Total Synthesis and Biological Activities of Polyhydroxy Flavonols: A Review

Abstract

Flavonols are an important class of flavonoids widely distributed across various plant species. They have garnered significant attention from synthetic chemists due to their extensive biological activities and medicinal value. This review provides a systematic overview of five classical synthetic methods, including the Auwers reaction, the Allan–Robinson reaction, the Baker–Venkataraman rearrangement, the Algar–Flynn–Oyamada (AFO) reaction, and DMDO-mediated oxidation. Each methodology is comprehensively discussed in terms of its advantages, limitations, and potential optimization strategies. Additionally, the biological activities, including antioxidant, anticancer, anti-inflammatory, and antiviral properties, are summarized and discussed in the context of structure–activity relationships (SARs).

1. Introduction

Flavonols represent a significant subclass of flavonoids, characterized by a core structure of 2-phenylchroman-4-one-3-ol with the typical C6-C3-C6 skeleton [1]. As significant secondary metabolites in plants, they are widely distributed in fruits, vegetables, and beverages [2]. The fundamental skeletal framework is shown in Figure 1, and representative compounds are depicted in Table 1.

Figure 1.

General structure of the flavonol scaffold (2-phenylchroman-4-one-3-ol) illustrating the C6-C3-C6 skeleton.

Table 1.

Common polyhydroxy flavonols.

Compound	R5	R6	R7	R8	R2′	R3′	R4′	R5′
kaempferol	OH	H	OH	H	H	H	OH	H
myricetin	OH	H	OH	H	H	OH	OH	OH
quercetin	OH	H	OH	H	H	H	OH	OH
galangin	OH	H	OH	H	H	H	H	H
morin	OH	H	OH	H	OH	H	OH	H
gossypetin	OH	H	OH	OH	H	H	OH	OH
quercetagetin	OH	OH	OH	H	H	H	OH	OH
fisetin	H	H	OH	H	H	H	OH	OH

Flavonols play diverse physiological roles in plants. Beyond their well-established functions in pigmentation, such as acting as co-pigments with anthocyanins [3,4], they serve as UV protectants, modulators of auxin transport for growth regulation, and defense compounds against herbivores and pathogens [5,6,7,8]. Due to their unique structural features, flavonols exhibit significant biological activities including antioxidant [9,10,11,12], anti-inflammatory [13,14,15,16], anticancer [17,18,19,20], antiviral [21,22,23,24], and anti-bacterial effects [25,26]. Consequently, these compounds hold extensive application prospects in the food [27,28], pharmaceutical [29,30], and cosmetic industries [31,32].

In recent years, flavonols and their derivatives have attracted increasing attention due to their novel structures and excellent pharmacological activities. However, systematic summaries of their total synthesis methods remain limited. The synthetic challenges primarily stem from regioselective hydroxylation [33], C3 oxidation state control [34], and susceptibility to oxidative degradation [35,36]. To address these challenges, several strategies have been developed, including the AFO reaction, the Baker–Venkataraman rearrangement, and the DMDO oxidation, while earlier approaches such as the Auwers and Allan–Robinson reactions have also contributed to the field. This review summarizes research progress in the total synthesis of flavonols, systematically examining these methodologies with emphasis on recent advances in efficiency, selectivity, and scalability. The SAR of polyhydroxy flavonols is also explored, aiming to provide references for structural optimization and drug development.

2. Total Synthetic Methods of Flavonols

2.1. Auwers Reaction

The Auwers reaction was first reported by Karl von Auwers in 1908, in this reaction benzofuran-3-one 1 undergoes an aldol condensation reaction with benzaldehyde, followed by bromination and rearrangement under alkaline conditions to yield flavonol 4 (Scheme 1) [37,38].

Scheme 1.

Synthetic route of flavonol 4 via the Auwers method.

This reaction has certain application value in the synthesis of flavonols. However, it imposes strict requirements on the structure of the reactants. Furthermore, the preparation of the dibromide intermediate is challenging, which often results in low overall yields and difficulties in scaling up production. Consequently, its application in synthesis remains limited.

