Recent advances of azomethine ylides for the synthesis of natural and synthetic bioactive pyrrolidines and spiropyrrolidines

Abstract

1,3-Dipole-based nitrogen containing carbanion, particularly azomethine ylide, is widely used in cycloaddition reactions. It has important and widespread application not only limited in the synthesis of core structural motifs like pyrrolidine and spiropyrrolidines but also in stereocontrolled dipolarophile for building important classes of heterocyclic compounds. Herein, we will explore the current development of azomethine ylide and all the possible applications of this small but elegant fragment using a green approach. This review article will also covers the application of this super energetic, pharmaceutically important moiety in the field of asymmetric, organocatalytic, and transition metal-catalyzed synthesis processes coupled with a theoretical approach. The development of an environmentally safe multicomponent, tandem synthesis method mediated by azomethine ylide will be discussed in detail in this current review article, which will be important for researchers in the field of organic synthesis, heterocyclic chemistry, and medicinal chemistry.

The current review article focuses on azomethine ylide as a key component for the strategic synthesis of pyrrolidine and spiropyrrolidine derivatives, which are important structural motifs in many natural and synthetic bioactive molecules.

1. Introduction

Cycloaddition is an important class of pericyclic reactions or chemical processes in which two or more unsaturated molecules (or portions of the same molecule) combine to generate a cyclic adduct. The backbone size of the participants typically characterises cycloadditions; hence, the 1,3-dipolar cycloaddition is a [3 + 2] cycloaddition, and the Diels–Alder reaction is a [4 + 2] cycloaddition. Unsaturated or partially saturated (hetero) cycles with distinct substitution patterns and frequently high stereocontrol are produced by the reaction. For example, there are numerous instances wherein the Diels–Alder reaction results in both heterocyclic and carbocyclic ring systems. Interestingly, [3 + 2] cycloaddition events produce 5-membered ring systems in a manner similar to that of the Diels–Alder reaction, whereas the former produces 6-membered rings. Dipolarophiles and 1,3-dipoles (cf. diene and dienophile in the Diels–Alder reaction) are the reactive partners in this reaction pathway. Additionally, these are 4π + 2π processes, where sp and sp² hybridized entities are generally used, including 1,3-dipoles. The stability of these 1,3-dipoles varies; while certain dipolar species must be used for reactions within 24 hours of production, some can be separated and stored. Others are produced and reacted in situ and are highly unstable. One of these sp²-type 1,3-dipoles, azomethine ylides (1, 1a), are planar molecules with four π electrons distributed over the three-atom C–N–C unit. They are made up of one nitrogen atom and two terminal sp² carbons and exist in two resonating structures (Fig. 1). Many pharmacologically significant heterocyclic compounds are synthesised using this class of potent reagents in [1,3]-dipolar cycloaddition processes.¹

Fig. 1. Resonating structures of azomethine ylide.

[1,3]-Dipolar cycloaddition of azomethine ylides with alkene or alkyne (Fig. 2) is one of the most efficiently used techniques for the synthesis of a variety of pyrrolidine-based (3) bioactive compounds, such as organic catalysts, natural alkaloids, and building blocks in organic synthesis.² Furthermore, the cycloaddition reaction of non-stabilized azomethine ylides with olefinic dipolarophiles offers a method for the synthesis of numerous novel spiroheterocycles (3a), having stereoisomers with up to four additional chiral centres.^3–9Scheme 1 delineates the overall key applications and the key advantages of the application of azomethine ylides. This review article will elucidate the significant uses of this potent moiety, which is primarily crafted in the twenty-first century, for the synthesis of different natural products as well as a number of other bioactive compounds (Table 1).

Fig. 2. General cycloaddition process of azomethine ylides.

Scheme 1. Schematic representing the overall key applications and the key advantages of the application of azomethine ylides.

Table 1. Summary of the application of azomethine ylide.

S. no.	Compound no.	Reagents and conditions applied	Reference
1	5	Sarcosine, (CH2O)n, 3 Å MS, PhMe, reflux	10
2	6	H2, 10% Pd/C, MeOH	10
3	8	3A MS, toluene	13
4	13	DIPEA, 1,2-dichloroethane, 83 °C, 24 h	14
5	14	AlCl3, DCM, rt, 12 h	14
6	16	HF·Py, THF, 50 °C	15
7	20	AlMe3, toluene, 0 °C	18
8	22	n-BuLi	18
9	25	N2H4·H2O, EtOH, reflux	20 and 21
10	26	AlMe3, PhH, 0 °C	20 and 21
11	28	n-BuLi, −78 °C H2O	20 and 21
12	31	n-BuLi, H2O	28
13	32	CH2O, HCl, CH3OH	28
14	33	HCl, mCPBA, heat	28
15	34	Ms2O, Py CsOAc, DMF 18-Crown-6, 125 °C, MeOH, K2CO3	28
16	38	o-DCB, 160 °C	39
17	47	PhNHNH2, AcOH, 105 °C	44
		NaBH4, MeOH, 0 °C to rt
18	52	Acryloyl chloride, Et3N, DCM	45
19	59a, 59b	ClCbz, K2CO3, THF	47
		Na(Hg), Na2HPO4 MeOH/THF
20	63	NBS/CHCl3, heat t-butyl chloride, Et3N, 0 °C	49
21	67	Et3N, EtOH, RT	50
22	72	NH2CH2CO2H, toluene	51
		Camphor sulfonic acid, heat
23	78	NH2OH, iPr2NEt, toluene, heat	52
24	78	Cl·NH2CH2CO2Et, iPr2NEt, toluene, heat	52
25	83	Sealed tube, heat	53
26	88	Toluene, reflux	54
27	94	10 mol% bis-PA	55
		Toluene, 3 A MS, 40 °C
28	99	Glycine, toluene, 110 °C	58
29	103	Cu(MeCN)4PF6, Et3N	59
		Ferrocene catalysts
30	108	Ag(i)F	64
31	112	PhMe, heat	65
32	115	CsF, THF	66
33	127	Cu(i)/DTBM-segphos	67
		Toluene, 80 °C
34	130	Methyl acrylate hydrocinchonine (6 mol%)	68
		AgOAc (3 mol%) MS4A, toluene, 0 °C
35	133	Tert-butyl acrylate	69
		Ag(i) or Au(i) catalyst
		Chiral phosphoramidite
		Et3N, toluene, RT
36	138	Toluene, heat	70
37	142	Bis-phosphoric acid (Bis-PA)	71
		3A MS, toluene
38	145	Cu(MeCN)4PF6, DBU DCM, −20 °C	72
39	149	N-Methyl glycine/HCHO	74
		Benzene, reflux
40	154	CH3I, CHCl3, reflux	75
41	157	HCHO, PhMe	76
		AcOH, reflux
42	160	CF3CO2H, DCE, 60 °C	77
		3A MS, toluene
43	163	Bis-phosphoric acid (Bis-PA)	78
		3A MS, RT, toluene
44	166	90 °C	79
		Aqueous media
45	171	EtOH, AcOH, rt	80
46	175	MeCN, reflux	81
47	183	[bmin]Br, 100 °C	82
48	190	MeOH, MW, 100 °C	84
49	194	EtOH, 30 °C	85
50	200	MeOH, reflux	86
51	205	MeOH, reflux	87
52	209	EtOH	88
53	213	MeOH, reflux	89
54	216	MeOH, reflux	90
55	219	MeOH, heat	91
56	224	MeOH, reflux	93
57	227	MeOH, reflux	94
58	229	d-Proline catalyst	95
59	233	MeOH, reflux	96
60	236	Dioxane/MeOH, reflux	97
61	240	EtOH, reflux	98
62	242	ACI/EG, 40 °C	99
63	246	MeOH, reflux	100
64	250	MeOH, reflux	101
65	252	Isatins and sarcosine	102
66	256	MeOH, reflux	103
67	258	DMF, 70–90 °C	104
68	262	MeOH, reflux	105
69	264	Cu(MeCN)4PF6, Et3N	106
70	269	DBU, DCM, room temp	107
71	271	DBU	108
72	274	EtOH/toluene, 80 °C	109
73	277	THF, 80 °C	110
74	281	Acetonitrile, reflux	111
75	285	MeOH, reflux	112

2. Application towards the synthesis of some natural products

2.1. (−)-Horsfiline

The plant Horsfieldia superba contains the oxindole alkaloid horsfiline (4), which is utilized in conventional herbal therapy. It has analgesic properties and has been studied for its synthetic production using simple techniques, as well as for the development of derivatives and analogues that may have better analgesic effects. Using chiral auxiliary-directed δ-face discrimination in an intermolecular [3 + 2] annulation of N-methylazomethine ylides with 2-(2-nitrophenyl) acrylate dienophiles 5, Cravotto et al.¹⁰ reported an effective asymmetric method for synthesizing (−)-horsfiline (Fig. 3).

