Spirooxindole: A Versatile Biologically Active Heterocyclic Scaffold

Spirooxindoles occupy an important place in heterocyclic chemistry. Many natural spirooxindole-containing compounds have been identified as bio-promising agents. Synthetic analogs have also been synthesized utilizing different pathways. The present article summarizes the recent development of both natural and synthetic spirooxindole-containing compounds prepared from isatin or its derivatives reported in the last five years. The spirooxindoles are categorized based on their mentioned biological properties.


Introduction
Spirocyclic compounds occupy a unique place within organic chemical compounds due to their rigidity and 3D-geometrical structure. A. Pictet and T. Spengler (1911) reported the first spiro-analog intermediate. Among all spirocyclic compounds, spiroindolecontaining compounds represent an important branch of this class. This is attributed to the versatile biological properties established by diverse natural and synthetic analogs that may originate from the C-2 or C-3 indolyl ring with many heterocycles affording various motifs [1] (Figure 1).

Introduction
Spirocyclic compounds occupy a unique place within organic chemical compounds due to their rigidity and 3D-geometrical structure. A. Pictet and T. Spengler (1911) reported the first spiro-analog intermediate. Among all spirocyclic compounds, spiroindole-containing compounds represent an important branch of this class. This is attributed to the versatile biological properties established by diverse natural and synthetic analogs that may originate from the C-2 or C-3 indolyl ring with many heterocycles affording various motifs [1] (Figure 1).
The unique chemical and bio-properties of spiroindolyl-containing alkaloids have attracted the great attention of many researchers. Many review articles have mentioned the synthetic protocols and bio-properties associated with compounds possessing this scaffold within the last few decades [1][2][3][4][5][6][7]. This study summarizes the important recent development of either naturally isolated or synthetically prepared spirooxindole-containing compounds within the last five years (2018-2022) based on their biological properties.

Antibacterial and Antifungal Spirooxindoles
A variety of 3-spirocyclopropyl-2-oxindoles 21 were synthesized through the methyleneindolinones 20 with the appropriate aromatic aldehyde and TsNHNH2 in MeCN. The reaction was assumed to take place via the formation of the corresponding hydrazone due to the reaction of an aromatic aldehyde with TsNHNH2 which afforded the aryl-diazomethane in presence of K2CO3. The latter due to the interaction with 20 finally furnished the spirooxindoles 21 (Scheme 1). Some of the targeted agents showed promising antibacterial properties. The most potent was 21d (R = Br, Ar = 3-C6H4-O-CH2-Ph) revealing considerable antimicrobial properties relative to Ciprofloxacin against Gram-positive (MIC = 0.49, 0.007; 0.24, 0.007 μM for 21d and Ciprofloxacin against S. pneumonia and B. subtilis, respectively) and mild behavior against Gram-negative bacteria (MIC = 7.88, 6.88; 3.9, 0.49 μM for 21d and Ciprofloxacin against P. aeruginosa and E. coli, respectively) [14]. Spirooxindole metabolites 15-19 were isolated from the marine fungus Penicillium janthinellum HK1-6 ( Figure 7). None of the isolated compounds reveal considerable antibacterial properties against Gram-positive (S. aureus, E. faecalis, E. faecium) and Gram-negative (E. coli) bacterial strains [13].

Antibacterial and Antifungal Spirooxindoles
A variety of 3-spirocyclopropyl-2-oxindoles 21 were synthesized through the methyleneindolinones 20 with the appropriate aromatic aldehyde and TsNHNH2 in MeCN. The reaction was assumed to take place via the formation of the corresponding hydrazone due to the reaction of an aromatic aldehyde with TsNHNH2 which afforded the aryl-diazomethane in presence of K2CO3. The latter due to the interaction with 20 finally furnished the spirooxindoles 21 (Scheme 1). Some of the targeted agents showed promising antibacterial properties. The most potent was 21d (R = Br, Ar = 3-C6H4-O-CH2-Ph) revealing considerable antimicrobial properties relative to Ciprofloxacin against Gram-positive (MIC = 0.49, 0.007; 0.24, 0.007 μM for 21d and Ciprofloxacin against S. pneumonia and B. subtilis, respectively) and mild behavior against Gram-negative bacteria (MIC = 7.88, 6.88; 3.9, 0.49 μM for 21d and Ciprofloxacin against P. aeruginosa and E. coli, respectively) [14].

