Sruthi Sudheendran
Leena
a,
Abdul
Akhir
b,
Deepanshi
Saxena
b,
Rahul
Maitra
b,
Sidharth
Chopra
*bc and
Ani
Deepthi
*a
aDepartment of Chemistry, University of Kerala, Kariavattom, Trivandrum-695581, India. E-mail: anideepthi@gmail.com
bDivision of Molecular Microbiology and Immunology, CSIR-Central Drug Research Institute, Sector 10, Janakipuram Extension, Sitapur Road, Lucknow-226031, Uttar Pradesh, India. E-mail: skchopra007@gmail.com
cAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
First published on 3rd May 2023
The synthesis of sixteen tryptanthrin appended dispiropyrrolidine oxindoles, employing [3 + 2] cycloaddition of tryptanthrin-derived azomethine ylides with isatilidenes, and their detailed antibacterial evaluation is described. The in vitro antibacterial activities of the compounds were evaluated against ESKAPE pathogens and clinically relevant drug-resistant MRSA/VRSA strains, from which the bromo-substituted dispiropyrrolidine oxindole 5b (MIC = 0.125 μg mL−1) was found to be a potent molecule against S. aureus ATCC 29213 with a good selectivity index.
Fig. 1 Tryptanthrin (1a) and a biologically active spirofused tryptanthrin-thiopyrano[2,3-b]indole hybrid (2). |
In this context, we synthesized tryptanthrin–spiropyrrolidine oxindole hybrid molecules and have evaluated their antibacterial properties in detail. The hybrid molecules were synthesized by 1,3-dipolar cycloaddition of azomethine ylides (AYs)8 generated from tryptanthrin to N-substituted isatilidenes. Except for very few reports,9 the generation of AYs from tryptanthrin and α-amino acids by decarboxylative routes is scarcely mentioned in literature and there has been no report on the synthesis of spiropyrrolidine oxindole cycloadducts from tryptanthrin. The spiropyrrolidine oxindoles constitute a privileged class of structural moieties on account of their enhanced biological properties.10 Therefore, hybridization of tryptanthrin with spiropyrrolidine oxindoles seems logical and intuitive.
Scheme 1 One-pot, three-component reaction of tryptanthrin 1a, N-ethyl isatilidene 3a and sarcosine 4. |
With the optimized reaction conditions, we tested the generality of the reaction. Initially, various substituted tryptanthrins 1b–g were employed for the reaction with N-alkyl isatilidene malononitriles 3a–c and sarcosine 4 (Scheme 2) to study the effect of different C8 substituents on tryptanthrin on the reaction. It was observed that when the C8 position is substituted with electron-withdrawing groups, like halo groups and the nitro group, the desired dispiro compounds 5b–5e and 5f were obtained in good yields, respectively. However, when electron-donating groups, such as the methoxy group, was employed in the 1,3-dipolar cycloaddition reaction, a slight decrease in the yield of the product (5g) was observed. No appreciable change in yield of the products 5h and 5i were observed using isatilidenes 3b (N-methyl) and 3c (N-propyl).
Scheme 2 Synthesis of dispirooxindole by a three-component reaction of substituted tryptanthrin 1, N-alkyl isatilidene malononitrile 3 and sarcosine 4. |
Scheme 3 depicts the reaction of azomethine ylide generated from tryptanthrin/ring-substituted tryptanthrins 1 and other amino acids such as proline/thioproline/tryptophan/phenylalanine 6 with 3 leading to the formation of the desired dispiro compounds 5j–o.
Scheme 3 Synthesis of dispirooxindoles by a three-component reaction of substituted tryptanthrin 1, substituted isatilidene malononitrile 3 and proline/thioproline/tryptophan/phenylalanine 6. |
It was also observed that the azomethine ylide generated from 8-chloro tryptanthrin 1c (representative example) and glycylglycine (Gly–Gly) 7 underwent facile [3 + 2] cycloaddition with N-ethyl isatilidene 3a to yield (8′′-chloro-3′,3′-dicyano-1-ethyl-2,12′′-dioxo-12′′H-dispiro[indoline-3,4′-pyrrolidine-2′,6′′-indolo[2,1-b]quinazoline]-5′-carbonyl)glycine 8 in 67% yield (Scheme 4). The reaction was conducted in the presence of acetic acid (10 equiv.) in EtOH–H2O (3:1) under reflux for 12 h. In the 1H NMR spectrum of compound 8, signals at δ 13.45 and 8.44 ppm were attributed to the COOH and NH protons of the peptide fragment while the signal of the NH-proton of the pyrrolidine ring appeared at 3.92 ppm. In the 13C NMR spectrum, the four carbonyl carbons of compound 8 were discernible at δ 182.2, 176.1, 167.3 and 157.8 ppm.
Scheme 4 One-pot, three-component reaction of 8-chloro tryptanthrin 1c, N-ethyl isatilidene 3a and glycylglycine (Gly–Gly) 7. |
The possible stereochemical approach of ylide I to dipolarophile 3a is depicted in Scheme 5. The transition states TS I and TS II are formed by exo and endo modes of approach of the ylide to both the faces of the dipolarophile 3a. In TS I, the dipole probably experiences a steric repulsion due to the presence of the benzene ring present in 3a when approaching it from a Si face, while no such steric effects operate in the approach through the Re face; hence the exo Re face approach is more favourable for the diastereoselective formation of the product. The endo approach of the dipole is not favourable due to the steric repulsion of the dipole with benzene ring (at the Si face of 3a) or with the nitrile group (at the Re face of 3a). As the cycloaddition was proceeding through the exo mode only, a single diastereomer was only observed in all cases.