2.2. Allan–Robinson Reaction

Allan–Robinson reaction was reported by Allan and Robinson in 1924 [39]. A typical representative reaction involves the synthesis of flavonol 6 through intramolecular condensation of a O-hydroxyaryl ketone 5 and an aromatic anhydride under alkaline conditions (Scheme 2). When aliphatic anhydrides are used as the reactant, it is termed the Kostanecki reaction.

Scheme 2.

Synthetic route of flavonol 6 via the Allan–Robinson method.

In 2024, Gu [40] improved the reaction conditions of the Allan–Robinson reaction. In her reaction the starting material was phloroglucinol 7, it underwent the Houben-Hoesch reaction with methoxyacetonitrile to form hydroxyaryl ketone 8. Intermediate 8 was then reacted with 3,4,5-trimethoxybenzoic anhydride via the Allan–Robinson reaction to yield 9. After demethylation with BBr₃/CH₂Cl₂, myricetin 10 was successfully synthesized (Scheme 3). This improved method allows the product to be purified through simple recrystallization, and achieves a total yield of up to 53%, demonstrating good potential for large-scale production. These characteristics significantly enhance the practicality and feasibility of the Allan–Robinson reaction.

Scheme 3.

Synthetic route of myricetin 10.

Despite the synthetic advantages of the Allan–Robinson reaction as a one-pot process for esterification, rearrangement, and condensation, it suffers from prolonged reaction times, generally low yields, and the frequent formation of 3-aryloxyflavones [41]. Moreover, when symmetric aromatic anhydrides are used, atom economy is poor: only one aryl unit is incorporated into the flavonoid skeleton; the other is lost as carboxylic acid.

2.3. Baker–Venkataraman Rearrangement Reaction

The Baker–Venkataraman rearrangement, developed as a refined alternative to the Allan–Robinson condensation, is a widely employed method for the synthesis of flavonols [42]. In this process, compound 11 undergoes intramolecular Claisen condensation to form the corresponding intermediate β-diketone 12, which is then cyclized under acid catalysis to yield flavonol 4 (Scheme 4). Notably, the β-diketone serves as a crucial intermediate in the Baker–Venkataraman rearrangement.

Scheme 4.

Synthetic route of flavonol 4 via the Baker–Venkataraman rearrangement.

The Baker–Venkataraman rearrangement enables the construction of the flavonoid backbone through intramolecular acyl migration via a well-defined mechanistic pathway. The reaction is initiated by deprotonation of the o-hydroxyl group in o-acyloxyacetophenone 13 under strong base conditions, generating the enolate 14, which undergoes intramolecular nucleophilic attack on the ester carbonyl to afford tetrahedral intermediate 15. In this step, the acyl group migrates to the oxygen of the former o hydroxyl, furnishing 1,3-diketone 16. Under acid catalysis, 16 undergoes protonation to give 17, which tautomerizes to enol 18. The enol then engages in an intramolecular nucleophilic attack, wherein the enolic hydroxyl attacks the ketone carbonyl carbon, leading to cyclic hemiketal intermediate 19. Finally, dehydration of the hemiketal intermediate 19 affords the flavonoid 20 (Scheme 5). Subsequent oxidation or hydroxylation can further convert it into flavonol [43].

Scheme 5.

The mechanism of Baker–Venkataraman rearrangement reaction.

Commonly used bases for the Baker–Venkataraman rearrangement include KOH, NaOH, LiHMDS, K₂CO₃, pyridine, TEA, and DBU, while the cyclization step is typically promoted by H₂SO₄, AcOH/H₂SO₄, p-TsOH, TFA, PPA, or Lewis acids [44,45]. It is also essential to design selective protection and deprotection steps for the preparation of flavonols. Commonly protecting groups such as MeOCH₂–, CH₃CO–, or CH₃O– exhibits stability under reaction conditions and can be efficiently removed in subsequent steps, thereby enabling high-yield synthesis of flavonols.