Antibacterial and Antifungal Spirooxindoles
A variety of 3-spirocyclopropyl-2-oxindoles 21 were synthesized through the methyleneindolinones 20 with the appropriate aromatic aldehyde and TsNHNH 2 in MeCN. The reaction was assumed to take place via the formation of the corresponding hydrazone due to the reaction of an aromatic aldehyde with TsNHNH 2 which afforded the aryl-diazomethane in presence of K 2 CO 3 . The latter due to the interaction with 20 finally furnished the spirooxindoles 21 (Scheme 1). Some of the targeted agents showed promising antibacterial properties. The most potent was 21d (R = Br, Ar = 3-C 6 H 4 -O-CH 2 -Ph) revealing considerable antimicrobial properties relative to Ciprofloxacin against Gram-positive (MIC = 0.49, 0.007; 0.24, 0.007 µM for 21d and Ciprofloxacin against S. pneumonia and B. subtilis, respectively) and mild behavior against Gramnegative bacteria (MIC = 7.88, 6.88; 3.9, 0.49 µM for 21d and Ciprofloxacin against P. aeruginosa and E. coli, respectively) [14].
A series of spirooxindolopyrrolidines 47 and 48 was prepared through dipolar cycloaddition of β-nitrostyrenes 43 and azomethine ylides (obtained from the condensation of isatin 44 and tryptophan 45 or L-histidine 46) in different organic solvents (Scheme 8). Enhanced/higher yields were observed upon considering ionic liquid ([bmim]Br, 1-butyl-3-methylimidazolium bromide) compared with the conventional solvents. Some of the synthesized agents revealed antifungal properties, of which 47 is the most notable, against C. albicans (MIC = 4-16 µg/mL) with inhibition of fungal hyphae and biofilm formation [21].

Antimycobacterial Spirooxindoles
Tuberculosis is one of the most severe infectious diseases threatening human life. Mycobacterium tuberculosis is a pathogenic bacterial microorganism responsible for infectious diseases. Although different therapeutics have been developed and clinically approved, novel agents are still in demand. This is due to the side effects of the used medications and drug resistance strains discovered [23].

Antimycobacterial Spirooxindoles
Tuberculosis is one of the most severe infectious diseases threatening human life. Mycobacterium tuberculosis is a pathogenic bacterial microorganism responsible for infectious diseases. Although different therapeutics have been developed and clinically approved, novel agents are still in demand. This is due to the side effects of the used medications and drug resistance strains discovered [23].

Antimycobacterial Spirooxindoles
Tuberculosis is one of the most severe infectious diseases threatening human life. Mycobacterium tuberculosis is a pathogenic bacterial microorganism responsible for infectious diseases. Although different therapeutics have been developed and clinically approved, novel agents are still in demand. This is due to the side effects of the used medications and drug resistance strains discovered [23].

Anticancer Spiroindoles
Cancer is one of the most deadly diseases threatening several millions of human lives every year. Chemotherapeutical approaches represent one of the major options besides radiotherapy, immunotherapy, and surgery for cancer treatment. Although advances have been achieved in discovering many chemotherapeutical agents, ideal therapeutics (high efficacy with limited side effects) are unreachable. Progress in research directed toward novel bioactive agents is still encouraged [29,30].
The MDM2 (human murine double minute-2) is an important target for cancer therapy. It is a cellular inhibitor for p53 (tumor suppressor). Overexpression of MDM2 was exhibited in many cancer types with wild p53. Due to protein-protein interaction, MDM2 is capable of p53 inhibition (negative regulation through direct binding or ubiquitination/degradation); it is considered a highly attractive target for developing antitumor active agents. The p53 has a circular role in cancer cell apoptosis. In other words, p53 inactivation is an important factor for cancer progression that may be achieved by blocking the interaction of MDM2-p53 [31,32]. Some spiroindole-containing compounds were discovered as MDM2-p53 inhibitors which are entered into human clinical trials ( Figure 8) [33].