Compound | MIC (μg mL−1) | ||||
---|---|---|---|---|---|
S. aureus ATCC 29213 | A. baumannii BAA-1605 | P. aeruginosa ATCC 27853 | E. coli ATCC 25922 | K. pneumoniae BAA 1705 | |
NT: not tested. | |||||
5a | 1 | >64 | >64 | >64 | >64 |
5b | 0.125 | >64 | >64 | >64 | >64 |
5c | 0.125 | >64 | >64 | >64 | >64 |
5d | 0.25 | >64 | >64 | >64 | >64 |
5e | 1 | >64 | >64 | >64 | >64 |
5f | 0.25 | >64 | >64 | >64 | >64 |
5g | 0.125 | >64 | >64 | >64 | >64 |
5h | 8 | >64 | >64 | >64 | >64 |
5j | 0.25 | >64 | >64 | >64 | >64 |
5n | 0.25 | >64 | >64 | >64 | >64 |
5o | 0.25 | >64 | >64 | >64 | >64 |
8 | 1 | >64 | >64 | >64 | >64 |
Levofloxacin | 0.25 | 8 | 1 | 0.0156 | 64 |
Vancomycin | 2 | NT* | NT* | NT* | NT* |
The structure activity relationship (SAR) of the molecular hybrids is depicted in Fig. 3. From the SAR studies, it was found that halogen substitutions at the R1 and R4 positions caused an enhancement in anti-bacterial activity while the same in R2 had a null effect. Further, anti-bacterial activity remained unaltered upon N-alkyl substitutions and on altering the pyrrolidine moiety (by using different amino acids for the reaction).
Compound | MIC (μg mL−1) | SI (CC50/MIC) |
---|---|---|
5b | 0.125 | 40 |
5c | 0.125 | >80 |
5f | 0.25 | <10 |
5g | 0.125 | >80 |
8 | 1 | 10 |
S. aureus | MIC (μg mL−1) | ||||||
---|---|---|---|---|---|---|---|
5b | 5c | 5g | Levofloxacin | Meropenem | Vancomycin | ||
MSSA | ATCC 29213 | 0.125 | 0.25 | 0.5 | 0.125 | 0.125 | 1 |
MRSA | NRS 119 | 0.125 | 0.125 | 0.25 | 16 | 64 | 1 |
NRS 129 | 0.125 | 0.25 | 0.25 | 0.25 | 4 | 1 | |
NRS 186 | 0.25 | 0.125 | 0.5 | 4 | 8 | 1 | |
NRS 191 | 0.0625 | 0.0625 | 0.25 | 16 | 64 | 1 | |
NRS 192 | 0.125 | 0.25 | 0.5 | 4 | 8 | 1 | |
NRS 193 | 0.25 | 0.25 | 0.5 | 32 | 64 | 1 | |
NRS 194 | 0.5 | 0.5 | 0.5 | 0.125 | 1 | 1 | |
NRS 198 | 0.125 | 0.25 | 0.5 | 32 | 64 | 1 | |
VRSA | VSR 4 | 0.5 | 0.25 | 0.5 | >64 | 64 | >64 |
VSR 12 | 0.25 | 0.25 | 8 | 32 | 8 | >64 |
As can be seen, 5b, 5c and 5g exhibited potent activities against various clinical isolates of MRSA and VRSA, although variations in MIC was observed with 5g against VRSA.12 The equipotent activity exhibited by 5b and 5c against MDR MRSA and VRSA indicates that these compounds possess a new mode of action and is able to bypass existing drug-resistance mechanisms, auguring well for their pre-clinical translation.
As 5b consistently exhibited more potent activity against clinical MDR MRSA VRSA isolates, it progressed further for time kill analysis.
Drug | MIC (μg mL−1) against S. aureus ATCC 29213 | MIC (μg mL−1) of 5b in presence of drug ‘A’ | MIC (μg mL−1) of drug in presence of 5b ‘B’ | FIC A | FIC B | ∑FIC = FIC A + FIC B | Inference |
---|---|---|---|---|---|---|---|
5b | 0.25 | ||||||
Ceftazidime | 8 | 0.25 | 8 | 1 | 1 | 2 | No interaction |
Daptomycin | 1 | 0.25 | 1 | 1 | 1 | 2 | No interaction |
Gentamycin | 0.25 | 0.25 | 0.25 | 1 | 1 | 2 | No interaction |
Levofloxacin | 0.125 | 0.25 | 0.125 | 1 | 1 | 2 | No interaction |
Linezolid | 2 | 0.25 | 2 | 1 | 1 | 2 | No interaction |
Meropenem | 0.0625 | 0.25 | 0.0625 | 1 | 1 | 2 | No interaction |
Minocycline | 0.125 | 0.25 | 0.125 | 1 | 1 | 2 | No interaction |
Rifampicin | 0.0078 | 0.25 | 0.0078 | 1 | 1 | 2 | No interaction |
Vancomycin | 1 | 0.25 | 1 | 1 | 1 | 2 | No interaction |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3md00017f |
This journal is © The Royal Society of Chemistry 2023 |