21].

Results and Discussion
The starting materials, 3,7-and 1,7-dibromodibenzosuberone 7 and 8 were prepared by the method previously developed in our laboratory, via regioselective bromination of commercially available dibenzosuberone [22]. As the regiochemistry of the substitution process in the dibenzosuberone ring generally results in ortho and para substituted isomers with respect to an alkyl group, our method yields para, para as well as ortho, para substituted isomers. In this manner the 3,7-and 1,7dibromoderivatives have been prepared as the main products and then used for the preparation of their respective N,N-dialkylaminopropyl and O-alkyloxime derivatives.
Among the other possible synthetic paths to the 3,7-dibromo derivative of noxiptiline (1), first we applied an indirect one to avoid the direct alkylation of the oxime 9 and possible formation of an N-alkyl oxime instead of expected O-alkyl one. Unfortunately this method, based on the O-alkyl hydroxylamine prepared by O-alkylation of hydroxylamine protected with a phtalimide group, failed at the N-hydroxyphtalimide alkylation stage. Abandoning the indirect approach we then used the selective O-alkylation of the oxime 9 under mild conditions (solvent, basic salt with an appropriate cation), as shown in Scheme 1. The oxime of 3,7-dibromodibenzosuberone (9) was thus prepared in five days by a method reported for hindered ketones [23], using two portions of four equivalents of hydroxylamine hydrochloride and pyridine. The next step, regioselective O-alkylation of oxime 9 [24][25][26], was performed in acetone with dimethylaminoethylchloride hydrochloride and potassium carbonate as the base. By this procedure O-alkylated compound 1 was obtained exclusively, with an overall yield of 87%. Since the alkylation of oximes as bidentate nucleophiles depends on the structural characteristics of the substrate, in the case of compound 9 alkylation at the less hindered oxygen atom was favored. Attachment of a dimethylaminopropylic functionality to the skeleton of brominated dibenzosuberone was achieved by a Grignard reaction [27][28][29][30]. Since the required Grignard reagent 3-dimethylamino-1-propylmagnesium chloride is not easy to obtain, very strictly controlled reaction conditions have to be employed. The free base is freshly prepared separately and the reaction is initiated with a more reactive alkyl halogenide and iodine. The reaction was carried out for 26 hours at room temperature and isolated compounds 10 and 11 were purified by chromatography on SiO 2 . After chromatographic separation 65% of the 3,7-and 32% of 1,7-dibromo derivatives 10 and 11 were isolated, respectively.

Scheme
Dehydration of compounds 10 and 11, obtained by the previous Grignard reaction, was carried out by stirring for a few hours at 4 ºC with 85% sulfuric acid. Starting with compound 10, 3,7dibromoamitriptyline (2) was obtained in 94% yield, but with the 1,7-dibromodibenzosuberone 11 a mixture of syn (Z) and anti (E) isomers with respect to the 1-Br atom, was obtained. We tried to separate the isomers by chromatography using various mixtures of solvents, but unfortunately without success. Although rotation around the double bond is impossible, the dimethylaminopropyl chain connected to the tricyclic ring in the carbocation 12 rotates freely (Scheme 3). Therefore, we tried to convert compound 3 to the thermodynamically more stable isomer by prolonged stirring in sulfuric acid. We anticipated that implied conclusion that in the thermodynamically controlled reaction the less sterically hindered anti-isomer 3E would be obtained. Indeed, under these conditions the pure E isomer 3E is formed in good yield. IR spectra were recorded on a Perkin-Elmer Spectrum One spectrophotometer, 1 H-NMR and 13 C-NMR spectra were recorded on a Bruker 600 instrument. Shifts are given in ppm downfield from TMS used as an internal standard. All the mass spectra were obtained on a FTMS Finnigan 2001 DD spectrometer. TLC analyses were performed on Merck (Darmstadt, Germany) DC-Alufolien with Kieselgel 60 254 . Elemental analyses were done at the Central Analytical Service (CAS) of the Ruđer Bošković Institute.

Preparation of the free base
Sodium hydroxide and 3-dimethylamino-1-propylchloride hydrochloride were dissolved separately in water (10 mL). These two solutions were mixed and the pH was adjusted to ~14. After extraction with dichloromethane (3x30 mL), the extracts were dried over anhydrous sodium sulfate and the solvent was removed to afford 3.34 g (53%) of the free base.

Grignard reaction
A solution of the free base was prepared by dissolving N,N-dimethylamino-1-propyl-chloride (2.5 g) in dry THF (20 mL). After that, CaH 2 (1.0 g) was added, the suspension was stirred for one hour, and filtered. This freshly prepared filtrate was added dropwise over 30 minutes to a small volume of dry THF (20 mL), a crystal of iodine and Mg-turnings (0.51 g). The reaction mixture was heated with stirring for 2 hours. The solution of the Grignard reagent was cooled to 0 °C and a solution of 3,7-(7) or 1,7-dibromodibenzosuberone (8) (3.66 g) in THF (50 mL) was added dropwise. The obtained mixture was stirred at room temperature overnight, then poured into a saturated solution of sodium chloride and extracted with dichloromethane (3x30 mL). The extract was dried over anhydrous sodium sulfate and the solvent was evaporated. The Grignard reaction products 10 and 11 were purified by column chromatography on SiO 2 with dichloromethane/methanol 9:1 as eluent.