Fig. 3. (−)-Horsfiline.

The procedure outlined by Cravotto et al. was used to prepare the necessary dipolarophile.¹⁰ Because of the quick availability of the chemicals, the process was straightforward, the reaction conditions were somewhat mild, and the product yields were high. Non-stabilized azomethine ylides were synthesised using the Tsuge route¹¹ (decarboxylation of R-amino acid iminium salts). Finally, in the presence of a chiral auxiliary-based dipolarophile, the azomethine ylide was reacted and the major product was the desired pyrrolidine (Fig. 4). The potential of the two-step cycloaddition/reductive heterocyclization protocol for the enantioselective synthesis of (−)-horsfiline was demonstrated by this discovery.

Fig. 4. Synthesis of (−)-horsfiline.

2.2. Spirotryprostatin B

The fungus Aspergillus fumigatus contains the indole alkaloid spirotryprostatin B (7) (Fig. 5). Due to their anti-mitotic qualities, spirotryprostatin B and a number of other indole alkaloids—including spirotryprostatin A, other tryprostatins, and cyclotryprostatins—have gained a lot of attention as potential anti-cancer medications.¹² Therefore, one of the main goals of organic chemists is the complete synthesis of this kind of chemical.

Fig. 5. Spirotryprostatin B.

It was expected that an asymmetric [1,3]-dipolar cycloaddition reaction may be used to produce the core pyrrolidine ring in the production of spirotryprostatin B. The reaction of a chiral azomethine ylide (produced by reaction of 9 with 10) with an oxindolylideneacetate 8 might generate four contiguous stereogenic centers in 11, which was finally extended to 7 by Williams and co-workers (Fig. 6).¹³

Fig. 6. Synthesis of spirotryprostatin B.

2.3. Lamellarin

The alkaloid lamellarin (12) (Fig. 7) has a pentacyclic planar chromophore, namely, 6H-[1] benzopyrano[4′,3′:4,5]pyrrolo[2,1-a]isoquinolinone. It can be cytotoxic to prostate cancer cells and to multidrug-resistant tumour cell lines. Therefore, to synthesize this important alkaloid, the solid-phase total synthesis of lamellarin L (12a) and lamellarin U (12b), the two naturally occurring alkaloids, was proposed by Cironi et al. in 2003 (ref. 14) (Fig. 8). Beginning with the Merrifield resin, an in situ-generated azomethine ylide 13 was produced from the solid-supported alkyne, which underwent an intramolecular [3 + 2] cycloaddition process to get the products. Cleavage of cycloadduct 14 with aluminium chloride produced 12a and 12b in an overall yield of 10% (R = Me, lamellarin L) and 4% (R = H, lamellarin U), respectively.

Fig. 7. Lamellarin.

Fig. 8. Synthesis of lamellarin.

2.4. Indolizidine 239CD

In order to prepare azomethine ylides for the synthesis of indolizidine 239CD (15) (Fig. 9), Pearson et al.¹⁵ investigated the utilisation of (2-azaallyl)stannanes 16. After being treated with HF·pyridine,¹⁶ (2-azaallyl)stannane yielded the extremely uncommon N-unsubstituted nonstabilized ylide 17. Pyrrolidine (18) was obtained via cycloaddition with phenyl vinyl sulfone as a combination of regio- and stereoisomers (Fig. 10), all of which exhibit the 2,5-trans relationship on the pyrrolidine ring, which is a desirable outcome. The penultimate indolizidine 239CD (15) was then obtained in a few steps from 18.

Fig. 9. Indolizidine 239CD.

Fig. 10. Synthesis of indolizidine 239CD.

2.5. Lapidilectine B

The first member of this class of Kopsia lapidilecta alkaloids is known as lapidilectine B (19) (Fig. 11).¹⁷ Pearson et al. most recently reported the first total synthesis of lapidilectine B (14) (Fig. 12).¹⁸ Using stannane 22, carbonyl compound 20 was converted to cycloadduct 23 in good yield through a one-flask imine formation/cycloaddition process. The desired natural product 19 was then obtained through functional group manipulation.

Fig. 11. Lapidilectine B.

Fig. 12. Synthesis of lapidilectine B.

2.6. Lepadiformine

The ambiguity around the true stereostructure of the marine alkaloid lepadiformine¹⁹ (24) has hindered its complete synthesis, which will be reported later on, as shown in Fig. 13. Lepadiformine skeleton was first synthesised by Pearson et al. using the 2-azapentadienyllithium cycloaddition technique^20,21 (Fig. 14). Amine 26 was produced via hydrazinolysis of phthalimide 25, and it was condensed with ketone 27 to yield imine 28. Pyrrolidine 29 was synthesised as a single isomer and in good overall yield from phthalimide via transmetalation of imine in the presence of phenyl vinyl sulphide. Three of the four potential diastereomers of lepadiformine could be produced at C2 and C13 via intramolecular reductive amination, oxidative cleavage of the propenyl side chain, and different functional group modifications. The fourth potential diastereomer was synthesised at those sites by Weinreb^22,23 and Kibayashi,²⁴ but none of these compounds were able to match the natural product. These findings led to the notion that lepadiformine was epimeric to the synthetic lepadiformine counterpart at the quaternary bridgehead position.²¹ This theory has been confirmed through total synthesis by other researchers.^25–27

Fig. 13. Lepadiformine.

Fig. 14. Synthesis of lepadiformine.

2.7. Coccinine

Coccinine (30) (Fig. 15) is an alkaloid belonging to the 5,11-methanomorphanthridine class of alkaloids. Pearson et al.²⁸ synthesised this compound via intramolecular cycloaddition of an in situ generated 2-azaallyllithium with vinylsulfide (Fig. 16). Following an aqueous workup, perhydroindole 32 was produced. This was then subjected to a Pictet–Spengler cyclisation to provide the cyclized product 33. After the sulfoxide was eliminated and the secondary alcohol was inverted, the desired coccinine alkaloid (30) was obtained.

Fig. 15. Coccinine.

Fig. 16. Synthesis of coccinine.

This azomethine ylide cycloaddition method has been used to synthesise a number of other alkaloids in addition to the ones already mentioned, like crinine,^29,30 amabiline,^30,31 augustamine,^30,31 and 6a-epipretazettine.³²

2.8. Mubironine C

Dendrobium nobile produces the sesquiterpenoid alkaloid dendrobine (25, Fig. 17), which is the main alkaloidal component of the Chinese folk remedy “Chin-Shih-Hu”.^33–36 Although it is not mechanistically related to picrotoxinin, this lactonic tetracycle has analgesic, antipyretic, and convulsant properties.^37,38 There are also closely related congeners from comparable orchid species that differ more infrequently in the methanolysis of the lactone, as in mubironine C (37), or more frequently in the oxidation at the 2-position, as in dendrine (36).

Fig. 17. Structures of mubironine and the related alkaloids.

Dendrobium Snowflake “Red Star” is the source of the alkaloid mubironine C. Trauner and colleagues devised the synthesis pathway by using an intramolecular 1,3-dipolar cycloaddition of an unstabilized azomethine ylide to detach the pyrrolidine ring. Their synthesis procedure started with R-carvone and proceeded in seven steps to the cyclisation precursor, which produced deoxymubironine C when it reacted with N-methylglycine via 1,3-dipolar cycloaddition (Fig. 18).³⁹

Fig. 18. Synthesis of deoxymubironine C.

2.9. Aspidospermidine

With over 250 naturally occurring alkaloids, the Aspidosperma alkaloids (Fig. 19) are the biggest family of monoterpene indole alkaloids.⁴⁰ They have been utilized in conventional therapies, including anti-inflammatory and anti-cancer medications, and exhibit a range of biological actions.^41–43 This big family's parent chemical is pentacyclic aspidospermidine (41).

Fig. 19. Aspidosperma alkaloids.

Its synthesis was started by alkylating δ-valerolactam 44 (Fig. 20) to yield lactam 46. Tricyclic ketone 47 was produced through a reductive Michael addition/[3 + 2]cyclo-addition cascade catalysed by an iridium catalyst. Ultimately, the complete synthesis of aspidospermidine (41) was achieved after Fischer indole cyclisation, followed by NaBH₄ reduction.⁴⁴

Fig. 20. Synthesis of the Aspidosperma alkaloids.

2.10. Pregnane (modified hybrid)

Using this method, Fedotcheva and associates created a modified hybrid using steroid pregnane as a chiral auxiliary. Starting with megestrol acetate (49), the study produced all of the novel hybrid compounds by modifying the C-3 pregnane position. An initial acryloylation produced 50, which yielded 51 on stereoselective cycloaddition with stabilised N-metalated azomethine ylide. On further repetition of these two steps, 52 was produced, which was again arylated to 53 (Fig. 21). The hybrids showed micromolar cytotoxic activity against breast carcinoma MCF-7 cell culture and human cervical epithelioid cancer HeLa cell culture in both natural and estradiol-stimulated forms. When compared with the hormonal tumour cells under study, the majority of hybrid chemicals were found to be less harmful to human skin fibroblasts (HSF).⁴⁵

Fig. 21. Synthesis of pregnane (modified hybrid).