Anticancer Spiroindoles
Cancer is one of the most deadly diseases threatening several millions of human lives every year. Chemotherapeutical approaches represent one of the major options besides radiotherapy, immunotherapy, and surgery for cancer treatment. Although advances have been achieved in discovering many chemotherapeutical agents, ideal therapeutics (high efficacy with limited side effects) are unreachable. Progress in research directed toward novel bioactive agents is still encouraged [29,30].
The MDM2 (human murine double minute-2) is an important target for cancer therapy. It is a cellular inhibitor for p53 (tumor suppressor). Overexpression of MDM2 was exhibited in many cancer types with wild p53. Due to protein-protein interaction, MDM2 is capable of p53 inhibition (negative regulation through direct binding or ubiquitination/degradation); it is considered a highly attractive target for developing antitumor active agents. The p53 has a circular role in cancer cell apoptosis. In other words, p53 inactivation is an important factor for cancer progression that may be achieved by blocking the interaction of MDM2-p53 [31,32]. Some spiroindole-containing compounds were discovered as MDM2-p53 inhibitors which are entered into human clinical trials ( Figure 8) [33].
A series of spirooxindoles 77 was prepared through the reaction of isatins with aroylacetonitriles 75 and 5-aminopyrazole 76 in refluxing AcOH/H 2 O (1:1) (Scheme 22). Promising antiproliferative properties were noticed by some of the synthesized agents (IC 50 = 6.9, 11.8; 0.12, 0.62 µM against HepG2 "liver" and PC3 "prostate" cancer cell lines for the promising agent synthesized "R = H; R' = Ph" and Doxorubicin, respectively). It has been noticed that the promising agent synthesized exhibited a high pro-apoptotic protein Bax level with low anti-apoptotic protein Bcl-2 in HepG2 cells, confirming its impact on apoptosis induction. The same phenomenon was also supported by testing the caspase-3/9 and p53 protein levels [42]. Additionally, the most promising agents discovered against MDA-MB-231 (triple-negative breast) cancer cell line are those with R/R' = H/Ph and Cl/Ph (IC 50 = 6.70 µM for both) relative to Doxorubicin (IC 50 = 0.12 µM) which also showed good affinity against caspase-3/9 and p53 protein supporting their capability for apoptosis induction [43].
Spiro    However, no process of reaction was detected with utilization of nitroalkenes possessing substituent 85 under the same mentioned reaction conditions. As a result, MeSO3H was used instead affording a mixture of the corresponding spiroindoles 86 and 3,3′bis(1H-indole)methane derivatives 87 [ However, no process of reaction was detected with utilization of nitroalkenes possessing substituent 85 under the same mentioned reaction conditions. As a result, MeSO 3 H was used instead affording a mixture of the corresponding spiroindoles 86 and 3,3 -bis(1Hindole)methane derivatives 87 [46] (Scheme 26).

Antimalarial Spirooxindoles
Malaria is one of the most endemic diseases worldwide. This is due to the suitable environment for mosquitoes in tropical and subtropical regions with a high global population. Many parasitic species of protozoa causing this disease have been identified as transmitted to humans through mosquito bites. Although several agents were investigated against malaria (Artemisinin 115, Nobel Prize in Physiology awarded to Professor Youyou Tu due to efforts in its discovery) [59] (Figure 9), there remains a need for newer ones. This is attributed to the resistance observed by some varieties of this parasite [60]. Artemisinin and its derivatives are fast-acting agents against the asexual blood stage parasites. Co-administration of artemisinin analog (fast-acting) with a long-acting drug as first-line therapeutics is recommended [61]. Cipargamin (Figure 2) is a promising antimalarial compound in clinical studies as a therapeutic inhibiting blood-stage P. falciparum. This is considered a promising agent to combat the artemisinin resistance parasite [62]. Hepatic safety behavior was achieved through clinical studies (phase II) across wide-

Antimalarial Spirooxindoles
Malaria is one of the most endemic diseases worldwide. This is due to the suitable environment for mosquitoes in tropical and subtropical regions with a high global population. Many parasitic species of protozoa causing this disease have been identified as transmitted to humans through mosquito bites. Although several agents were investigated against malaria (Artemisinin 115, Nobel Prize in Physiology awarded to Professor Youyou Tu due to efforts in its discovery) [59] (Figure 9), there remains a need for newer ones. This is attributed to the resistance observed by some varieties of this parasite [60].

