A combined experimental and theoretical study of the synthesis of quinazolino[3,2-a ][1,5]benzodiazepin-13-ones

A simple and efficient general approach to various tetracyclic 6,7-dihydroquinazolino[3,2-a ][1,5]benzodiazepin-13(5 H )-ones has been demonstrated by reductive N-heterocyclization of 5- alkyl-or benzoyl-1-(2-nitrobenzoyl)-1,3,4,5-tetrahydro-2 H -1,5-benzodiazepin-2-ones. These 2-nitrobenzoylamides were obtained by acylation of the starting 5-alkyl-or benzoyl-1,5-benzodiazepin-2-ones with 2-nitrobenzoyl chloride. A theoretical understanding of the features of the reductive N-heterocyclization reaction was provided by means of quantum chemical reactivity descriptors calculations

As a part of our ongoing research activity, [8][9][10] we were interested in the synthesis of some quinazolino [1,5]benzdiazepine derivatives as possible analogues of natural products.The main synthetic routes to compounds with quinazolino [1,4]benzodiazepine moiety utilize the implementations of 2-azidobenzoylamides in aza-Wittig methodology 4,5,11 and transition metalinduced reductive N-heterocyclization 2,3,6,7,12 for the construction of a variety of heterocyclic compounds.Although these methods have emerged as versatile strategies, they have some disadvantages such as cost and availability of the reagents such as 2-azidobenzoyl chloride and noble metal catalysts.
Thus, benzoylation of lactams 1a-i with freshly prepared 2-nitrobenzoyl chloride in the presence of N,N-diisopropylethylamine (DIPEA) and the catalytic amount of 4dimethylaminopyridine (DMAP) in dry dichlorethane (DCE) at room temperature afforded corresponding nitro benzoylamides 2a-i.The compounds of structure 2 were isolated in acceptable 42-65% yields after chromatographic purification.It should be noted that several experimental parameters were explored with the aim to obtain much higher yields of products 2. The attempts to achieve a better yield of this transformation by increasing the reaction temperature (refluxing in different solvents) and prolonging the reaction time and usage of various quantities of the catalyst (DMAP) and acid chloride were not successful.Continuing our interest in this benzoylation reaction, we investigated the interaction of 1,5-benzodiazepin-2-ones 1a-c with 5-acetylamino-2-nitrobenzoyl chloride under analogous conditions.This interaction led to acetylamino-substituted amides 4a-c in 20-25% yields.We think that rather low yields of isolated products 4a-c is a result of their very poor solubility in usual solvents (DCE, CHCl3, ethyl acetate, benzene) and their complicated separation from the side products (e.g., DIPEA .HCl salt).In addition, 5-acetylamino-2-nitrobenzoyl chloride was prepared according to the procedure 15 and used without further purification.
As the second step in this investigation, we studied the reductive N-heterocyclization of precursors 2a-i and 4a-c.Recently, only a few examples of the catalytic reductive cyclization of N-(2-nitrobenzoyl)amides to the corresponding quinazolino [1,5]benzodiazepines have been reported by us. 16Herein, we report the use of zinc dust in glacial acetic acid at room temperature for the reduction of nitro compounds 2 and 4. As planned in Scheme 1, this process in the case of compounds 2a,b,d,e,g,h was nicely accompanied by a simultaneous N-heterocyclization to give 6,7-dihydroquinazolino[3,2-a][1,5]benzodiazepines 3a,b,d,e,g,h.It should be noted that during this process the formation of the corresponding amino derivatives was not observed (TLC).The polycyclic compounds 3a,b,e,h were obtained in good 68-97% yields, whereas compounds 3d and 3g were isolated only in 30-32% yields.Due to their poor solubility and instability in heterocyclization reaction conditions (acetic acid), acetylamino-substituted amides 4a-c were unsuitable models for the study of this reaction.
Starting compounds 1a-c were easily obtained by the benzoylation of the corresponding 1,3,4,5-tetrahydro-2H-1,5-benzodiazepin-2-ones according to the procedure. 17Precursors 1d-i were previously described by us. 18he structures of the studied compounds were investigated using IR and NMR spectroscopy.The IR spectra of amides 2a-i and 4a-c show typical bands at 3361-3383 cm -1 for the NH (compounds 4a-c), 1645-1729 cm -1 for two or three C=O and 1500-1530 cm -1 and 1332-1350 cm -1 for the NO2 stretching vibrations.All examined quinazolinones 3 showed one or two (compounds 3a,b) bands in the region of carbonyl group absorption at 1641-1691 cm -1 and C=N bond absorption peak at 1609-1611 cm -1 .