2.11. Aloracetam analogue (modified hybrid)

Mandal et al. reported the use of a novel class of enal-functionalized azomethine ylides (54, EAYs) in 2024 for the synthesis of an aloracetam analogue (Fig. 22). The Rh-catalyzed [3 + 2] annulation of ylide with N-alkyl imines (55) yielded tetrasubstituted N-alkyl pyrrole-3-carbaldehyde derivatives (56). Notably, despite the side chain's competing NH-insertion reaction, the N-Boc-protected 1,2-ethylenediamine-derived aldimine was able to provide pyrrole 56, an analogue of the medication aloracetam (57), in 49% yield.⁴⁶

Fig. 22. Synthesis of the aloracetam analogue (modified hybrid).

2.12. Nicotine

The direct enantioselective synthesis of α-heteroarylpyrrolidines using the [3 + 2] cycloaddition of heteroarylsilylimines with activated alkenes has been demonstrated as a feasible and effective process by Javier and coworkers in 2014. With a broad range of azomethine precursors and dipolarophiles, strong enantioselectivity and moderate-to-high diastereoselectivity have been achieved using Cu(CH₃CN)₄PF₆ as the catalyst system. The cycloaddition of azomethine precursors 56, with phenyl vinyl sulfone, also showed remarkable reactivity, yielding a mixture of both regioisomers 59a and 59b in 88% yield (Fig. 23),⁴⁷ which were then converted to 60, the known precursor⁴⁸ for α-nicotine 61.

Fig. 23. Synthesis of nicotine.

2.13. Tetrazomine

Scott et al. described the synthesis (Fig. 24) of the powerful antitumor antibiotic tetrazomine for the first time in 2002. To obtain tetracyclic intermediate essential for the synthesis of tetrazomine, the synthetic protocol began with an anisole-derivative, 62, to produce the allylic amine precursor 63. An azomethine ylide was then created and utilised in an effective [1,3]-dipolar cycloaddition to give 64, which lead to tetrazomine 65 in eight additional steps.⁴⁹

Fig. 24. Synthesis of tetrazomine.

2.14. Lamellarin skeleton

A novel, all-encompassing pathway (Fig. 25) to 1,2-diaryl-substituted pyrrolo[2,1-a]isoquinoline (68) has been established through the 1,5-dipolar electrocyclization processes of azomethine ylide (67), which is obtained from easily accessible stilbenic acid (66). The lamellarin skeleton (69) was subsequently constructed from 68.⁵⁰

Fig. 25. Synthesis of the lamellarin skeleton.

2.15. Aspidospermine, aspidospermidine, and quebrachamine

Through a tandem cyclization/cycloaddition cascade reaction, Coldham et al. reported the synthesis of three new rings while maintaining total control over stereochemistry and regiochemistry (Fig. 26). The protocol began with the conversion of a dibromoketone (71) into a precursor of a cyclic azomethine ylide (72). The formation of a cyclic azomethine ylide followed by an intramolecular cycloaddition onto the tethered alkene yielded the tricycle (73). This tricycle then further extrapolated to aspidospermine (74), aspidospermidine (75), and quebrachamine (76).⁵¹

Fig. 26. Synthesis of aspidospermine, aspidospermidine, and quebrachamine.

2.16. Stemona alkaloid cores

In 2010, Burrell et al. demonstrated the synthesis (Fig. 27) of the stenine and neostenine core ring systems, which may be accomplished using the chemistry of cascade condensation, cyclisation, and cycloaddition. The study began with propyl nitrile (77), which was converted into an acyclic aldehyde (78). The stemona alkaloid cores (79, 80) with an azepine ring bonded to a pyrrolidine and a cyclohexane ring were then produced from 78.⁵²

Fig. 27. Synthesis of Stemona alkaloid.

2.17. Quinocarcin core

In 2011, Huck et al. investigated whether azomethine ylides could be produced from 82, through iminium salt, 83. He described a catalyst-free dehydration method (Fig. 28) that produced anti- and syn-cycloadducts that included the quinocarcin core (81) and thereby unveiled a novel synthetic approach that enables access of hemiaminals 84 and 85 to yield highly functionalised cycloadducts.⁵³

Fig. 28. Synthesis of the quinocarcin core.

2.18. Lycoposerramine-S

The complete synthesis of lycoposerramine-S (86) from the known alkyl iodide 87 was described (Fig. 29) by Fukuyama and coworkers in 2012. They have achieved total synthesis in 14 stages. The azomethine ylide precursor 88 was first produced from 87 and reacted with 89, which upon 5-exo-trig radical cyclisation and intramolecular 1,3-dipolar cycloaddition simply produced the desired polycycle 90, which finally led to the production of lycoposerramine-S (86), in a few steps.⁵⁴

Fig. 29. Synthesis of lycoposerramine-S.

2.19. 18-Epispirotryprostatin A and 9,18-bis-epispirotryprostatin A

Cheng et al. (2011) have shown (Fig. 30) a 1,3-dipolar cycloaddition reaction between azomethine ylides and methyl 2-(2-nitro-phenyl)acrylate (94) that produces highly enantioenriched pyrrolidine derivatives (95). The cycloaddition was catalysed by a bis-phosphoric acid derivative (91). The application of this process was demonstrated by the synthesis of diastereoisomers of spirotryprostatin A (96 and 97).⁵⁵

Fig. 30. Synthesis of the diastereoisomers of spirotryprostatin A.

2.20. (±)-Crispine A

In order to synthesise (±)-crispine A (98), Coldham et al. (2009)⁵⁸ utilised a cascade pathway of condensation–cyclization–cycloaddition process. The aldehyde (99) required for this method was first easily produced in two steps by following the described protocol.^56,57 After that, glycine and phenyl vinyl sulfone were initially used in the cascade chemistry; however the resultant mixture of two isomers produced the required cycloadduct in a low yield. Therefore, using the highly active bis-sulfone (100) as a dipolarophile, we performed the crucial step. In this instance, an enhanced yield (70%) of the intended product 101 was attained (Fig. 31), which was used to synthesize alkaloid crispine A in a few steps.⁵⁸

Fig. 31. Synthesis of the diastereoisomers of crispine A.

2.21. Spirotryprostatin A analogue

Waldmann and coworkers, in 2011, synthesised the polycyclic spirotryprostatin scaffold (102) containing the 3,30-pyrrolidinyl-spirooxindole core in a highly enantioselective and high-yielding manner (Fig. 32). The cycloaddition step was facilitated by copper/ferrocene dual catalysis, which involved an isatin-type dipolarophile (103), to yield spiropyrrolidine 105. The spirotryprostatin A analogue was then produced from 103 in a few steps. The synthesis is effective and very useful and provides easy access to a range of compounds inspired by natural products.⁵⁹

Fig. 32. Synthesis of the spirotryprostatin A analogue.

2.22. Epibatidine

In 1992, Daly et al. isolated epibatidine (106), a novel alkaloid with the 7-aza-bicyclo[2.2.1]heptane ring system and a 6-chloro-3-pyridyl substituent in exo orientation,⁶⁰ from the skin extracts of Equadorian poison frogs, Epipedobates tricolour. It has been demonstrated to have 200–500 times the nonopiod analgesic activity of morphine. According to recent research, epibatidine is a very strong agonist of the acetylcholine receptors and has been implicated in the mediation of a number of human illnesses, including Parkinson's and Alzheimer's.^61–63 Therefore, to address this growing attention, its synthesis was attempted by Pandey and coworkers in a stereoselective fashion using the crucial step of [3 + 2] cycloaddition of azomethine ylide (Fig. 33). The methodology involves the use of a silver-catalysed cycloaddition step with a pyridine derivative (108) and an azomethine ylide generated in situ from 107. The cycloadduct (109) was then extended to 106 in a few steps. This method has opened up the possibility of using it to synthesise the different analogues of 106 and homoepibatidine.⁶⁴

Fig. 33. Synthesis of epibatidine.

2.23. Homoerythrina alkaloids

Pearson et al. described the synthesis (Fig. 34) of the demethoxy derivative of the homoerythrina skeleton (110) in 2007, as well as other synthetic attempts aimed at the complete synthesis of homoerythrina alkaloids. The study began with a monoprotected diol (111), which was converted to 112, in a few steps for a tandem N-alkylation/azomethine ylide [3 + 2] cycloaddition reaction. The effective synthesis of the alkaloids' A–C rings is the crucial aspect of the syntheses. This crucial step involves N-alkylation with 113, followed by the intramolecular cycloaddition of a tethered dipolarophile (112), yielding the demethoxy derivative of the homoerythrina skeleton (110).⁶⁵

Fig. 34. Synthesis of the homoerythrina skeleton.

