Design and Synthesis of the Candesartan Key Intermediate

This paper presents experimental data regarding the synthesis and structural characterization by: 1H-NMR, 13C-NMR, IR spectral analysis, melting point and thin layer chromatography of the candesartan key intermediate: methyl 2-[(tert-butoxycarbonyl)amino]-3-nitrobenzoate. In addition, a computational study of predicted molecular parameters, vibrational wavenumbers, frontier molecular orbitals energy diagram, molecular electrostatic potential map and other electronic distributions maps using restricted hybrid HFDFT SCF calculation has been performed for obtaining the most stable conformer. For the most Stable conformer has been made a series of DFT calculations using the B3LYP levels using the 6-31G * basis set.

Cardiovascular disease or heart disease is a category of diseases involving the heart or blood vessels. From a technical point of view, the term cardiovascular disease refers to any disease that affects the cardiovascular system, commonly referred to as diseases related to atherosclerosis and/or hypertension. Cardiovascular disease remains the leading cause of death worldwide. Over the past two decades, the mortality rate caused by these diseases has evolved differently, while have been falling in many high-income countries, in low-income countries, deaths and cardiovascular disease have been rapidly growing. Every year, heart disease kills more people than cancer. In recent years, cardiovascular risk in women has been rising and killed more women than breast cancer. Emotional studies among young people showed that early atherosclerosis can occur in adolescence and primary prevention efforts have been made since childhood.
The development of angiotensin receptor blockers (ARBs) is a major advance for the treatment of hypertension and potentially for other cardiovascular disorders [1]. One of the angiotensin receptor blockers is the candesartan. Candesartan is administered as candesartan cilexetil, which has better bioavailability than candesartan. The prodrug is rapidly and completely hydrolyzed to candesartan during absorption by the gastrointestinal tract [2,3]. Candesartan cilexetil is indicated for the treatment of hypertension and also for the management of chronic heart failure [4]. and the reduction of the derivative (4), compound (5) (alkyl 3-amino-2 -[[(2'-cyanobiphenyl-4-yl) methyl] amino] benzoate) have been generated. By the cyclocondensation of the compound (5) with tetraethyl orthocarbonate and acetic acid or with triethyl orthoformate have been obtained alkyl ester of 1-[(2'-cyanobiphenyl)-4-yl-methyl]-2ethoxybenzimidazole-7-carboxilyc acid (6). Compound (6) is further treated with trimethyl tin azide in toluene at the reflux generating compound (7) which after hydrolysis in the basic medium leads to the formation of candesartan methyl]benzimidazole-7-carboxylic acid) has been prepared by the reaction of (7) compound with the trityl and the triethyl amine in the appropriate solvent. After the esterification of the (9) compound with 1-haloethyl cyclohexyl carbonate has been obtained candesartan cilexetil trityl (9), compound which, following the hydrolysis reaction (in the acidic condition), leads to the formation of candesartan cilexetil.

Experimental part
Melting points have been determined in opened capillary on Melting point apparatus OptiMelt and are uncorrected. Elemental analysis have been performed on a Perkin Elmer CHNS/O Analyzer 2400 Series II. UV-Vis have been recorded on an instrument UV-Vis LAMBDA 12. FT IR have been recorded on an instrument Bruker Vertex 70 with diamond optic. 1 H-and 13 C-NMR spectra have been recorded in DMSO-d 6 , on two instruments Varian, Varian Gemini 300 BB (operating at 300 MHz for proton and 75 MHz for carbon) and UNITY 400 Plus(operating at 400 MHz for proton and 100 MHz for carbon). Tetramerthylsilane as internal standard have been the reference for the chemical shifts. All chemical shifts are given in the delta scale (ppm vs internal TMS).