2.3.1. Synthesis of Polyhydroxy Flavonols 24

Despite its reliability, the Baker–Venkataraman rearrangement involves three steps, esterification, rearrangement, and ring closure, rendering the process cumbersome. To streamline the synthesis, Kim et al. [46] developed a concise approach to prepare flavonol derivatives as selective JAK1 inhibitors in 2014. In this method, 22 was prepared from 21 by debenzylation followed by THP protection, and 21 was obtained by base-promoted fragmentation of perbenzylated quercetin. Subsequently, 22 was esterified with substituted benzoic acids using EDC and DMAP to afford 23. The cyclization reaction was then carried out under basic conditions using TBAB as a catalyst, followed by deprotection with THP to yield the flavonol skeleton. Finally, the 4-bromo functionality was reduced using LiAlH₄ to afford polyhydroxy flavonols 24 (Scheme 6). Notably, this approach streamlines the three-step sequence by integrating the rearrangement and cyclization into a single operation, thereby improving step economy and reducing reaction time. Direct cyclization proceeds under mild conditions using K₂CO₃/TBAB in toluene at 90 °C, avoiding strong acids or Lewis acids required in conventional protocols. Final deprotection proceeds without compromising the flavonol core, underscoring the practicality of this method.

Scheme 6.

Synthetic route of polyhydroxy flavonols 24.

2.3.2. Synthesis of Polyhydroxy Flavonols 28

Despite these advances, stepwise approaches involving protection, acylation, cyclization, and deprotection suffer from lengthy procedures and moderate yields. To address these limitations, streamlined synthetic strategies have been developed. In 2014, Forbes et al. [47] reported a modified one-pot Baker–Venkataraman rearrangement for the synthesis of flavonols, adapting the method originally developed by Ichikawa et al. [48]. Methoxy phloroacetophenone 8 was treated with substituted benzoic acid 25 and the corresponding benzoyl chloride 26 in the presence of triethylamine and DMF. This mixed anhydride strategy facilitated acylation and in situ cyclization under basic conditions, directly affording the flavonol methyl ether intermediate without isolating the triester intermediate. Demethylation to yield polyhydroxy flavonol 28 was initially attempted using BBr₃/CHCl₃ or Na₂S/DMF, but both proved unsatisfactory: BBr₃ afforded only partial demethylation due to its water sensitivity; Na₂S failed because deprotonation of the phenolic hydroxyl groups rendered the aromatic ring electron-rich and the methoxy groups unreactive. Complete demethylation was ultimately achieved using HBr/AcOH (Scheme 7). This protocol offers advantages over traditional stepwise approaches, including elimination of intermediate isolation, reduced solvent consumption, simplified workup, shorter reaction time, and improved yields, making it suitable for scalable synthesis of diverse flavonol analogs.

Scheme 7.

One-pot synthetic route of polyhydroxy flavonols 28.

2.3.3. Synthesis of Houttuynoid B 35

In 2016, Kerl et al. [49] reported the first total synthesis of Houttuynoid B 35, employing a Baker–Venkataraman rearrangement on a pre-glycosylated substrate to construct the flavonoid skeleton. The synthesis started from chrysin 29, which was converted to 31 via bis o-benzylation, retro-aldol reaction, and Rubottom α-oxidation. Subsequent glycosylation with peracetylated β-galactose afforded 32, which was coupled with the HOBT-activated benzofuran derivative to give aryl ester 33. Using potassium carbonate as a base in the presence of TBAB as a phase-transfer catalyst, ester 33 underwent the desired Baker–Venkataraman rearrangement with concomitant in situ cyclization of the intermediate 1,3-diketone to afford 34. Finally, hydrogenolysis followed by deacetylation yielded Houttuynoid B 35 (Scheme 8). Notably, this work achieved the first total synthesis of a glycosylated flavonoid via a pre-glycosylated Baker–Venkataraman rearrangement, a transformation previously considered challenging due to the instability of glycosidic bonds under basic conditions. The mixed Bn/Ac protection strategy enabled orthogonal deprotection. However, the nine-step route (11% overall yield) features a modest 57% yield in the key rearrangement step and requires sequential deprotection, indicating room for further optimization.