2.24. Rolipram

A novel method for the enantioselective synthesis of 1,3,4-trisubstituted and 1,4-disubstituted pyrrolidin-2-one derivatives was described by Olano and coworkers in 2001. The authors synthesized Fischer alkoxy alkenyl carbene complexes (115) from the different derivatives of menthol, which underwent 1,3-dipolar cycloaddition with functionalised azomethine ylides (116) to produce cycloadducts (117) as chelated tetracarbonyl Fischer carbene complexes. The cycloadducts then underwent oxidation and further transformations, which makes pyrrolidin-2-ones easily accessible. Using this methodology, the authors easily showcased (Fig. 35) its implication by producing the anti-inflammatory and depressive medication rolipram (114) in four steps with an overall yield of 20%.⁶⁶

Fig. 35. Synthesis of rolipram.

3. Application towards the synthesis of some synthetic bioactive pyrrolidines

Apart from the naturally occurring pyrrolidine and spiropyrrolidine-based products, azomethine ylide-mediated cycloadditions also provide a simple and modular pathway to highly substituted and enantiomerically enriched lab-grown pyrrolidines and spiropyrrolidines of immense biological significance. These synthetic molecules are generally produced from azomethine ylides of imines or isatins with dipolarophiles, such as acrylates or maleimides.^2–9 Aryl-based dipolarophiles have been of keen interest to a number of potentially active research groups in recent years.

For example, Ayan et al. (2013) described (Fig. 36) the development of a successful synthetic route for phenylpyrrolidines (122, 123) using enantiopure organocatalysts (120, 121). These pyrrolidines were then converted into putative chiral ligands (124, 125) using a structure-based methodology. The usefulness of this chemical class for the development of selective thrombin inhibitors is indicated by the bioinformatic approaches used to clarify the ligand–enzyme interactions between well-defined pyrrolidines 124 and 125 and the thrombin/trypsin pair.⁶⁷

Fig. 36. Synthesis of the enantiomerically pure pyrrolidine derivatives with potential antithrombin activity.

Also, very recently in 2022, Teng and coworkers created a variety of new chiral 3,3-diuoro- and 3,3,4-triuoropyrrolidinyl derivatives (128) by asymmetrically adding arylated azomethine (126) ylides to less-active 1,1-gem-difluoro/1,1,2-trifluorostyrenes (126) via a Cu(i)-catalyzed 1,3-dipolar cycloaddition (Fig. 37). High yields (up to 96% yield), stereoselectivity (up to >20 : 1 dr and 97% ee), and a wide substrate scope (55 instances) are characteristics of this technology. Antifungal experiments were used to assess the biological activity of these recently synthesised compounds, and some of these fluorinated pyrrolidines showed excellent antifungal activity against Rhizoctonia solani, Sclerotinia sclerotiorum, Pestalotiopsis, and brown-rot fungi.⁶⁸

Fig. 37. Synthesis of the antifungal fluorinated pyrrolidines.

Apart from these, uses of heteroaryl-substituted azomethine ylides have also been reported. Using AgOAc and hydrocinchonine (129), Xie and colleagues, in 2008, created an operationally straightforward catalytic system. In this methodology, thiazole-based azomethine ylide was prepared from 130 and reacted with methylacrylate to yield pyrrolidine core 131, which was extended to 132, a HCV polymerase inhibitor. This asymmetric synthesis was also performed in multikilogram scale (Fig. 38).⁶⁹

Fig. 38. Synthesis of the HCV polymerase inhibitors.

Later in 2011, a similar report was published by Cossío and coworkers, where enantioselective 1,3-dipolar cycloaddition involving a leucine-derived iminoester (133) and tert-butyl acrylate was used to synthesise a second-generation inhibitor of hepatitis C virus (Fig. 39). Using chiral phosphoramidites or chiral BINAP in the presence of silver(i) and gold(i) catalysts in this reaction, pyrrolidine (134) was produced in high yields. When extended, the hepatitis C virus inhibitor (135) was synthesised in four steps under optimal reaction conditions, resulting in a 63% overall yield and 99% ee. DFT simulations also supported the origin of the chiral gold(i) catalyst's enantioselectivity, with one of the gold atoms and the nitrogen atom of the thiazole moiety forming a stabilising coulombic connection.⁷⁰

Fig. 39. Synthesis of the second-generation inhibitor of hepatitis C virus.

Similar aryl-based azomethine ylides can also be generated in situ and be further reacted in the medium to produce the desired pyrrolidine core. For example, the 1,3-dipolar cycloaddition between an N-substituted maleimide (138) and an azomethine ylide, which is made in situ using the decarboxylative technique from a carbonyl compound (136) and an a-amino acid (137), is a crucial step in the synthesis of pyrrolidine, 139, which in three steps yields 140, a new class of nonpeptidic, active, and selective thrombin inhibitor (Fig. 40). The compounds were also tested for activity against the related serine protease trypsin in biological assays.⁷¹

Fig. 40. Synthesis of the selective thrombin inhibitors.

Another three-component reaction involving aldehydes (141), amino-esters (142), and substituted alkyl (143) catalysed by phosphoric acid was reported in 2011 by Tu and coworkers, which was used to create a number of novel chiral 2,5-dihydropyrrole derivatives (144) with strong enantioselectivity. High enantioselectivity (up to 98% ee), atom economy, a wide range of substrate tolerance, and operational simplicity are the main characteristics of this method, which make it easy and simple to obtain biologically significant chiral 2,5-dihydropyrroles. Furthermore, a number of compounds with effective cytotoxicity to the cancer cell line MCF7 have been identified as a result of the initial assessment of the cytotoxic activity of this class of chiral 2,5-dihydropyrrole derivatives (Fig. 41).⁷²

Fig. 41. Synthesis of the anticancer pyrrolidines.

Another interesting case of aryl-substituted azomethine ylide was reported (Fig. 42) by Waldmann and coworkers in 2013, where an effective ferrocene/Cu dual catalysis led to highly diastereoselective and enantioselective [3 + 2] cycloaddition process of 1,3-fused cyclic azomethine ylides (145) with nitroalkenes (146) to yield azabicyclo-octanes (147). In their report, an S-shaped azomethine ylide was successfully employed in an enantioselective catalytic method for the first time. The authors also investigated their hedgehog-signaling inhibitory activities and found them to be highly potent.⁷³

Fig. 42. Synthesis of a novel hedgehog-signaling inhibitor.

Apart from aryl-substituted azomethine ylides, the utilization of other categories of such 1,3-dipoles has also been reported. For example, Hanessian et al. in 2005 described a synthetic route for a peptide-linked pyrrolidine in an enantiopure form (Fig. 43). The process began with R-amino aldehyde (148) originating from l-leucine, which led to the azomethine ylide precursor 149. A subsequent 1,3-dipolar addition then produced pyrrolidine 150, which was further elaborated to link a dipeptide to achieve the intended prototype pseudopeptide inhibitor 151. Its inhibitory activity against BACE1, an enzyme implicated in the cascade of events leading to plaque formation in Alzheimer's disease, was found and later further tuned with pyrrolidinone analogues, which showed low nanomolar inhibition.⁷⁴

Fig. 43. Synthetic route for a BACE1 inhibitor.

Another interesting case was reported by Menon and coworkers in 2013, where cationic fullerene–s-triazine conjugates were synthesized (Fig. 44), which may cleave supercoiled DNA when exposed to photoirradiation and NADH. Starting from a triazine (152), the azomethine ylide precursor 153 was achieved in a few steps, which upon cycloaddition with C60 produced the fullerene conjugate (154). Further reaction with MeI led to the production of cationic fullerene–s-triazine, 155. The mechanism of fullerene-conjugate binding over the minor groove of DNA was further comprehended through computational modeling. The docked conformation was found to be in good agreement with the obtained experimental data. In the area of DNA cleavage by this new class of fullerene–s-triazine derivatives, this discovery presents intriguing opportunities.⁷⁵

Fig. 44. Synthesis of the cationic fullerene–s-triazine conjugate DNA binder.

Also, macrolides can be functionalized with the help of azomethine ylides to produce bioactive pyrrolidines. For example, Gu et al. reported (Fig. 45) a stereoselective route towards novel antibacterial agents via an azomethine ylide in 2004.⁷⁶ In this process, the macrolide 156 was first converted to 157, which then underwent (3 + 2) cycloaddition intramolecularly to yield 158.

Fig. 45. Synthesis of the novel macrolide antibacterial agents.

