Asymmetric Synthesis and Antimicrobial Activity of Some New Mono and Bicyclic Β-lactams

Reaction of the amino acid D-phenylalanine ethyl ester (4) with cinnamaldehyde gave chiral Schiff base 5, which underwent an asymmetric Staudinger [2+2] cycloaddition reaction with phthalimidoacetyl chloride to give the monocyclic β-lactam 6 as a single stereoisomer. Ozonolysis of 6 followed by reduction with lithium aluminum tri(tert-butoxy) hydride afforded the hydroxymethyl β-lactam 8. Treatment of 8 with methansulfonyl chloride gave the mesylated monocyclic β-lactam 9, which was converted to the bicyclic β-lactam 10 upon treatment with 1,8-diazabicyclo[5,4.0] undec-7-ene (DBU). Deprotection of the phthalimido group in β-lactams 6 and 10 by methylhydrazine and subsequent acylation of the free amino β-lactams with different acyl chlorides in the presence of pyridine afforded mono and bicyclic β-lactams 14a-d and 15a-d respectively. The compounds prepared were tested against Escherichia coli, Staphilococcus citrus, Klebsiella pneumanie and Bacillus subtillis. Some of these compounds showed potential antimicrobial activities.


Introduction
The β-lactam skeleton is the key structural element of the most widely used class of antimicrobial agents, the β-lactam antibiotics [1].Among the various methods for constructing this pharmaceutically important four-membered ring, the ketene-imine cycloaddition, also known as the Staudinger reaction [2] has provided useful and economical entries to β-lactams, mainly due to the ready availability of both Schiff bases and ketenes [3].The asymmetric syntheses of α-amino β-lactams can be divided into five categories: a) asymmetric induction from the imine component; b) asymmetric induction from the ketene component; c) double stereodifferentiating cycloadditions; d) carbacephem intermediates and e) 2-oxaisocephems and 2-isocephems [1].The asymmetric induction in the reaction of achiral ketenes with chiral imines has been effected from imines derived from chiral aldehydes and achiral amines and also from imines derived from chiral amines and achiral aldehydes.Although it is believed that the former case is better for diastereoselectivity, it has been reported that the reaction of the imine derived from D-glucosamine and cinnamaldehyde with phthalimidoacetyl chloride and triethylamine furnished the β-lactam 1 as a single isomer [4].It has also been found that the imine derived from the amino acid D-threonine upon treatment with azidoacetyl choride and triethylamine affords β-lactams 2 and 3 in a 95:5 stereoisomeric ratio [5].Gunda has reported the use of imines derived from chiral amines in βlactam syntheses giving various degrees of diastereoselectivity [6].The reaction of fluoroacetyl chloride with the imine derived from p-anisidine and D-glyceraldehyde acetonide afforded the corresponding cis β-lactam as a single diastereomer [7].High levels of asymmetric induction in reactions between Evans-Sjogren ketenes and imines derived from (R)-and (S)-α-amino acid esters have been reported [8].
Hashimoto and his coworkers have used chiral imines derived from erythro 2-methoxy-1,2diphenylethylamine and aromatic aldehydes to prepare β-lactams in good yields with high diastereoselectivity [9].Recently, high levels of asymmetric induction have also been reported for monocyclic β-lactam formation [10][11].Thus, we decided to synthesize some diastereoselective mono and bicyclic β-lactams using the asymmetric induction by the imine component and their antimicrobial activities against some pathogenic microorganisms have been tested.

Scheme 1
In addition, the absence of a singlet peak for the benzylic protons in 1 H-NMR spectrum of 11 confirms the formation of bicyclic β-lactam 10.The IR spectrum showed the β-lactam carbonyl absorption at 1780 cm -1 for 10 which has increased a little due to fusing two rings and subsequently the amide resonance is lowered.We next turned to the deprotection of the phthalimido protecting groups of β-lactams 6 and 10 (Scheme 2).

Scheme 2
Among the different methods available for this kind of deprotection, the one using methylhydrazine worked best in this case [19][20][21][22] and a significant improvement in the yield and in the ease of isolation of the free amino β-lactam is noted when methylhydrazine is employed instead of the usual hydrazine.This is due to the decreased acidity of the by-product N-methylphthalhydrazide with respect to phthalhydrazide and hence no complex is formed with the free amine.Consequently there is no need to heat or for acid treatment of the reaction mixture.N-methylphthalhydrazide separates from a chloroform solution of the methylhydrazine-phthalisoimide adduct leaving the free amine in solution.Therefore the methylhydrazine method is most suitable for sensitive substrates [21].Upon treatment with methylhydrazine, β-lactams 6 and 10 were converted to free amino β-lactams 12 and 13 respectively.Treatment of these mono and bicyclic β-lactams with benzoyl, phenoxyacetyl, cinnamoyl and phenylacetyl chloride in the presence of pyridine afforded β-lactams 14a-d and 15a-d in moderate yields

Biological Screening. Antimicrobial Activity Tests
The antimicrobial activity test was performed using the disk diffusion method [23] using ampicillin and gentamycin as the reference compounds.Some of the prepared compounds: 6, 7, 8, 10, 14a-d and 15a-d were tested against one strain each of a Gram positive bacteria (Staphylococcus citrus), a Gram negative bacteria (Escherichia coli), a Gram negative containing capsule (Klebsiella), and a Gram positive spore (Bacillus subtilis).As shown in Table 1, it was found that compounds 6, 14b and 14c were highly active against Bacillus subtilis and moderately active against Staphilococcus citrus.Other compounds were all inactive against these four pathogenic microorganisms.

General
Melting points were determined in open capillary tubes in a Buchi 510 circulating oil apparatus and are uncorrected.Infrared spectra were recorded on a Perkin Elmer 781 spectrophotometer. 1 H-NMR spectra were recorded at room temperature for CDCl 3 solutions, (unless otherwise stated) on a Jeol JNM-EX 90A FT NMR and Bruker Avance DPX 250 MHz spectrometers using tetramethylsilane as an internal standard.All chemical shifts are reported as δ values (ppm).Mass spectra were obtained on a GCMS-QP 1000 EX mass spectrometer (70 eV).The microorganism strains used in this study were β-lactam sensitive ones.

Table 1 .
Results of antimicrobial activity tests of the synthetic monocyclic and bicyclic β-lactams.