Antimalarial Spirooxindoles
Malaria is one of the most endemic diseases worldwide. This is due environment for mosquitoes in tropical and subtropical regions with a hig lation. Many parasitic species of protozoa causing this disease have bee transmitted to humans through mosquito bites. Although several agents gated against malaria (Artemisinin 115, Nobel Prize in Physiology awarde Youyou Tu due to efforts in its discovery) [59] (Figure 9), there remains a n ones. This is attributed to the resistance observed by some varieties of this Artemisinin and its derivatives are fast-acting agents against the asexu parasites. Co-administration of artemisinin analog (fast-acting) with a long Artemisinin and its derivatives are fast-acting agents against the asexual blood stage parasites. Co-administration of artemisinin analog (fast-acting) with a long-acting drug as first-line therapeutics is recommended [61]. Cipargamin ( Figure 2) is a promising antimalarial compound in clinical studies as a therapeutic inhibiting blood-stage P. falciparum. This is considered a promising agent to combat the artemisinin resistance parasite [62]. Hepatic safety behavior was achieved through clinical studies (phase II) across wide-range doses [63,64].

Antihyperglycemic Spirooxindoles
Multi-component azomethine (formed from isatin and thioproline) cycloaddition with 4-arylidene-5(4H)-oxazolones 127 in refluxing methanol afforded the corresponding spirooxindoles 128 and 129 as a mixture of two diastereoisomers (Scheme 43). Promising antihyperglycemic properties were observed by some of the synthesized agents. Compound 128 with R = 4-MeC 6 H 4 is the most potent (IC 50 = 1.76, 4.81 µM against α-amylase from human saliva and α-glucosidase from Saccharomyces cerevisiae, respectively) [69]. Spirooxindoles 130 were obtained through azomethine (obtained through condensation of isatin and benzylamine) cycloaddition with chalcones 88 in refluxing methanol (Scheme 44). Some of the synthesized agents revealed AGE (advanced glycation end, which is the formation of sugar-derived substances) inhibitory properties in the BSA (bovine serum albumin) glucose assay, supporting the suitability for diabetes. The most promising agent synthesized is that with R = Ph (IC50 = 11.37 μM) relative to the aminoguanidine "standard reference" (IC50 = 40.54 μM) [70]. Spirooxindoles 130 were obtained through azomethine (obtained through condensation of isatin and benzylamine) cycloaddition with chalcones 88 in refluxing methanol (Scheme 44). Some of the synthesized agents revealed AGE (advanced glycation end, which is the formation of sugar-derived substances) inhibitory properties in the BSA (bovine serum albumin) glucose assay, supporting the suitability for diabetes. The most promising agent synthesized is that with R = Ph (IC 50 = 11.37 µM) relative to the aminoguanidine "standard reference" (IC 50 = 40.54 µM) [70]. Spirooxindoles 130 were obtained through azomethine (obtained through condensation of isatin and benzylamine) cycloaddition with chalcones 88 in refluxing methanol (Scheme 44). Some of the synthesized agents revealed AGE (advanced glycation end, which is the formation of sugar-derived substances) inhibitory properties in the BSA (bovine serum albumin) glucose assay, supporting the suitability for diabetes. The most promising agent synthesized is that with R = Ph (IC50 = 11.37 μM) relative to the aminoguanidine "standard reference" (IC50 = 40.54 μM) [70].

Conclusions and Future Directions
Development of new potential therapeutics is always a challenge for medicinal chemistry research. Spirooxindoles represent an important class of heterocyclic compounds and have emerged as attractive scaffolds with unique structural architecture and diverse pharmacological properties. Many natural and synthetic compounds have been identified as potential pharmacophores. Even though there have been several important breakthroughs and encouraging results on spirooxindoles as potential therapeutic agents as discussed above, challenges and opportunities remain for medicinal chemistry research. Several investigations on spirooxindole scaffolds were reported and studied in recent years [73][74][75][76][77]. The current compiled synthetic protocols of pharmacologically active spirooxindole scaffolds will provide an efficient platform to create a new generation of potential spirooxindole analogues for various diseases.

Conclusions and Future Directions
Development of new potential therapeutics is always a challenge for medicinal chemistry research. Spirooxindoles represent an important class of heterocyclic compounds and have emerged as attractive scaffolds with unique structural architecture and diverse pharmacological properties. Many natural and synthetic compounds have been identified as potential pharmacophores. Even though there have been several important breakthroughs and encouraging results on spirooxindoles as potential therapeutic agents as discussed above, challenges and opportunities remain for medicinal chemistry research. Several investigations on spirooxindole scaffolds were reported and studied in recent years [73][74][75][76][77]. The current compiled synthetic protocols of pharmacologically active spirooxindole scaffolds will provide an efficient platform to create a new generation of potential spirooxindole analogues for various diseases.