4. Application towards the synthesis of some synthetic bioactive spiropyrrolidines

Bioactive spiropyrrolidines are often produced by using 1,3-dipolar cycloaddition of azomethine ylides with an exocyclic double/triple bond. Pairing an isatin derivative and an α-amino acids or an amine is quite common and effective. In a single step, azomethine ylide forms highly functionalized spiropyrrolidine frameworks via cycloaddition to electron-deficient alkenes as a reactive dipole.

4.1. Isatin-based azomethine ylides

In 2013, an effective three-component reaction (Fig. 46) of isatin (159), amino-ester (160) and alkyne (161) was reported by Shi and coworkers to produce a 2,5-dihydropyrrole scaffold based on spiro-oxindole (162) with potential bioactivity. This procedure is the first 1,3-dipolar cycloaddition of electron-deficient alkynes with azomethine ylides generated from isatin. It offers a simple way to obtain structurally diverse 2,5-dihydropyrroles based on spiro-oxindole in high yields (up to 99%). Additionally, sixteen compounds with promising cytotoxicity to MCF-7 cells were discovered via bio-screening of novel spiro-dihydropyrroles.⁷⁷

Fig. 46. Synthesis of the 2,5-dihydropyrrole scaffolds.

However, the employment of alkenyl dipolarophiles to synthesize bioactive scaffolds is much more abundant than that of alkenyl dipolarophiles. In this context, another interesting report was published in the previous year by the same group, where they had utilized alkene (164), instead of alkyne. They reported an organocatalytic 1,3-dipolar cycloaddition of isatin (163)/amino-ester (160)-based azomethine ylides (Fig. 47), which has been used to establish the catalytic asymmetric construction of a biologically significant spiro[pyrrolidin-3,2′-oxindole] scaffold (165) with contiguous quaternary stereogenic centres in excellent stereoselectivities (up to >99 : 1 d.r., 98% ee). In order to comprehend the stereochemistry, theoretical calculations were also carried out on the reaction's transition state. Using these spiro[pyrrolidin-3,2′-oxindole], preliminary bioassays demonstrated that a number of compounds exhibited considerable cytotoxicity to SW116 cells.⁷⁸

Fig. 47. Organocatalytic synthesis of the spiro[pyrrolidin-3,2′-oxindole] scaffolds.

Galvis et al. (2013) replaced aliphatic alkene dipolarophiles with its aryl variant and developed a highly regio- and stereoselective method for the synthesis of a 20-membered library of spirooxindole 1′-nitro pyrrolizidines (169) in aqueous media through the 1,3-dipolar cycloaddition of azomethine ylides (Fig. 48). The ylides are produced in situ via a decarboxylative route from a common set of diverse isatins (167) and l-proline derivatives (168) and reacted with substituted β-nitrostyrenes (166). The products were tested with Zebrafish embryos to determine their ADME properties and LC₅₀. They were also subjected to computational studies to (i) determine the ADME properties using Lipinski's rule, (ii) screen the toxicological profile, and (iii) forecast the synthetic compounds' ability to pass through the blood–brain barrier (BBB).⁷⁹

Fig. 48. Synthesis of spirooxindole 1′-nitro pyrrolizidines.

In the same year, Kumar et al. reported an easy-to-follow and regioselective protocol for the efficient synthesis of a novel series of spiro[indoline-3,5′-pyrrolo[1,2-c]thiazoles] (173) in good yield (65–85%) from the reactions of isatins (170), thioprolene (171) (Fig. 49) and substituted chalcones (172) using a catalytic amount of acetic acid as a catalyst in ethanol at room temperature. The anti-proliferative properties of all the synthesized compounds were evaluated via MTT assay against three breast cancer cell lines: MDA-MB-231, T47D, and MCF-7. The majority of the substances that were synthesised exhibited notable anti-cancer properties.⁸⁰

Fig. 49. Synthesis of spiro[indoline-3,5′-pyrrolo[1,2-c]thiazoles].

Similar reaction with thioprolene was also reported in 2016 by Cao and coworkers to synthesize a new class of isoxazole-functionalised spiropyrrolidine-fused oxindoles (179–181) without the use of a catalyst (Fig. 50). The reaction was done between substituted isatins (174), isoxazole derivative (175) and l-proline (176)/thioprolene (177)/N-methylglycine (178) in acetonitrile under refluxed conditions at 80 °C (Fig. 50). The synthesised products were also shown to be highly effective against all the tested cancer cell lines, such as lung (A549), prostate (PC-3), and leukaemia (K562) cancer cells.⁸¹

Fig. 50. Synthesis of the isoxazole-functionalised spiropyrrolidine-fused oxindoles.

Later in 2019, Kumar et al. described the synthesis of several 3-(aryl)-5-(4-hydroxybenzyl)spiro[indoline-3′.2-pyrrolidin]-2′-ones (185) in excellent yields (88–94%) in just one hour using [3 + 2] cycloaddition reactions of substituted isatins (182), tyrosine (183), and nitrostyrenes (184) in the presence of ionic liquid ([bmim]Br) at 100 °C (Fig. 51). The anti-cancer activity of the cycloadducts (185) against the lung cancer cell line A549 was found to be nearly identical to that of the reference medication, camptothecin.⁸²

Fig. 51. Synthesis of 3-(aryl)-5-(4-hydroxybenzyl) spiro[indoline-3′,2-pyrrolidin]-2′-ones.

In the same year, Biju et al. constructed a unique spiro(oxindole-3,2′-pyrrolidine) scaffold (188) with a broad range of biological activity. The traditional 1,3-dipolar cycloaddition reaction of azomethine ylide, made in situ from isatin (186)/N-methylglycine (178), with heterocyclic ylidenes (187) has been used to create a number of these compounds with heterocyclic rings joined to the pyrrolidine unit (Fig. 52). Cell toxicity and anticancer efficacy of these entitied were assessed and it has been found that, by activating the pro-apoptotic genes p53 and caspase 7, they demonstrated strong anticancer action while remaining non-cytotoxic up to 100 mg ml⁻¹.⁸³

Fig. 52. Synthesis of spiro(oxindole-3,2′-pyrrolidine).

There have also been reports on the intramolecular cyclisation of azomethine ylides that allow the targeted synthesis of structurally complex and highly functionalised spiro-oxindoles (193), particularly access to octahydropyrrolo[2,3-c]pyrrol-4-ones and octahydro-pyrrolo[2,3-c]pyridine-4-ones, in extremely effective 5-step sequences (Fig. 53). The reaction began with arene 189 and was converted to the precursor 190, which upon treatment with isatin 191 yielded cycloadduct 192 and was further extended to highly functionalised spiro-oxindoles (193). The synthesized spiro-oxindoles demonstrated high potency in the wild-type osteosarcoma SJSA-1 cell line proliferation experiment.⁸⁴

Fig. 53. Intramolecular cyclisation of azomethine ylides.

Later in 2021, Kutyashev et al. demonstrated the three-component reaction of 3-nitro-2-(trifluoro(trichloro)methyl)-2H-chromenes (194) with azomethine ylides produced in situ from isatins (195) and l-(thia)proline (196) in EtOH at 30 °C to regio- and stereoselectively produce (trifluoro(trichloro)methyl)-substituted spiro[chromeno(thia)pyrrolizidine-oxindoles] (197) in high yields (Fig. 54). The resulting compounds showed strong cytotoxic effect against RD human embryonic rhabdomyosarcoma cells and HeLa human cervical carcinoma cells.⁸⁵ All the candidates showed high selectivity for cancer cells, and cytotoxicity testing showed that each drug's IC₅₀ exhibited low toxicity to normal human dermal fibroblasts (HDF). The low micromolar cytotoxic activity and selectivity of these spirooxindole analogues make them promising anticancer medicines, particularly for the targeted destruction of cervical cancer cells with little adverse effects on healthy tissues.

Fig. 54. Synthesis of spiro[chromeno(thia)pyrrolizidine-oxindoles].

Around the same time in 2021, Aziz et al. reported two new spirooxindole compounds (202, 203) that target the p53–MDM2 relationship, which inhibits Bcl-2 signalling. Aziz et al. used an effective synthetic approach (Fig. 55), i.e., by enabling an aldol condensation reaction between N-methyl-2-acetylpyrrole (198) and substituted benzaldehydes (199) to produce corresponding N-methyl pyrrole chalcones (200) regio- and stereoselectively. The required spirooxindole derivatives (202, 203) were then formed by subjecting these chalcones to a one-pot cycloaddition reaction with 5-chloroisatine (201) and sarcosine (178)/thioproline (177). These compounds demonstrated significant anticancer effects through Bcl2 signalling and the p53–MDM2 interaction, respectively.⁸⁶

Fig. 55. Synthesis of the anticancer spirooxindoles.

