Synthesis and Spectral Characterization of Benzo-[6,7][1,5]diazocino[2,1-a]isoindol-12-(14H)-one Derivatives

A simple synthetic route affording 27%–85% yields of benzo[6,7][1,5]diazocino[2,1-a]isoindol-12(14H)-one ring systems from readily available 3-(2-oxo-2-phenylethyl) isobenzofuran-1(3H)-ones and 2-(aminomethyl)aniline starting materials in toluene and catalysed by p-toluene-sulfonic acid is developed. The 1H- and 13C-NMR spectra of the final products were assigned using a variety of one and two-dimensional NMR experiments. The distinction between the two potential isomers of the final products was made on the basis of heteronuclear multiple bond connectivity (HMBC) NMR spectra.


Introduction
In recent years the synthesis and chemistry of medium ring heterocycles has attracted considerable attention because they are often present in biologically active natural products and because of their broad pharmacological profile [1]. Nitrogen-containing eight-membered heterocycles such as azocines and diazocines are known to exhibit a number of important biological properties [2][3][4][5]. The general strategies towards the synthesis of eight-membered heterocycles remain an active area of research and the most common synthetic approach to construct diazocine rings involves the conventional condensation reaction of 2-aminobenzophenones. However, this method is time-consuming, and the yield varies with different substrates. Furthermore, the syntheses of 2-aminobenzophenones can be fairly complicated and expensive [6]. As a result, synthetic strategies for diazocines preparation are limited [7]. The isoindole ring system plays a key role in many pharmaceutical agents owing to their broad range of biological activities [8][9][10][11][12][13][14]. Isoindolinones are the core of many natural products and biologically active compounds such as the benzazepine alkaloids lennoxamine (1) and jamtine (2) (Figure 1). The literature shows that the occurrence of medium-sized rings with two nitrogen atoms in bio-active compounds increases the pharmaceutical strength and activities of the compounds. The [1,5]benzodiazocines are known as homologs of 1,4-benzodiazepines and inhibitors of 17β-hydroxysteroid dehydrogenase type 3. The 17β-hydroxysteroid dehydrogenases play key roles in the formation of active intracellular sex steroids [15]. Impairment of this testosterone-converting enzyme has been shown to be responsible for male pseudohermaphroditism [16], moreover, [1,5]benzodiazocine derivatives showed low to moderate ability to inhibit the 17β-hydroxysteroid dehydrogenase type 3 enzyme which can be used in the treatment of hormone-dependent cancer.
Interest and research in the preparation of compounds containing eight-membered rings has increased considerably in recent years. However, the formation of these ring systems is a challenge for synthetic chemistry researchers. Due to unfavourable entropic and enthalpic effects, the ring closure to form eight-membered rings by intramolecular cyclisation reactions is often difficult in comparison to smaller sized rings. As a result, the usual synthetic strategies for the preparation of other ring systems cannot always be applied to eight-membered rings. Several conventional approaches such as intramolecular cyclization, intermolecular cyclization, palladium-catalyzed, Ugi Four-Center Three-Component coupling reaction (U-4C-3CR), use of microwave radiation, Morita-Baylis-Hillman reaction and intramolecular Friedel-Crafts strategies have been reported in the literature [17][18][19][20][21][22][23][24][25][26][27]. Despite the large number of literature reports on the conventional synthesis of diazocine skeletons, these conventional methods suffer from some drawbacks such as long reaction times, harsh reaction conditions, low-product yields, high cost, toxic by-products and use of toxic catalysts. Therefore, there is a need to introduce new and more efficient methods in order to develop the synthesis of medium-sized rings in the pharmaceutical industry. Interesting biological activities are shown by compounds containing five or six membered heterocyclic rings fused to diazocines [28][29][30][31][32][33][34]. Compound 3, a potent and orally bioavailable Smac mimetic, inhibits cell growth and induces apoptosis in cancer cells and has been shown to be a potent antagonist of inhibitor of apoptosis proteins (IAPs). It is in phase 1 clinical trials for the treatment of human cancer [35][36][37].

