Synthesis and antimicrobial activity of new 2-azetidinones from N-( salicylidene ) amines and 2-diazo-1 , 2-diarylethanones

This paper describes the reactions of N-salicylideneamines with diarylketenes generated from thermal decomposition of the 2-diazo-1,2-diarylethanones. An equimolar reaction of the Nsalicylidenebenzhydrylamine with diphenylketene affords a mixture of 2[(benzhydrylimino)methyl]phenyl-2,2-diphenylacetate as a major product and 1-benzhydryl-3,3diphenyl-4-[2’-(O-diphenylacyl)hydroxyphenyl]-2-azetidinone as a minor product. The reactions of various N-salicylideneamines with 2.2 molar equivalents of 2-diazo-1,2-diarylethanones have been carried out to afford 1-substituted-3,3-diaryl-4-[2’-(O-diarylacyl)hydroxyphenyl]-2azetidinones as sole product in excellent yields. The products, characterized on the basis of satisfactory analytical and spectral data, have shown moderate to good antimicrobial activity against some bacteria and fungi. Among the fourteen compounds in the series, the 2-azetidinone 4l with a 4-chlorophenyl group on β-lactam ring nitrogen and 4-methylphenyl groups on βlactam ring C-3 position is the most active compound.


Introduction
2-Azetidinones, commonly known as β-lactams, are well-known heterocyclic compounds among the organic and medicinal chemists. 1The activity of the famous antibiotics such as penicillins, cephalosporins and carbapenems are attributed to the presence of 2-azetidinone ring in them.Recently, some other types of biological activity besides the antibacterial activity have been reported in compounds containing 2-azetidinone ring. 2,3Such biological activities include antifungal, antitubercular, antitumor, cholesterol absorption inhibition and enzyme inhibition activity.The β-lactams also serve as synthons for many biologically important classes of organic compounds. 4][3][4] The most common method for the synthesis of 2-azetidinones is the Staudinger keteneimine cycloaddition, which involves the reaction of imines with acid chloride in the presence of a tertiary base. 5This reaction, however, depends, on many factors including temperature, which often needs to be optimized. 6We have been using α-diazoketones as precursors of diarylketenes and carbenoids, and investigating the reactions of these intermediates with organic compounds containing nitrogen atom in different structural environments. 7These reactions are simple to carry out, versatile and don ot require the use of any acidic or basic reagent.Using 2diazoketones as precursors of the diarylketenes, we have recently reported the synthesis of 2azetidinones and spiro-2-azetidinones from the imines of the thiophene-2-carbaldehyde and indoline-2,3-dione, respectively, as possible antimicrobial agents. 8,9The 2-azetidinones, obtained from the imines of thiophene-2-carbaldehyde and diarylketenes, showed moderate antibacterial and antifungal activity. 8The reactions of ketenes with ambident substrates, however, depend on the structural environment of the particular functional group.For example, diphenylketene adds on to the carbon-nitrogen double bond of the benzil monoimines and camphor monoimines leaving the carbon-oxygen double bond on these compounds. 10,11The reaction of diphenylketene with isatin imines has been observed to occur at the amido nitrogen and not at imino nitrogen. 12ur group has recently undertaken a project on the reactions of salicylaldehyde derivatives with α-diazocarbonyl compounds in quest for new aspirin analogs.We have carried out an equimolar reaction of the 2-diazo-1,2-diphenylethanone 1a with N-salicylideneanilines 2 and observed that the regiochemistry of the reaction depended significantly on the substituents on aniline moiety (Scheme 1). 13The reaction of diphenylketene (A), generated in situ from thermal decomposition of the 2-diazo-1,2-diphenylethanone 1a, with 2 having an electron withdrawing chloro group on the phenyl ring attached to imino nitrogen occurred mainly at phenolic hydroxyl group giving rise to [(4-chlorophenylimino)methyl]phenyl-2,2-diphenylacetate 3 whereas the reaction with substrates 2 having an electron-donating groups afforded a mixture of 2-[(arylimino)methyl]phenyl-2,2-diphenylacetates 3 and 1-substituted-3,3-diphenyl-4-[2'-(Odiphenylacyl)hydroxyphenyl])-2-azetidinones 4; the latter being in appreciable amounts.In continuation of this study, the present paper reports an equimolar reaction of the 2-diazo-1,2diphenylethanone 1a with an N-alkylimine, N-salicylidenebenzhydrylamine 2g, and a 2.2:1 molar reaction of the 2-diazo-1,2-diarylethanones 1a,b with various N-salicylideneamines 2a-h affording new 1-substituted-3,3-diaryl-4-[2'-(O-diarylacyl)hydroxyphenyl])-2-azetidinones 4a-n in excellent yields.The products, characterized on the basis of satisfactory analytical and spectral data, have been screened for their antibacterial and antifungal activity.
The results of the 1:1 molar reaction of the diphenylketene with N-salicylideneamines led us to infer that the hydroxy group had higher reactivity in comparison to imino group towards diphenylketene.However, the imines with strong electron-donating ethoxy group competed efficiently with the hydroxyl group to afford the 2-azetidinone in appreciable amount.Diphenylketene, formed from thermal decomposition of the 2-diazo-1,2-diphenylethanone, reacted first with the hydroxyl group forming an intermediate 5 (Scheme 2).A 1,3-proton transfer in the latter intermediate afforded the product 3g.A second mole of diphenylketene then reacted with imino nitrogen of the 3g forming the zwitterionic intermediate 6.Similar zwitterionic intermediates have been proposed earlier in the reactions of ketenes with imines.These intermediates may cyclize to form the product 4g.
The reaction of imine 2a with 2.2 molar equivalents of 2-diazo-1,2-diphenylethanone 1a afforded a white crystalline compound as the sole product in good yield (Table 1) (Scheme 3).This product has been identified as the 2-azetidinone 4a on the basis of similar spectral data (see experimental) and undepressed mixed melting point with the sample obtained from an equimolar reaction of the 1a with 2a. 13 Similar reactions of 1a with 2b-h, and of 2-diazo-1,2-bis(4methylphenyl)ethanone 1b with 2 also afforded the 2-azetidinones 4b-m in good to excellent yields (Table 1).The significant features of the spectra are described briefly in the succeeding paragraph.