In another report, almost simultaneously, researchers demonstrated a new class of pyrooxindole derivatives (207) with thiochromene (205) dipolarophiles. According to Barakat et al., 206 was condensed with different isatin derivatives (204) to produce azomethine ylides. The required spirooxindole derivatives were then formed through a 1,3-dipolar cycloaddition process with different chalcones that included thiochromene scaffolds (Fig. 56). Promising results from the anti-cancer assay made them excellent candidates for further research. These entities are most effective against breast cancer (MCF-7) and cervical cancer (HeLa) cell lines.⁸⁷

Fig. 56. Synthesis of the thiochromone-based pyrooxindoles.

In the following year, Mayakrishnan et al. described a very effective and eco-friendly in situ 1,3-dipolar cycloaddition procedure (Fig. 57) to create new spirooxindole hybrids (210, 211). This synthetic approach involved the one-pot reaction of 5-bromoisatine derivatives (208) with quinoline-indole-based chalcones (209) in the presence of thioproline (177) or sarcosine (178). These hybrids are very effective in blocking the Bcl-2 receptor, which was further supported by molecular docking experiments. They also demonstrated significant cytotoxic action against HepG-2 cell lines.⁸⁸

Fig. 57. Synthesis of the quinoline-indole-based spirooxindole hybrids.

Independently, around the same time in 2022, a novel set of spirooxindole derivatives was created by Rajaraman et al., who also assessed the compounds' antioxidant capacity against superoxide and hydroxyl radicals. As part of the synthetic approach, isatin (159) and 2-(piperazin-1-yl)ethanamine (212) underwent decarboxylative condensation to produce an azomethine ylide precursor (213), which in situ produced reactive azomethine ylide intermediates, that also underwent a 1,3-dipolar cycloaddition reaction (Fig. 58) with different chalcones (214) to create the target spirooxindole derivatives (215). The antitumor effectiveness of these entities against KB oral cancer cells was determined to be promising. This spirooxindole scaffold's overall ability to target malignancies that begin in the epithelium is also promising, as they also maintained negligibly harmful effects on non-cancer cells even at high doses. This information suggests a lot of room for additional structural and preclinical development.⁸⁹

Fig. 58. Synthesis of the antitumor spirooxindoles.

Another similar chalcone-based dipolarophile was used by Zhao et al. to create a new class of spirooxindole derivatives (218) that include pyrrolothiazolidine (Fig. 59). Using chloroisatin (191) and thioproline amino acid (177), azomethine ylides were produced at the start of the synthesis. These intermediates were in situ exposed to a [3 + 2] cycloaddition process with different chalcones (216), producing the desired spirooxindole derivatives (217). Using peptide coupling reagents, such as HATU/DIEPA, the nitro group on the benzene ring was later reduced to an amine, which was further linked with acid-linker derivatives (218). These entities demonstrated significant inhibitory action against MDM2 and HDAC. Additionally, they outperformed the reference medications SAHA and Nutlin-3 in terms of efficacy, exhibiting strong antiproliferative activities against MCF-7 cell lines. These studies were further confirmed by molecular docking investigations that showed specific inhibition against HDAC1 and HDAC2 and therefore may be considered for the development of viable anti-cancer drugs in the near future.⁹⁰

Fig. 59. Synthesis of the anticancer spirooxindoles.

Nivetha et al. found that the 1,3-dipolar cycloaddition reaction between the chalcone dipolarophiles (219) and the azomethine ylide dipole, which is created in situ by the reaction of amino acids (220) and isatin (159), can be performed in a highly stereoselective three-component manner (Fig. 60) to produce pyrrolidine-based spirooxindoles (221). Following the creation of spirooxindole derivatives, the authors used the enzymes α-glucosidase and α-amylase to investigate the derivatives' anti-diabetic qualities. Their substantial inhibitory effect against enzymes α-glucosidase and α-amylase was further confirmed by molecular docking tests.⁹¹

Fig. 60. Synthesis of the anti-diabetic spirooxindoles.

Interestingly, in the following year, Alshahrani et al. used another type of chalcone with benzimidazole link (222) as the dipolarophile and described the synthesis of a new class of spirooxindole compounds (223). These cycloadducts were created regio- and stereoselectively in a one-pot [3 + 2] cycloaddition process (Fig. 61), where azomethine ylides were generated in situ from 5-chloroisatin (201) and (2S)-octahydro-1H-indole-2-carboxylic acid (206). On biological assessment, these compounds demonstrated excellent anticancer activity with considerably lower IC₅₀ (mM) values.⁹²

Fig. 61. Synthesis of the benzimidazole-based spirooxindoles.

In the same year, Islam et al. employed phenylimidazole-based chalcones (224) as dipolarophiles and reacted with in situ generated azomethine ylide from isatin (159) and l-thioproline (177) to construct (Fig. 62) a novel series of spirooxindole derivatives with a spiro[3H-indole-3,20-pyrrolidin]-2(1H)-one framework (225). Upon investigation of their potential as MDM2 inhibitors and in vitro anti-tumorigenic activity on different cell lines, many of these entities were found to be highly active against A549 lung cancer cells and to have strong cytotoxic activity against A2780 ovarian cancer cells. Some of them have also shown efficacy against MDA-MB-453 breast cancer cells, demonstrating their potential, particularly in multi-target cancer treatment approaches, as evidenced by the modest IC₅₀ values in the micromolar range, selectivity against cancer cells and ability to restore p53 tumour suppression.⁹³

Fig. 62. Synthesis of phenylimidazole-based spirooxindoles.

In order to explore more complex dipolarophiles, very recently, triazine-based chalcones (227) were utilized by Shawish et al. to construct triazole–spirooxindole derivatives (228). The azomethine ylides, which were produced in situ by the decarboxylative condensation of isatin derivatives (226) with thioproline (177), were reacted with the aforementioned chalcones to produce cycloadducts (Fig. 63). The authors then investigated their anticancer properties using HepG2 and MDA-MB-231 cell lines and found them to be effective cytotoxic agents against these cell lines. The good binding interactions of these adducts in the EGFR active site were further confirmed by 60-hour molecular docking experiments, suggesting the possibility of dual inhibition. Due to their significant selectivity and structural flexibility, these spirooxindoles may lead to the potential development of EGFR inhibitors in anticancer therapy.⁹⁴

Fig. 63. Synthesis of the triazole–spirooxindole derivatives.

Abdessadak et al. have attempted to carry out a d-proline catalysed reaction between acrolein (230) and a thiophene-linked imine diester (229). The reaction led to the formation of thiophene-linked pyrrolidines (231), which was further extended to 232, in a few steps (Fig. 64). The final product was examined in the search for substances that are effective against hepatitis C virus. Additionally, molecular docking simulations were used to assess the antiviral activity of the pyrrolidine against cyclophilin A, the co-factor that causes hepatitis C virus. These simulations revealed interesting interactions and a high C-score, which were further validated by molecular dynamics simulations that showed stability over a 100-ns simulation period. Additionally, compared with the hepatitis C virus inhibitor, the cis-cycloadduct pyrrolidine shows advantageous drug-like characteristics and a superior ADMET profile.⁹⁵

Fig. 64. Synthesis of the triazole-spirooxindole derivatives.

In this context, most recently, Islam et al. reported a new series of spirooxindoles to target bacteria and non-small cell lung cancer (Fig. 65). This novel family of pyrazole-linked spirooxindoles (235) was inspired by the fusion of some structural elements from marine anticancer and antibacterial drugs.⁹⁶ Cycloadducts were produced from a chalcone-based dipolarophile (233) via reaction with azomethine ylides, generated in situ from isatins (234) and sarcosine (178). Upon biological investigation, these entities have shown strong antibacterial and anticancer properties against Staphylococcus aureus and non-small cell lung carcinoma (NSCLC), respectively, having low IC₅₀ values with tumor-specific cytotoxicity. These results received further support from docking experiments that revealed strong intercalative binding within DNA strands.

Fig. 65. Synthesis of the pyrazole-linked spirooxindoles.

Some recent reports focused on the application of the exocyclic variants of spiropyrrolidines. For example, Periyasami et al. described a reaction between dipolarophile (E)-2-arylidine-1-ketocarbazole (236) and azomethine ylide derived from proline (176) and isatin (159), which produced dispirooxindolopyrrolizidine derivatives (237) demonstrating antifungal and antibacterial properties (Fig. 66).⁹⁷

Fig. 66. Synthesis of the antifungal and antibacterial dispirooxindolopyrrolizidines.

Later Huang et al. deduced a productive method for the 1,3-dipolar cycloaddition of isatin-derived exocyclic dipolarophiles (240) with 1,3-dipoles derived from 1,2,3,4-tetrahydroisoquinoline (239) and isatin derivatives (238). Excellent yields of the expected di-spirooxindoles (241) with strong regio- and stereoselectivities were obtained from this three-component process (Fig. 67). Additionally, many of these cycloadducts were found to demonstrate promising cytotoxicity to HepG2 cells.⁹⁸

Fig. 67. Synthesis of di-spirooxindoles.