The 1 H NMR spectra of starting compounds 1a-c, as well as of previously described 1d-i, 18 exhibited the signals of aliphatic protons (CH3, CH2 or CH groups) of the heptatomic ring, the characteristic NH singlet at about 9.0 ppm and the resonances of aromatic protons in the range of 6.77.2 ppm integrated for nine protons. 13C NMR spectra of 1a-c, in addition to the unambiguously assigned aromatic and aliphatic carbon resonances, revealed two appropriate downfield lying lines at 170.2171.0ppm (5-CO) and 173.6175.7 ppm (C-2).
The missing NH singlet and the appearance of four additional aromatic protons in 1 H NMR spectra pointed to the formation of compounds 2a-i.The characteristic most downfield signal in the aromatic region at ~ 8.3 ppm was ascribed to CH-3 using the HMBC spectra. 13C NMR spectra of 2a-c, compared to spectra of 1a-c, showed the typical resonances with almost the same chemical shift values (169.7170.9ppm) for 5-CO, upfield shifted 1.93.0ppm for C-2 and the new signals at ~134.7 ppm, ~144.5 ppm and ~167.7 ppm assigned to C-1, C-2 and 1-CO, respectively.The presence of diastereotopic methylene protons (5-CH2) which resonances formed characteristic AB quartet at ~ 4.3 ppm and ~ 4.5 ppm in 1 H NMR spectra and the absence of the resonance of 5-CO group carbon in 13 C NMR spectra were intrinsic to compounds 2d-f compared to 2a-c. 1 H and 13 C NMR spectra of compounds 2g-i showed the presence of 5-CH3 group which resonated at 2.8 ppm and 39.4 ppm, respectively.Broadened resonances of aliphatic and some of aromatic moieties were observed in the case of compounds 2d, 2g, 2h in 1 H and 13 C NMR spectra possibly due to the slow dynamic processes of heterocycle ring compared to the NMR time scale.
The above mentioned facts can be applied to prove the structure of compounds 4a-c.In addition, two new resonances, singlets at 2.1 ppm (COCH3) and 10.7 ppm (5-NH), were observed in 1 H NMR spectra of compounds 4a-c.The resonances in 13 C NMR spectra of compounds 4a-c at 136.5 ppm, 137.8 ppm and 145.4 ppm were attributed to 1-, 2-, 5-aryl ring carbons, respectively, and the resonance at 169.6 ppm was attributed to 5-NHCO carbon atom.These assignments were made by detailed analysis of HMBC spectra.
The essential feature of the formation of the compounds of structure 3 was the missing signal of C-2 (carbonyl group) at ~172.8 ppm and the appearance of resonance at ~ 154.0 ppm (3a,b) and ~156.1 ppm (3d,e,g,h) attributed to C=N group carbon compared to the corresponding 13 C NMR spectra of compounds 2. The resonances of 1-CO group were shifted upfield by approximately 7 ppm and were observed in the range of 160.6-160.9ppm, the resonances of C-1' carbon were shifted upfield ~ 13.4 ppm and those of C-2' downfield ~ 2.2 ppm.The most deshielded proton (at ~ 8.3 ppm) in the aromatic region was assigned to CH-6 in 1 H NMR spectra in the case of compounds 3.
It should be noted that the reductive cyclization carried out with derivatives 2c,f,i under analogous conditions was unsuccessful.Nitro derivatives 2c,f,i (~50%) and starting lactams 1c,f,i (~20%), as a result of partial hydrolysis of nitrobenzoylamides 2, were obtained, while the corresponding amino compounds were not isolated.Thus, 1-nitrobenzoylated derivatives 2c,f,i bearing the methyl substituent in the 3-position of the diazepine nucleus behaved differently in comparison with nitrobenzoylamides 2a, 2b, 2d, 2e, 2g, 2h which had no substituents or had the methyl substituent in the 4-position of heptatomic moiety.These differences can not be explained entirely by the steric hindrance caused by 3-methyl group.The steric influence of 3methyl group was not observed in our previous study of the synthesis of imidazo[1,2-a] and thiazolo[3,2-a][1,5]benzodiazepine derivatives. 8,9Moreover, it is well known that the steric hindrance effects of bulky methyl group most often affects reactivity behaviors in the less flexible aromatic system or in the molecular systems bearing conjugated delocalized π bonding. 19,20While in this reductive heterocyclization the heptatomic diazepine ring bearing 3methyl group can easily change configuration due to flexible σ bonding system.In this way molecular self-regulation of reacting molecular system (in our case 1-nitrobenzoylated derivatives 2c,f,i) could lead to the geometry changes that avoids the steric hindrance effects of reacting centre.