Scheme 3
The IR spectra of the compounds showed strong absorption band at 1750-1765 cm -1 characteristic of the β-lactam and ester carbonyl groups.The broad band around 3400 cm -1 observed in the IR spectra of imines 2a-h also disappeared in the IR spectra of the products indicating that the reaction has occurred at the hydroxyl group.The 1 H NMR spectra of 4 showed the disappearance of the singlet azomethine signal at around δ 8.0 ppm observed in the 1 H NMR spectra of the substrates 2 indicating the reaction of the diarylketenes with the ARKAT USA, Inc. azomethine group.In most of the products, the β-lactam ring methine proton and O-acyl proton appeared respectively at around δ 5.8 and 5.3 ppm.These assignments were confirmed by 1 H- 13 C correlation studies by HMBC (Figure 1).A typical HMBC spectrum is shown in Figure 2. The 13 C NMR spectra showed two downfield signals at around δ 171.0 and 166.0 ppm corresponding to the ester and amido carbonyl carbons, respectively.The other four membered ring carbons appeared at around 72.0 (C-3) and 56.0 ppm (C-4).The formation of the products can be explained by same path as shown in Scheme 2. However, in case of imines with electron donating groups, the product 2-azetidinones might not be formed only from the reaction of diarylketenes with 1:1 molar product 3.In these cases the imino group can compete with the phenolic group for the diaryketenes and reaction at both sites can occur simultaneously.
Ten of the fourteen 2-azetidinones 4 have been screened for their antibacterial and antifungal activity.The compounds exhibited moderate activity (Table 2).Usually the monocyclic 2-azetidinones show better activity against Gram (-) bacteria.These compounds, however, showed almost parallel activity against Gram (-) bacteria and Gram (+) B. subtilis.Although no significant effect of substituents on the 2-azetidinone ring was observed on the activity of the compounds the product 4l with 4-chlorophenyl group on ring nitrogen was the most active compound in the series.The 2-azetidinones 4j and 4m with methoxy and benzhydryl group, respectively, paralleled the antibacterial activity of 4l but their activity on fungi was lower.The compounds having 4-methylphenyl groups on C-3 position of the aztidinone ring ARKAT USA, Inc.
showed significantly higher activity in comparison to those with phenyl rings on C-3 position of the 2-azetidinones against S. aureus.All the compounds showed better antifungal activity against C. mycoderma compared to S. cerevisiae.2-Diazo-1,2-diarylethanones were prepared by oxidation of the benzil monohydrazones. 14The latter compounds were synthesized by the condensation of benzils with hydrazine hydrate by reported method. 14N-(2-Salicylidene)amines were prepared by reacting an equimolar amount of salicylaldehyde and amines. 15eneral procedure for the reaction of 2-diazoketones with N-salicylideneamines A solution of either equimolar amount (1 mmol of each of 1a and 2g) or 2.2 mmol of 1 and 1 mmol of the imines 2 in dry benzene (10 ml) was refluxed for 6-8 hrs under continuous stream of nitrogen.The solvent was removed under reduced pressure and the residual matter was purified by fractional crystallization with ethanol to afford white crystalline products.

Antimicrobial activities
Using microlitre syringes, different amounts (0.05 µg, 0.1 µg, 0.5 µg, 5 µg, 10 µg, 50 µg and 100 µg) of each compound and standards were spotted on different glass-backed plates coated with silica-gel (Merck) 60 F 254 .Nutrient agar was inoculated with respective pure microbial cultures, overlaid on the above plates and incubated appropriately.Inhibition of the bacterial and fungal growth was evaluated based on a simple and rapid bioautographic agar overlay method using procedures described earlier.

Table 1 .
Physical data of 2-azetidinones 4a-n * All the compounds gave satisfactory elemental analysis for C, H and N.

Table 2 .
13tibacterial and antifungal activity of 4 (MIC µg/mL) Melting points have been recorded on a Stuart Scientific melting point apparatus and are uncorrected.The IR spectra were recorded on a Perkin-Elmer-781 IR spectrophotometer using KBr disc of the sample.The 1 H and13C NMR spectra were recorded in a CDCl 3 solution at 300 MHz and 75.4 MHz, respectively, on a Bruker TM 300 MHz spectrometer.Benzil, 4,4'-dimethylbenzil, hydrazine hydrate, salicylaldehyde and amines were Aldrich products.Benzene was dried by refluxing over sodium hydride.