Within a couple of years, through the use of an environmentally friendly deep eutectic solvent, Sathi et al. described the regio- and diastereospecific syntheses (Fig. 68) of new biologically relevant thiazolo[3,2-b]indole derivatives (245). The [3 + 2] cycloaddition reaction was done between a thiazolidine dipolarophile (242) and azomethine ylides, obtained from isatins (243) and different amino acids (244). These entities were then investigated for their potential to inhibit MDM2-p53 via molecular docking and in vitro investigations and found to have promising inhibitory properties.⁹⁹

Fig. 68. Synthesis of the MDM2-p53 inhibiting di-spirooxindoles.

In this context, an interesting seven-membered azepane-based exocyclic dipolarophile (246) was described by Kumar et al., which was used in a one-pot [3 + 2] cycloaddition procedure to design and synthesise a novel spirooxindole derivative with a pyrrolidine motif. These chalcones were reacted with an azomethine ylide [produced in situ by the decarboxylative condensation of isatin (159) with phenylglycine (247)] afforded 248. As shown in Fig. 69, this method effectively produced the intended new bis-spirooxindole derivatives (248). These products were then tested for their antiproliferative potential and were found to have strong activity with low IC₅₀ values. Their potency was also found to be increased over time, suggesting a potential cumulative or delayed mode of action. According to these findings, more pharmacokinetic and structural optimisation would be required to attain adequate potency and selectivity for anticancer treatment.¹⁰⁰

Fig. 69. Synthesis of the anticancer bis-spirooxindoles.

In the following year, Al-Majid et al. adopted the (3 + 2) cycloaddition reaction to synthesise a new series of di-spirooxindole derivatives with a cyclohexanone moiety in a single pot. As shown in Fig. 70, this procedure involved in situ reaction of isatins (249) with (2S)-octahydro-1H-indole-2-carboxylic acid (206) to produce azomethine ylides, which were subsequently reacted with cyclohexanone-based chalcones (250) to produce di-spirooxindole derivatives (251). The synthesised di-spirooxindole derivatives showed high activity with an IC₅₀ value in the low micromolar range, and all of the investigated compounds have shown significant antiproliferative capabilities against PC3 human prostate cancer cell lines. Some of them also demonstrated significant cytotoxicity in cervical (HeLa) and triple-negative breast carcinoma (MDA-MB-231) cancer cell lines. Additionally, these substances demonstrated a high degree of cytotoxic selectivity between normal and malignant cells at comparable dosages. The strong affinity and selectivity for tumor-related targets may be due to the structural stiffness of the di-spirooxindole structure. The practical significance of this scaffold for investigating additional anticancer medications is demonstrated by the possible dual antitumor effects in a number of malignancies.¹⁰¹

Fig. 70. Synthesis of the anticancer bis-spirooxindoles.

A similar investigation was done by Fawazy et al. in 2021, where a novel series of alkylsulfonyl-substituted di-spirooxindole derivatives (254) showing anti-SARS-CoV-2 characteristics was successfully developed and synthesised (Fig. 71). Azomethine ylides, which were produced in situ from isatins (253) and sarcosine (178), were used in the 1,3-dipolar cycloaddition with 1-(alkylsulfonyl)-3,5-bis(ylidene)-piperidin-4-ones (252) to accomplish the synthesis. These substances showed a favourable selectivity index towards normal RPE1 cells while being tested for activity against a number of human cancer cell lines, including MCF7, HCT116, A431, and PaCa2. The di-spirooxindole derivatives were assessed for their antiviral, anticancer, and cholinesterase-inhibiting properties and were found to demonstrate strong potency. Furthermore, a small number of the analogues demonstrated significant dual activity against both BChE and AChE and therefore hold great promise as multi-target treatments for viruses, cancer, and neurological illnesses.¹⁰²

Fig. 71. Synthesis of di-spirooxindoles for multi-target treatment.

Also, very recently in 2024, Nafie et al., in search of a possible anti-breast cancer drugs, created a novel family of di-spirooxindole hybrids (257) based on pyrrolidinyl-bis-spirooxindole (Fig. 72). Here, the azomethine ylides were produced from isatins (256) and thioprolene (177) and then reacted with thiazolidine-based dipolarophiles (256) to produce cycloadducts. The entities showed good anticancer activity, with low IC₅₀ values. They were shown to have strong cytotoxicity against MCF-7 and MDA-MB-231 breast cancer cells. Additionally, compounds containing alaninate moieties showed good efficacy against HeLa and MDA-MB-231 cancer cell lines.¹⁰³

Fig. 72. Synthesis of the anticancer bis-spirooxindole derivatives.

This 1,3-dipolar cycloaddition was also explored in sugar-based systems. For example, the reaction of uracil polyoxin C (UPoC)-derived azomethine ylides for obtaining analogues of nikkomycin is notable in this context, where the thermal reactions of uracil polyoxin C (UPoC) (258) with isatin 159 in the presence of N-methylmaleimide (NMM) 259 resulted in good yields of polyoxin cycloadducts 260 and 261 (Fig. 73).¹⁰⁴

Fig. 73. Reaction of the polyoxin C (UPoC)-derived azomethine ylides.

The have also been reports on steroid-based systems. For example, modified bis-spiropyrrolidine derivatives of oestrone steroid 263 were synthesised by reacting (Z)-16-arylidene-estrone derivatives 262 as 2π components with azomethine ylides (Fig. 74).¹⁰⁵ Three distinct sets of circumstances were used to conduct the reactions. Additionally, innovative bis-spiro oxindole/pyrrolidine 267 has been synthesised in moderate yields using 2,5-bis(arylmethylidene)-cyclopentanone 265 as a dipolarophile.

Fig. 74. Synthesis of a steroid-based bis-spiropyrrolidine.

4.2. Non-isatin-based azomethine ylides

Among non-isatin-based azomethine ylides, Waldman's report may be cited here. Using 1–3 mol% of a chiral catalyst produced from an N,P-ferrocenyl ligand (266) and CuPF₆(CH₃CN)₄, Waldman and colleagues reported a highly enantioselective synthesis of 3,3′-pyrrolidinyl spirooxindoles (267) (Fig. 75) inspired by natural products. This was accomplished by an asymmetric Lewis acid-catalyzed 1,3-dipolar cycloaddition of an azomethine ylide, generated in situ from 265, to a substituted 3-methylene-2-oxindole (264). It was discovered that the cycloadduct interfered with the p53-MDM2 interaction, causing mitotic arrest.¹⁰⁶

Fig. 75. An enantioselective synthesis of 3,3′-pyrrolidinyl spirooxindoles.

Later, in 2021, the potential of a novel series of nitroisoxazole-based spiro[pyrrolidin-3,20-oxindoles] (270) with a –CF₃ moiety as dual inhibitors of GPX4 and MDM2 was assessed by Liu et al. In this reaction, 3-methyl-4-nitro-5-styrylisoxazole (269) and 6-chloro-isatin imine (268) (Fig. 76) reacted in a basic medium of DBU to produce cycloadducts. Adducts were targeted for their potential application in breast cancer owing to their potent dual inhibitory activities against GPX4 and MDM2. They induced ferroptosis and apoptosis in MCF-7 cells by suppressing the MDM2-mediated degradation of p53 and lowering GPX4 expression. Lipid peroxidation, caspase-9 processing, and ROS generation were all involved in the process.¹⁰⁷

Fig. 76. Synthesis of the nitroisoxazole-based spiro[pyrrolidin-3,20-oxindoles].

A similar DBU-enabled reaction was recently reported by Zhou et al. to successfully design and synthesise a novel series of CF3-substituted pyrrolidinyl spirooxindoles (273) with exceptional diastereoselective 1,3-dipolar [3 + 2] cyclo-addition of N-2,2,2-triuoroethylisatin-based ketimines (271) with chalcones (272) (Fig. 77). Using MTT assay, the synthesised compounds' anticancer activity was evaluated in vitro against human gastric cancer SCG7901 cells. Many of them showed cytotoxicity, with moderate IC₅₀ values. The biological evidence supports the phenotypic significance of the –CF₃ motif and spirooxindole scaffold, despite the fact that they are not sub-micromolar.¹⁰⁸

Fig. 77. Synthesis of the CF3-substituted pyrrolidinyl spirooxindoles.

In another interesting report by Leena et al., dispiropyrrolidineoxindoles (276) were synthesised from tryptanthrin (274) and isatilidenes (275) (Fig. 78) via [3 + 2] cycloaddition of azomethine ylide units. A thorough in vitro evaluation against ESKAPE pathogens and clinically relevant drug-resistant MRSA/VRSA strains showed the antibacterial properties of these compounds. The findings demonstrated that the bromo-substituted dispiropyrrolidineoxindole had good selectivity and a strong case against S. aureus ATCC 29213.¹⁰⁹

Fig. 78. Synthesis of antibacterial oxindoles.