It is worth mentioning that the mechanism for the reduction of nitro compounds has been the subject of many investigations and there is considerable evidence that this reaction proceeds stepwise through a number of intermediates including nitroso and hydroxylamine derivatives that were detected in the reaction mixture. 21,22However, the theoretical understanding of the reduction occurring with simultaneous heterocyclization process is not established well.To identify which reaction step is important for the initiation of heterocyclization reaction, we focused our efforts on the estimation of possible reaction intermediates by means of quantumchemical reactivity descriptors calculations.
We suggested the possible reaction mechanism scenario for this heterocyclization and presented it in Scheme 2.

Scheme 2. Mechanism of reductive heterocyclization of nitroamides 2a-i.
We assumed that the studied reductive cyclization reaction starts in a similar way to the reduction reaction of aromatic nitro compounds to amines. 21,22After the nitro group is activated by metal mediated electrons and two hydrogen atoms, the elimination of water molecule occurs leading to nitroso intermediates Ia-i which after the reductive addition of two hydrogen atoms form hydroxylamine intermediates IIa-i.The formation of hydroxylamine intermediates IIa-i enables the nucleophilic attack of the 2'-N atom on the C(2) atom of the protonated carbonyl group.After the reductive addition of a hydrogen atom, intermediates IIIa-i possessing the weak interaction between the C(2) and 2'-N atoms are formed.The subsequent elimination of two water molecules afforded the C=N double bond and final products 3a, 3b, 3d, 3e, 3g, 3h.The presented reaction mechanism (Scheme 2) demonstrates that the initiation of heterocyclization reaction becomes possible after intermediate III is formed.Thus, the insight into the electronic structure of intermediate III can be of great importance for the explanation of different reactivity pattern of nitrobenzoylamides 2a-i in the reductive heterocyclization reaction.To confirm our assumptions, the theoretical investigation results of intermediates IIIa-i by means of quantum chemical reactivity descriptors calculations are presented in this article.Frontier molecular orbitals densities 23 and Mulliken (M), natural (N) and electrostatic potential derived charges (ESP) 19,[23][24][25][26][27][28][29] reflect the different behavior of these intermediates in the reductive heterocyclization process.
It is known that frontier orbital densities on atoms allow the estimation of donor-acceptor interactions that exist between different atoms in the same molecule. 244][25][26][27][28] Mulliken charges and natural charges show how much electron density is associated with each atom's orbitals.The ESP charges at the atom are chosen to best describe the electrostatic potential surrounding the molecule.The molecular ESP charges on the molecular electron density iso-surface are a good indicator for the interpretation of chemical reactivity. 25- 27,29Hence, it gives a suitable description of molecular properties, such as strong noncovalent interactions that are predominantly electrostatic in nature.
In our earlier quantum chemical reactivity descriptors studies of the substituted 1,5benzodiazepine-2-thiones interaction with bromoketones, the performance of the AM1 method and DFT B3LYP functional with two different basis sets (6-31G* and 6-31+G*) was compared and it was stated that both methods provided very similar results. 29The advantage of the AM1 model is that it is less time consuming.However, the DFT B3LYP model allows the calculation of more molecular quantum properties, offers better accuracy in the estimation of reactivity descriptors, ensures high reliability, and includes a broader diversity of descriptors.Hence, in the present computational study of heterocyclization reaction we used the AM1 and DFT B3LYP 6-31G* methods.The first optimization of plausible intermediate structures was carried out with AM1 method.Consequently, the AM1 geometry optimized structures were used as initial coordinates for energy optimization at the DFT level using the B3LYP functional and 6-31G* basis sets. 30The vibrational frequencies were computed for optimized intermediate structures and checked to present no imaginary vibrational frequency to ensure that they were local minima points on the potential energy surface. 20,31The HOMO and LUMO densities were calculated according to the methods described in the literature. 