Synthesis of 3-nitrophthalic acid
A mixture of phthalic anhydride (148g, 1 mol), nitric acid (247 mL), sulfuric acid (202 mL) was stirred at 75 0 C, then slowly at 80 0 C and then 2 h at 100 0 C. At the end of the reaction, the mixture was cooled and was poured over 500 g ice and 100 mL of water. The precipitate formed (a mixture of 3-nitrophthalic acid and 4-nitrophthalic acid) was filtered off, washed with cold water, and after drying, was recrystallized from water to yield 3-nitrophtalic acid. One of the ways of the synthesis of Candesartan cilexetil is described in scheme 1. [5][6][7][8]. The synthesis pathway consists in the condensation of the alkyl 2-tertbutoxycarbonylamino-3-nitrobenzoate compound (1) (R = methyl or ethyl) with the 4-(2-cyanobiphenyl)-benzyl bromide (2). After the acid hydrolysis of the compound (3) Synthesis of methyl 2-carboxy-3-nitro-benzoate 17.21 mL Trimethyl ortoformiate (16.98 g; 0.16 mol) and 2.2 mL sulfuric acid 98 % was added to a solution of 3-nitrophtalic acid ( 25 g; 0.118 mol) in 60 mL methanol, and the mixture was stirred at reflux temperature for 24 h, and then was evaporated to dryness. The crude compound was recrystallized from water to yield methyl 2-carboxy-3-nitro-benzoate (mp 162.5-165°C; yield 87%). 1  Synthesis of methyl-2-(chlorocarbonyl) -3-nitrobenzoate 2.7 mL SO 2 Cl 2 was added to a solution of methyl 2carboxy-3-nitro-benzoate (4.5 g; 0.02 mol), N,Ndimethylformamide (1 mL), in chloroform (25 mL), and the mixture was stirred at the room temperature 30 min. and then 4 hours at the refux temperature. The mixture was washed with water and sodium carbonate 20%. The organic layer was dried over Na 2 SO 4 , and will be used in the next step of the synthesis without isolation of the compound from the reaction mixture.

Synthesis of methyl 2-(azidocarbonyl) -3-nitrobenzoate
The chloroformic solution containing methyl 2-(chlorocarbonyl) -3-nitrobenzoate was added to a mixture of sodium azide (1.82 g; 0.028mL) in N, N-dimethylformamide (10 mL), and the mixture was stirred at the 5 0 C for 1 h. The mixture was washed with 40 mL water. The organic layer was dried over Na 2 SO 4 , and will be used in the next step of the synthesis without isolation of the compound from the reaction mixture.
Synthesis of methyl 2-(tert-butoxycarbonylamino)-3nitrobenzoate 9 mL (0.095 mol) tert-butanol was added to a solution of methyl 2-(azidocarbonyl)-3-nitrobenzoate in chloroform and the mixture was stirred at 30 0 C 15 min., and then at 95 0 C, 30 min. At the end of the reaction, the mixture was cooled and was diluted with water to precipitate the product. The precipitate formed was filtered off, washed with water, and after drying, was recrystallized from methanol to yield methyl 2-(tert-butoxycarbonylamino)-

Molecular mechanics calculations
The molecular modeling study of predicted molecular parameters, vibrational wavenumbers, frontier molecular orbitals energy diagram, molecular electrostatic potential map and other electronic distributions maps using restricted hybrid HF-DFT SCF calculation have been performed for obtaining the most stable conformer. For the most stable conformer have been made a series of DFT calculations using the B3LYP levels using the 6-31G * basis set [9]. The most important topological, conformational characteristics and QSAR properties has been calculated: weight, no. of conformers and tautomers, area, volume, ovality, polarizability, log P, energy of solvation, dipole moment, energy of the HOMO and LUMO orbitals, angles and distances, dihedral angles. NMR, UV and IR spectra of the methyl 2-[(tert-butoxycarbonyl)amino]-3nitrobenzoate compound have been calculated with Spartan 14 software.

Results and discussions
The key intermediate 2-(N-tert-butoxycarbonylamino) -3-nitrobenzoate was obtained according to scheme 1.The synthesis starts from phthalic anhydride, by nitration reaction. Recrystallization from hot water afforded 3nitrophthalic, which further by esterification with trimethyl orthoformate (TOF) in the presence of concentrated sulfuric acid in methanol, leads to obtaining the methyl 2-carboxy-3-nitrobenzoate. The methyl ester is subjected to chlorination reaction with thionyl chloride in the presence of N, N-dimethylfomamide. Methyl 2-(chlorocarbonyl)-3nitrobenzoate is further treated with sodium azide, and the corresponding azide, methyl-2-(azidocarbonyl)-3nitrobenzoate, by heating under reflux in tert-butanol leads to the formation of the key intermediate, methyl 2-(N-tertbutoxycarbonylamino)-3-nitrobenzoate.