Figure 2.
Numbering scheme of 5a for the purpose of NMR assignments.   The assignments were based on a combination of proton and carbon homo-and heteronuclear 2D NMR experiments. There is potential to form two isomers (compounds 5 and 6, Scheme 1) in the final step of the reaction and the chemical shift assignments were made to establish the structure of the products formed. Many of the long range proton-carbon correlations observed in the heteronuclear multiple bond connectivity spectra (HMBC: see supplementary material) fit either of the two isomers ( Figure 3). The assignments were based on a combination of proton and carbon homo-and heteronuclear 2D NMR experiments. There is potential to form two isomers (compounds 5 and 6, Scheme 1) in the final step of the reaction and the chemical shift assignments were made to establish the structure of the products formed. Many of the long range proton-carbon correlations observed in the heteronuclear multiple bond connectivity spectra (HMBC: see supplementary material) fit either of the two isomers ( Figure 3). However, correlations from the methylene group protons H14 to the carbonyl group C12 are indicative of structure 5. The assignment of C12 is confirmed by the long range proton-carbon correlation between H11 and C12.
Protons H7 and H14 from the methylene groups correlate with carbon 7a. Typically, correlations spanning two to three bonds are observed in the HMBC experiment and the methylene group protons next to the nitrogen in structure 6 are too far removed from carbon atoms C7a and C12 and, therefore, highly unlikely to be observed if structure 6 were formed in the reaction. The NMR analyses fully support the conclusion that isomer 5 was formed in this reaction. Other derivatives, where no 2D-NMR data were acquired follow the proton and carbon chemical shifts patterns of the fully characterized derivatives 5a, 5b, 5f and 5h and, therefore, the same regiochemistry was inferred.

General Information
All chemicals were purchased form Sigma Aldrich (Dorset, UK) or Merck (Nottingham, UK) and were used without further purification. Melting points were determined using a Gallenkamp melting point apparatus (Thermo Fisher Scientific, Paisley, UK) and are uncorrected. NMR spectra (at 600 MHz for protons and 151 MHz for 13 C) were recorded on a ECA 600 MHz NMR instrument (JEOL Co Ltd., Tokyo, Japan) equipped with a 5 mm gradient broadband probe. Tetramethylsilane was used as internal standard and solvents as indicated. Chemical shifts were measured in ppm (δ) relative to TMS (0.00 ppm) or the residual solvent peaks. Coupling constants (J) are reported in Hertz (Hz). LC-MS spectra were obtained with a spectrometer equipped with an ESI source (Varian: 210 However, correlations from the methylene group protons H14 to the carbonyl group C12 are indicative of structure 5. The assignment of C12 is confirmed by the long range proton-carbon correlation between H11 and C12.
Protons H7 and H14 from the methylene groups correlate with carbon 7a. Typically, correlations spanning two to three bonds are observed in the HMBC experiment and the methylene group protons next to the nitrogen in structure 6 are too far removed from carbon atoms C7a and C12 and, therefore, highly unlikely to be observed if structure 6 were formed in the reaction. The NMR analyses fully support the conclusion that isomer 5 was formed in this reaction. Other derivatives, where no 2D-NMR data were acquired follow the proton and carbon chemical shifts patterns of the fully characterized derivatives 5a, 5b, 5f and 5h and, therefore, the same regiochemistry was inferred.

General Information
All chemicals were purchased form Sigma Aldrich (Dorset, UK) or Merck (Nottingham, UK) and were used without further purification. Melting points were determined using a Gallenkamp melting point apparatus (Thermo Fisher Scientific, Paisley, UK) and are uncorrected. NMR spectra (at 600 MHz for protons and 151 MHz for 13 C) were recorded on a ECA 600 MHz NMR instrument (JEOL Co Ltd., Tokyo, Japan) equipped with a 5 mm gradient broadband probe. Tetramethylsilane was used as internal standard and solvents as indicated. Chemical shifts were measured in ppm (δ) relative to TMS (0.00 ppm) or the residual solvent peaks. Coupling constants (J) are reported in Hertz (Hz). LC-MS spectra were obtained with a spectrometer equipped with an ESI source (Varian: 210 LC pumpsˆ2, 1200 L Quadrapole MS/MS, 410 autosampler) (Varian (now Agilent), Oxford, UK) using a gradient solvent system of A: Water/0.1% formic acid and B: acetonitrile/0.1% formic acid. Infrared spectra were recorded with a Varian 800 FT-IR spectrophotometer (Varian).