23he calculated reactivity descriptors -M, N and ESP charges, HOMO and LUMO densities, for 3,4,5-substituted intermediates IIIa-i on the C(2) and 2'-N atoms are the most significant for the reaction progress.They are presented in Table 1.To get the overall insight to the observed reactivity, the typical and the most important for the reaction progress HOMO and LUMO shapes of IIIa-c were selected from all calculated results of IIIa-i and pictured in Figure 2. Computation reveals that HOMOs of IIIa-i consist of identical shapes.Figure 2 demonstrates that the HOMO shapes for IIIa-c consist of out of plane π orbitals located on the benzene ring of benzoylamide moiety and the lone pair orbital situated on the nitrogen 2'-N atom of the hydroxylamine group.The data presented in Table 1 supports the results of the pictured HOMO shapes showing that HOMO density values for 2'-N atom of all studied intermediates differ slightly and are in the range of 0.24-0.38,whereas differences in the shapes of LUMO of IIIa-i are observed.As shown in Figure 2, the LUMOs of 3,4-unsubstituted and 4-methyl substituted IIIa-b mainly consists of the antibonding out of plane π orbital located on the C(2)-O bond of the diazepine skeleton.The minor contribution to the LUMO shape occupancy of IIIa-b is located between the nitrogen N(1) atom of the diazepine skeleton and the carbon atom of 1-CO group in benzoylamide moiety.The magnitudes of densities and the phase of HOMO on the nitrogen 2'-N atom and on the C(2) atom of LUMO demonstrate the tendency in phase overlap between those molecular orbitals and allow bonding interaction between C(2) and 2'-N atoms for IIIa-b.The LUMO shapes of the 3-methyl substituted IIIc mainly consist of the antibonding out of plane π orbital located on the carbonyl group of benzoylamide moiety and partly on the benzene ring annulated with the diazepine cycle (Figure 2).Meanwhile, the C(2)-O bond on diazepine skeleton does not have noticeable shape for IIIc.Thus, in this case, the bonding overlap between the nitrogen 2'-N atom and the carbon C(2) atom is not promoted.This phenomenon is also revealed by the calculated values of LUMO densities for C(2) atom of IIIa-i presented in Table 1.There is a significant difference between the two intermediate groups: LUMO density values for IIIa,b,d,e,g,h are in the range of 0.35-0.53,whereas those for IIIc,f,i in the range of 0.05-0.15.
The results presented in Table 1 show that the calculated M, N and ESP charges for IIIc,f,i bearing 3-methylgroup in heptatomic nucleus do not differ from those for the rest investigated intermediates IIIa,b,d,e,g,h.Therefore, it is possible to suggest that the charges do not play a significant role for this reaction step, whereas the calculated HOMO and LUMO shapes and density values suggest that heterocyclization reaction is controlled by frontier molecular orbitals.Moreover, our computational results reveal that the position of the substituents on the diazepine skeleton have the effect on the LUMO density changes on the IIIa-i.This suggests that the presence of the electron donating 3-methyl substituent decreases the electrophilicity of the C(2) atom and evokes resistance for further intramolecular rearrangements.In conclusion, a series of new 6,7-dihydroquinazolino[3,2-a][1,5]benzodiazepin-13(5H)-ones was successfully synthesized using the reductive N-heterocyclization of 1-(2-nitrobenzoyl)-1,3,4,5-tetrahydro-2H-1,5-benzodiazepin-2-ones.It was established that the course of this heterocyclization depended on the presence of substituents in the heptatomic ring of the starting heterocycles.The possible heterocyclization reaction mechanism was suggested.It was shown that the initiation of heterocyclization reaction in the reduction process became possible with the formation of intermediate IIIa-i.The calculated reactivity descriptors (frontier molecular orbitals densities, Mulliken, natural and electrostatic potential derived charges) of intermediates IIIa-i suggest that heterocyclization reaction is an intermolecular rearrangement strongly controlled by frontier molecular orbitals, whereas charges do not play a significant role for this reaction step.Moreover, our computational results reveal that the position of the substituent on the diazepine skeleton affect the changes in LUMO densities of IIIa-i, therefore this reactivity descriptor can be useful for characterization and prediction of the studied heterocyclization process.

Experimental Section
General.Melting points were determined in open capillaries on a MEL-TEMP 1202D apparatus and are uncorrected.The IR spectra (potassium bromide) were taken on a Perkin Elmer Spectrum GX FT-IR spectrometer. 1 H and 13 C NMR spectra were recorded on a Varian Unity Inova 300 and Bruker Ascend tm 400 at 302 K.Chemical shifts (δ) are reported relative to tetramethylsilane (TMS) with the solvent reference: CDCl3 (δ 7.26 ppm), DMSO-d6 (δ 2.50 ppm) for 1 H NMR and CDCl3 (δ 77.0 ppm), DMSO-d6 (δ 39.50 ppm) for 13 C NMR.The values of chemical shifts are expressed in ppm and coupling constants (J) in Hz.The assignments of 13 C NMR spectra were made with the aid of APT and HMBC experiments.Elemental analyses (C, H, N) were performed on an Elemental Analyser CE-440.The reactions were controlled by the TLC method and performed on a Merck precoated silica gel aluminum roll (60F254) with chloroformethyl acetate-methanol (/, 14:7:1) as the eluent and was visualized with UV light.Dry column vacuum chromatography 32 was performed with silica gel 60 (0.015-0.040 mm, Merck).

Table 1 .
Calculated M, N and ESP charges, HOMO, LUMO densities for intermediates IIIa-i on C(2) and 2'-N atoms