On the synthetic way to novel peri-annelated imidazo[1,5]benzodiazepinones as the potent non-nucleoside reverse transcriptase inhibitors

A series of novel imidazo[1,5]benzodiazepines were synthesized.


Introduction
It is widely accepted that the primary etiological agent for the acquired immunodeficiency syndrome is the human immunodeficiency virus type 1 (HIV-1). 1 The anti-HIV chemotherapy era has started a decade ago.11 These compounds act by a mechanism distinct from that of nucleoside analogs such as AZT, DDC or DDI.Some of the highly active compounds which inhibit the replication of HIV-1, e.g.R82150, were confirmed in vitro to be relatively nontoxic.Several analogs of the parent TIBO (R81250) halogenated in the aromatic ring have been studied and demonstrated anti-HIV activity at low concentrations. 12,13 ccording to these findings, we initiated a preparation and testing of similar structures based on a 1,5-benzodiazepine skeleton (isoTIBO) isomeric to TIBO.

Strategy
The synthons suitable for the construction of a tricyclic imidazo [1,5]benzodiazepine system incorporating the imidazole nucleus condensed both to the seven-membered heterocyclic and aromatic rings appeared to be molecules carrying the 9-nitrodihydro(or tetrahydro)-1,5benzodiazepinone (A, B) fragments.The subsequent extension of the bicyclic 1,5-benzodiazepines to the desired tricyclic heterosystem can be achieved according to the previously 13 described "capping" procedure 10, including the reaction of the corresponding intermediate triamines with C-electrophilic reagents.The realization of this strategy requires 9-nitro substituted 1,5-benzodiazepinones.In this context, some synthetic routes for the preparation of these compounds were employed.Aromatic ring substituents could be incorporated prior to the synthesis of a desired heterocycle (A), and when this was not convenient or feasible the desired targets could be realized via direct electrophilic aromatic substitution in the tetrahydrobenzodiazepinone system (B).

Synthesis of tetrahydroimidazo[1,5,4-e,f][1,5]benzodiazepine-2(1H)-thiones(or ones).
This paper reports the synthesis of tricyclic 1,5-benzodiazepine derivatives 1, 2 and 3 8 incorporating a bromo substituent at C-9. Dihydrobenzodiazepinone 9 has been obtained in moderate yield by the condensation reaction of commercially available 3-nitro-1,2-phenylenediamine with ethyl acetoacetate. 14reatment of compound 9 with an eight-equivalent excess of LiAlH4 led to the reduction of the amide group, the N=C bond and the nitro group to afford the air-sensitive triamine 10, which without isolation, was utilized in the cyclizations for the synthesis of 1 and 2 (Scheme 1).Conversion of 10 to thione 1 was accomplished with CS2 in an overall yield of 31% from 9. 13 Carbonylation of 10 was carried out with 1.1 mol equivalent of trichloromethyl chloroformate (diphosgene) in the presence of NMM. 10 Subsequently, a hydrolytic workup of the carbamoyl chloride derivative formed in the course of this reaction was used to regenerate product 2.This procedure led to the 2 in an overall 11% yield.

Scheme 1
The synthetic approach to the efficient synthesis of 9-bromo substituted title compounds lead to the expectation of substitutent effects for electrophilic substitution of the aromatic ring of tetrahydrobenzodiazepinones.5-Acetyl-7-bromobenzodiazepinones 11a c which have been previously prepared 15  Application of this procedure gave the desired products 13a c but was less effective than in the case of product 10.Monitoring the progress of the reactions with LiAlH4 by TLC showed that prolonged heating caused a complete loss of starting materials and formation of several byproducts.Intending to accumulate larger quantities of the substances, first of all the nitro group in compounds 12 was catalytically reduced.The resulting 9aminobenzodiazepinones were not isolated but successively used in the reactions with LiAlH4.The modification of our reduction methodology also afforded triamines 13a c.These crude products similarly, as described above, were subjected to "capping" with CS2 or diphosgene to effect cyclization.So, we were able to complete the synthesis of 9bromo substituted tricyclic derivatives 3 5 and 6 8.

Biological Screening-Results
Some of compounds 1, 3 5 have been selected by National Cancer Institute (NCI) and evaluated in the preliminary screen of the In Vitro Anti-AIDS Drug Discovery Program for their suppression of HIV-induced cytopathogenicity.This procedure involves the killing of T-4 lymphocytes in a CEM-SS cell line by HIV-1 and is designed to detect antiviral agents acting at any stage of the virus reproductive cycle.Structure-activity data are shown in Table 1 and represent values for 50% effective concentration (EC50) against HIV cytopathic effects, 50% inhibitory concentration (IC50) for cell growth and therapeutic Index (TI = IC50/EC50).All four compounds were relatively nontoxic to cells.Among the compounds tested, two 9-bromo derivatives 4, 5 were confirmed to be active while compounds 1 and 3 to have moderate anti-HIV activity.The compounds 4, 5 are less potent than the 8(or 9)-halogen TIBO analogs 13 , however, these compounds inhibit viral replication in the same submicromolar range as "isoTIBO" derivative, which has a prenyl side chain at the N-7 position. 16The resulting therapeutic index (TI) for compounds 4, 5 is not sufficient for further in vivo testing.These findings indicate that imidazo [1,5]benzodiazepinethiones share structural similarities with TIBO's, thus suggesting that the introduction of an unsaturated prenyl side chain is crucial for the higher anti-HIV activity.
Compounds 6 8 (NSC 711581, 711582, 711583) have been tested in the primary In Vitro Anti-tumor Screen of NCI.The cell panel consists of 60 lines against which the compounds were tested.None of the evaluated imidazo [1,5]benzodiazepinone derivatives exhibited anti-tumor activity.

Experimental Section
General Procedures. 1 H NMR spectra were taken on a Hitachi R-22 spectrometer operating at 90 MHz (35 o ).Chemical shifts (δ) are reported in ppm from TMS. IR spectra were obtained on a Specord 75 IR instrument.Mass spectra were recorded on a Hewlett Packard HP 5890 gas chromatography with a mass-selective detector HP 5971A (70eV).Ascending TLC was performed on Silufol UV254 silica gel plates.Column chromatography was performed with silica gel Chemapol L 40/100.Melting points were determined in open capillaries with a PTP apparatus and are uncorrected.

4,5,6,7-Tetrahydro-6-methylimidazo[1,5,4-e,f][1,5]benzodiazepine-2(1H)-thione (1).
A suspension of LiAlH4 (1.4 g, 36.8 mmol) in 50 mL of dry THF was brought to reflux and maintained for 2 h.It was then cooled to rt.To a stirred suspension a solution of dihydrobenzodiazepinone 9 (1.0 g, 4.6 mmol) in 20 mL of THF was added dropwise over 20 min.The mixture was heated at reflux for 6 h.It was then recooled with an ice bath and quenched with a careful sequential addition of 1.4 mL of H2O in 4.2 mL of THF, 1.4 mL of 15% NaOH solution, and 4.2 mL of H2O.The mixture was warmed to rt and stirred for 0.5 h before the solid was filtered off.The filtrate was concentrated.The resultant dark oil was dissolved in 10 mL of dry EtOH and immediately used without further purification for synthesis of 1. Carbon disulfide (1.8 mL, 30 mmol) was added and the solution was stirred under argon for 2 h at rt.After cooling the formed precipitate was filtered.The crude solid was purified by recrystallization (EtOH) to give 310 mg (31%) of 1 as a white solid.