Very recently, Pan et al. reported a three-component [3 + 2] azomethine ylide cycloaddition approach using benzofuran-based dipolarophiles (277), ninhydrin (278) and sarcosine (178) to create a spiro-pyrrolidine molecule based on benzofuran (279) (Fig. 79). With a wide variety of substrates, excellent yields, and simple operation under mild circumstances, this reaction can quickly produce potentially beneficial compounds. These spiro-heterocyclics also demonstrated outstanding antitumor capabilities.¹¹⁰

Fig. 79. Synthesis of the antitumor benzofuran scaffolds.

Ninhydrin was also used by Ren and coworkers to produce a novel series of spirooxindole–indenoquinoxaline compounds (284) that function as tryptophanyl-tRNA synthetase (TrpRS) inhibitors. However, in this case, an indeno-quinoxalinone (281) was first produced from ninhydrin and orthophenylene diamine (280). The ketone was then further used to produce azomethine ylides from amino acids (283). Finally, through a 1,3-dipolar cycloaddition reaction with 282 (Fig. 80), dispirocycloadducts were produced. Through in vitro tests against a variety of Gram-positive and Gram-negative bacterial strains as well as diffuse large B-cell lymphoma (DLBCL) cell lines, these drugs' biochemical TrpRS inhibitory activity was assessed, and they showed excellent antibacterial activity against Staphylococcus aureus. Some of them also demonstrated significant anti-proliferative activities against DLBCL cell lines in cancer models, where they can serve as a tool compound for the discovery of dual antibacterial and anticancer agents.¹¹¹

Fig. 80. Synthesis of spirooxindole–indenoquinoxaline.

The same indeno-quinoxalinone (281) was used by Barakat et al., where a novel series of spiro-indeno[1,2-b]quinoxaline derivatives (286) (Fig. 81) with a benzimidazole moiety was logically developed and synthesised. The imidazolyl chalcones (285) were reacted in a one-pot, multicomponent [3 + 2] cycloaddition process with the indeno-quinoxalinone (281) and octahydroindole-2-carboxylic acid (206). Upon testing their anticancer potential in human lung cancer cells, they have shown significantly high activity, suggesting significant selectivity towards cancer cells. Such a selectivity is consistent with earlier observations of spirooxindole compounds that contain indenoquinoxaline and benzimidazole moieties. Significantly, at concentrations that were effective for A549 cells, normal cardiovascular fibroblasts did not show significantly reduced viability, demonstrating excellent selectivity against CDK2 and pointing to its potential as an anti-lung cancer medication.¹¹²

Fig. 81. Synthesis of spiro-indeno[1,2-b]quinoxaline.

5. Conclusion

In this review article, we have focused on exploring recent impacts of azomethine ylide chemistry for the strategic synthesis of pyrrolidine and spiropyrrolidine derivatives, which are important structural motifs in many natural and synthetic bioactive molecules. Azomethine ylides' inherent reactivity as adaptable 1,3-dipoles permits extremely effective and stereocontrolled [3 + 2] cycloaddition reactions with a wide range of dipolarophiles, facilitating the quick building of structurally varied and stereochemically complicated heterocycles. Also, significant advancements have been made in asymmetric, organocatalytic, and transition-metal-catalyzed processes to improve regio-, chemo-, and enantioselectivity control. These, coupled with computational research and mechanistic understanding, have facilitated researchers in the logical design of substrates and catalysts, increasing the accuracy and range of these transformations. Of note, atom economy and operational simplicity have also been enhanced by the development of multicomponent, tandem, and environmentally safe synthetic methods, which are consistent with the concepts of sustainable and green chemistry. All of these developments have made it easier to synthesise complex natural products and pharmacologically active compounds, demonstrating the critical role of azomethine ylides in contemporary synthetic and medicinal chemistry. The major challenges of the application of azomethine ylides include the control of stereoselectivity and regioselectivity issues. It also deals with the limited confined space of dipolarophiles, which are in general very reactive in nature. Therefore, there is an urgent need for a specific generation method to restrict the application of harsh chemicals and to get rid of unwanted side products. In a nutshell, regio- and stereoselectivity issues, limited dipolarophile scope, the need for harsh reaction conditions like high temperature, the incorporation of stabilizing group, catalyst specificity, and related competing reactions like proton transfer reaction, condensation reaction etc., are the major limitations of the application and challenges of azomethine ylides. These limitations may be overcome by further investigation into their reactivity in conjunction with cutting-edge catalytic systems, which will open up new possibilities for the effective synthesis of intricate nitrogen-containing frameworks with increased biological potential.

Conflicts of interest

There are no conflicts to declare.

Abbreviations

MCF-7

Breast cancer cell line

MDA-MB-231

Breast cancer cell line

T47D

Breast cancer cell line

A549

Lung cancer cell line

PC-3

Prostate cancer cell line

K562

Leukaemia cancer cell line

SJSA-1

Wild-type osteosarcoma cell line

HeLa

Cervical cancer cell line

HepG-2

Hepatocellular carcinoma

KB

Oral cancer cell line

MDA-MB-453

Breast cancer cell line

NSCLC

Non-small cell lung carcinoma

HCT116

Colon cancer cell line

A431

Human epidermoid carcinoma cell line

PaCa2

Epithelial cell line

Biographies

Biography

Priyankar Paira.

Dr Priyankar Paira, MRSC, Associate Professor, VIT, Vellore, Tamilnadu, India, received his PhD degree from the Indian Institute of Chemical Biology (IICB), Jadavpur University, Kolkata. He has carried out his Post Doctoral work under the supervision of Prof. Giorgia Pastorin and Prof. Ang Wee Han from National University of Singapore. His current research focused on bioorganometallic and medicinal chemistry. He has over 100 publications in reputed international journals having citations (3063) and h-Index (29) along with 3 patents. He has successfully completed three SERB projects, two ICMR projects and currently working on one CSIR project. He is associated with professional bodies like ACS, RSC, SBIC, CRSI, CRS, ISCBC, CBS.

Biography

Rinku Chakrabarty.

Dr Rinku Chakrabarty embarked on her academic journey at IIEST, Shibpur, India for her PhD under the guidance of Prof. Shyamaprosad Goswami in 2011, focusing on Supramolecular Chemistry, followed by postdoctoral studies at the University of Witwatersrand, Johannesburg, South Africa, where she was involved in the synthesis of rare-earth-based molecular systems as MRI agents. Her passion for academia led her to assume the role of an Assistant Professor in the Department of Chemistry at Alipurduar University. Her research pursuits are centered on nanomaterials, molecular sensing and bioinorganic chemistry, showcasing her unwavering commitment to the advancement of the field of Chemistry.

Biography

Piyali Deb Barman.

Dr Piyali Deb Barman earned her MSc degree in Organic Chemistry from IIT Kharagpur. She carried out her doctoral studies in the application of 1,3-dipolar azomethine ylide cycloaddition reactions as well as in enantioselective total synthesis of bioactive natural products and received her PhD in Organic Chemistry from Jadavpur University in 2014. Presently Dr Deb Barman is employed as a Senior Chemist in the Chemical Division of the Geological Survey of India (GSI), Eastern Region, Kolkata. Her current research is focused on elemental speciation and quantitative analysis of geological materials. She has published in several reputed international journals including Elsevier, Wiley and ACS.

Biography

Bhagat Singh.

Dr Bhagat Singh completed his Master of Science in Chemistry at the University of Delhi in 2011. He earned his PhD in 2021 from the Indian Institute of Science Education and Research Kolkata, where his doctoral research focused on aryl carbon–halogen bond functionalization using phenalenyl-based radicals. Following his PhD, Dr Singh held a postdoctoral position at the University of North Carolina at Greensboro, USA (2022–2024). He subsequently pursued a second postdoctoral appointment with the Medicinal Chemistry Team at Wake Forest University, (2024–2025). He is currently undertaking his third postdoctoral position at Eastern New Mexico University, where he has been based since 2025.

Biography

Rupankar Paira.

Dr Rupankar Paira earned his Master of Science degree in Organic Chemistry from the Indian Institute of Technology (IIT), Kharagpur. After that, he carried out his doctoral studies on bioactive N-heterocycles and graduated with a PhD degree from Jadavpur University, Kolkata. For his post-doctoral studies, he then went to National University of Singapore (NUS) and Nanyang Technological University (NTU), Singapore and worked on C–H activation chemistry. He is currently employed as an Assistant Professor at Maharaja Manindra Chandra College (affiliated to University of Calcutta). He has received the DST-SERB Young Scientist start-up award (YSS/2014/000877) in 2014, and also a UGC-MRP grant (ROMRP-ERO-CHEM-2015-16-68950) in 2015. He has published in several reputed journals including Elsevier, Wiley, RSC, and ACS.

Data availability

No new data was generated.