Study On The Synthesis And Density Functional Theory Of Cephalosporin Antibiotics' Intermediate-AE-active Ester

AE-active ester [S-2-Benzothiazolyl 2-amino-alpha-(methoxyimino)-4-thiazolethiol acetate] is an important intermediate of many third and fourth generation cephalosporin antibiotics. In this paper, AE-active ester's synthesis process has been studied. The optimum reaction conditions are found and the yield is increased by designing rational experimental project. Density functional theory (DFT) is also used and AE-active ester is studied by quantum chemistry calculation. The equilibrium molecular geometry of AE-active ester is determined. In addition, the components and energies of molecular orbital, NBO charge distribution and transfer, vibration frequency and IR spectra of the compound have been analyzed. These works can offer valuable reference to the QSAR (quantitative structure-activity relationship) and other relevant theoretical research of cephalosporin antibiotics.1. There are two synthesis processes of AE-active ester, triethyl phosphate synthesis method and triphenyl phosphine recovery method. Both of them are optimized by orthogonal experiment design in this paper. The optimum reaction conditions result are as follows: 1) Triethyl phosphate synthesis method.In this method, the materials are AT(Ethyl-aminothiazoly Loximate) and DM(bisbenzothiazole sulfide) and the cheap triethyl phosphate is used to instead of the expensive triphenyl phosphine. The mixed solvent of dichloromethane and acetonitrile is used as the reaction's solvent system. Catalysts are pyridine and triethylamine. The whole reaction is under low temperature environment. Except temperature and catalyst, the main influence factors to reaction yield are the proportion of the materials, the volume ratio of the mixed solvent and reaction time. The optimum reaction conditions optimized by orthogonal experiment design are: the reaction temperature is 5~10℃, the volume ratio of dichloromethane and acetonitrile is 1:2, the reaction time is 2 hours and the proportion of AT and DM is 1:1.4. Then the production yield is 90.8% and the purity≥98.5 % (LC).2) Triphenyl phosphine recovery method.In this method, AT, DM and triphenyl phosphine are used as materials to synthesize AE-active ester. This method is classic and traditional. The product yield can reach to 88%. This paper's main work is to recover the expensive triphenyl phosphine and DM. For this aim, bis (trichloromethyl) carbonate is added into the residual liquid of AE-active ester's synthesis reaction above. Triethylamine is used as the initiator. Finally, triphenyl phosphine and DM are recovered by two reactions and the aim of the materials'recovery and reuse is realized. The optimum reaction conditions are: the reaction temperature is 30℃, the reaction time is 4 hours, the addition of bis (trichloromethyl) carbonate is 8.9 g(the molar ratio with the theoretical dose is 1.5:1) and the dosage of triethylamine is 2 mL(be proportional to the dose of bis (trichloromethyl) carbonate). Then the recovery yield of DM is 64.2% and the recovery yield of triphenyl phosphine is 63.1%. Both of their purity is beyond 98 % (LC). The DM and triphenyl phosphine recovered can be used to synthesize AE-active ester.Parallel experiments are taken for verification on the base of the optimum reaction condition. The synthetic product is characterized by HPLC, IR spectroscopy, MS analysis and 1H-NMR analysis. The reaction mechanism has been discussed. The carboxylic H of AT is deleted under the action of triethylamine, and the carboxylic acid oxygenic negative ion is formed which attacks the P atom's empty orbit in triethyl phosphate to form O&P single bond. The lone pair electronic of S atom in DM attacks the carbonyl carbocation to form transition state, and then the oxygenic negative ion converts to carbonyl again. Because it's easy to form P & O double bond, the C-O bond breaks, triethyl phosphate is removed finally and the target product AE-active ester is obtained.2. A quantum chemistry calculation study and density functional theory study on AE-active ester are carried out by Gaussian 03 calculation software and DFT B3LYP method. Two different basis sets 6-31g and 6-31g (d, p) are selected. The equilibrium molecular geometry of AE-active ester is determined after 40 steps optimizing calculation. According to the equilibrium molecular geometry, the components and energies of molecular orbital, NBO charge distribution and transfer, vibration frequency and IR spectra of the compound have been analyzed. The relationship of molecular structure and activity is also discussed combined with the pharmaceutical synthesis reaction and medical effect mechanism.AE-active ester is obtained by the thio-esterification condensation reaction of AT and DM. The equilibrium molecular geometry is the vertical connection of AT molecular fragment and M (2-mercapto benzothiazole) molecular fragment. At the connection place, the carboxyl C atom of AT connected with the thiol group S atom of M.The zero-point energy corrections molecular total energy is -5.619×104 eV, and the compound has better stability. The frontier molecular orbital analysis shows that the main active part centralizes at the AT molecular fragment, especially at the amido connected with the thiazole ring and it is easy to react while acting with other bio-molecule.Compare with the experiment test IR spectroscopy and the calculation simulation IR spectroscopy of AE-active ester. The result shows that the data of the characteristic absorption peak is consistent and comparability is better. So that it's reliable to study the compound molecular vibration frequency analysis by calculation simulation.When to synthesize cephalosporin antibiotics, the ester bond of active ester molecule breaks and the M molecular fragment is removed. And then active with amido connected with theβ– lactam in the 7-ACA molecule. The C atom accepts the electrons offered by -NH2 as electron acceptor. In conclusion the theory calculation can explain the reaction mechanism well.