Molecular mechanics calculations
The molecular modeling study of predicted molecular parameters have been performed for obtaining the most stable conformer [7]. In this paper, the DFT/B3LYP/6-31 G * level of basis set has been used for the computation of molecular structure, vibrational frequencies and energies of optimized structures ( fig. 1). In order to perform structure-activity relationship (SAR) studies, some electronic properties (table 1), such as HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy values, HOMO and LUMO orbital coefficients distribution, molecular dipole moment, polar surface area (PSA), the ovality, polarizability, the octanol water partition coefficient (logP), the number of hydrogen-bond donors (HBDs) and ad acceptors (HBAs) and acceptor sites (HBAs) and positive and negative ionizable sites derive from CFD assignments. HBA/HBD and +/-Centers, Hydrophobe Centers including aromatic centers, can be viewed in figure 2. The polarizability is useful to predict the interactions between non-polar atoms or groups and other electrically charged species, such as ions and polar molecules having a strong dipole moment.
Molecular polar surface area (PSA), is a descriptor that has been shown to correlate well with passive molecular transport through membranes and therefore allows the prediction of transport properties of the drugs. LogP is Scheme 2. Preparation of methyl 2-(tertbutoxycarbonylamino)-3-nitrobenzoate  [8]. A few important graphical quantities resulted from quantum chemical calculations were displayed, manipulated and interrogated. Another indicator of electrophilic addition local map is provided by the ionization potential, overlapping energy to remove electrons (ionization) the electron density. In addition, the electrostatic potential map ( fig. 3a), an overlay of the electrostatic potential (the attraction or repulsion of a positive charge for a molecule) on the electron density, is valuable for describing the overall distribution of molecular charge, as well as to predict the sites of electrophilic addition. Another indicator of the electrophilic addition is supplied by the local ionization potential map ( fig. 3b), an overlapping of the energy of electron removal (ionization) on the electron density. In the end, an indicator of nucleophilic addition is offered by the |LUMO| map ( fig. 3c), an overlap of the absolute value of the lowestunoccupied molecular (LUMO).

Atomic net charges
Three different atomic charges, Electrostatic, Mulliken and Natural, have been calculated (table 2) using DFT/ B3LYP/6-31G (d, p) level and have been given in units of  electrons. A positive charge indicates a deficiency of electrons on an atom and a negative charge, an excess of electrons [10]. More charge density has been found at C4 (carbon atom) than the other carbon atoms. The high positive charge (+0.824e) are due to the O10 (oxygen atom) attached with C4. The high negative charge charge (-0.749e) has been found nitrogen atom N5.
The calculation of the molecular orbital geometry (calculated with Spartan software) shows that, the visible absorption maxima correspond from HOMO to LUMO (91 %). In the electronic absorption spectrum of the studied compound, there are four absorption Banda with a maximum 351, 326, 290 and 264 nm.
The UV-Vis experimental spectrum of the compound methyl 2-(tert-butoxycarbonylamino)-3-nitrobenzoate is shown in the figure 4. Table 3 UV/Vis ALLOWED TRANSITIONS NMR spectral analysis NMR spectra of the methyl 2-(tert-butoxycarbonylamino)-3-nitrobenzoate has been calculated with DFT/ B3LYP/6-31G (d, p) level. After analyzing the experimental and calculated spectra (H-NMR, C-NMR) the correlation between experimental and calculated data has been observed.
Experimental spectrum : 1 H-NMR(dmso-d6, δ ppm, J Hz): 9.53(bs, 1H, NH, deuterable); 8.11(dd, 1H, H-4, 1. The molecular orbital analysis of the Frontier molecular orbital's (FMOs) play an essential role in the chemical stability of a molecule and in the interactions between atoms. They provide information that can be used to predict the characteristics of molecules such as optical properties and biological activities. Between them the most important are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) ( fig. 5) The HOMO represents the ability of a molecule to donate an electron, while the LUMO represents the ability to accept an electron [8,9,11,12].

Conclusions
In the present study, we have reported the synthesis of the methyl 2-(tert-butoxycarbonylamino)-3-nitrobenzoate, an intermediate used in the synthesis of an angiotensin receptor blockers (ARBs), candesartan cilexetil [13]. Its structure has been determined and confirmed by the following methods: elemental analysis, IR spectral analysis, H-NMR, C-NMR, thin layer chromatography. The molecular modeling study of predicted molecular parameters, vibrational wavenumbers, frontier molecular orbitals energy diagram, molecular electrostatic potential map and other electronic distributions maps using restricted hybrid HF-DFT SCF calculation have been performed for obtaining the most stable conformer. For the most stable conformer have been made a series of DFT calculations using the B3LYP levels using the 6-31G * basis set. The most important topological, conformational characteristics and QSAR properties has been calculated: weight, no. of conformers and tautomers, area, volume, ovality, polarizability, log P, energy of solvation, dipole moment, energy of the HOMO and LUMO orbitals. NMR, UV and IR spectra of the methyl 2-[(tert-butoxycarbonyl) amino]-3-nitrobenzoate compound have been calculated with Spartan 14 software.