Scheme 8.

Synthetic route of Houttuynoid B 35.

2.3.4. Synthesis of (±)-Sanggenol F 51

In 2017, Sheng et al. [50] reported the total synthesis of (±)-sanggenol F 51. The synthesis commenced from commercially available 2,4,6-trihydroxyacetophenone 36, which was methylated with Me₂SO₄ and K₂CO₃ in refluxing acetone to afford 37. Subsequent esterification with 2,4-dimethoxybenzoyl chloride in the presence of NaH in THF provided ester 38. α-Bromination of 38 with PTT was carried out with batchwise addition of the reagent to prevent hydrolysis of the ester 39. Nucleophilic substitution with potassium benzoate in refluxing acetonitrile afforded 40. The Baker–Venkataraman rearrangement of 40 with NaH in refluxing THF proceeded to afford 41. Cyclization under acidic conditions gave 42, followed by saponification to remove the benzoate group, affording 43. Finally, complete demethylation with pyridine hydrochloride at 220 °C yielded morin 44, which was then elaborated to sanggenol F 51 through sequential prenylation and Claisen rearrangement (Scheme 9). This route to morin 44 features the Baker–Venkataraman rearrangement to construct the flavonoid skeleton without strong oxidants, and the final demethylation delivers the natural product in high yield. However, the overall sequence is relatively lengthy, and the harsh conditions required for the demethylation may limit its scalability.

Scheme 9.

Synthetic route of (±)-sanggenol F 51.

2.3.5. Synthesis of 4′-Substituted Kaempferols 59

In 2020, Kim et al. [51] reported the synthesis of 4′-substituted kaempferols 59 via the Baker–Venkataraman rearrangement. The synthesis commenced with Lewis-acid-catalyzed acylation of phloroglucinol 7 to afford 2,4,6-trihydroxyacetophenone 36, which was subsequently bis-benzylated to provide 30. Diversification at the 4′-position was introduced via EDCI-mediated coupling of 30 with various substituted benzoic acids to form phenolic esters 52. The resulting methyl ketones 52 were subjected to α-bromination, most efficiently achieved via a bis-bromination-selective reduction sequence to give the mono-bromide intermediates 54, followed by nucleophilic displacement with potassium benzoate to afford benzoates 55. Subsequent treatment with LiHMDS promoted the Baker–Venkataraman rearrangement to generate 1,3-diketone intermediate 56. Then 56 was subjected to dehydrative cyclization under acidic conditions with heating to afford the perbenzylated kaempferol intermediate 57. Finally, hydrolysis of the benzoate ester 57 furnished perbenzylated kaempferol 58, which underwent global deprotection via hydrogenolysis over Pd/C to afford the target flavonol, 4′-substituted kaempferols 59 (Scheme 10). This route features benzyl protecting groups to shield the C5 and C7 hydroxyl groups, preventing side reactions, and the Baker–Venkataraman rearrangement constructs the flavonol skeleton under mild conditions with LiHMDS. However, the overall sequence is relatively lengthy, and the moderate yield of the cyclization step may limit overall synthetic efficiency.

Scheme 10.

Synthetic route of 4′-substituted kaempferols 59.

2.3.6. Synthesis of Cycloicaritin 71

In 2019, Liu et al. [52] reported a total synthesis of cycloicaritin 71. The synthesis commenced with 2,4,6-trihydroxyacetophenone 36. Methylation with DMS afforded 37. Esterification of 37 with 4-methoxybenzoyl chloride gave the phenolic ester 60. α-Bromination of 60 using PTT afforded dibromide 61. Selective debromination with DEP and TEA gave monobromide 62. Treatment of 62 with benzoyl chloride gave the corresponding benzoate, which underwent Baker–Venkataraman rearrangement with NaH to afford 1,3-diketone 63. Prenylation of 63 with prenyl bromide gave 64, which underwent ortho-Claisen rearrangement with Bi(OTf)₃ to afford 65. Subsequent cyclization under acidic conditions furnished 66. Removal of the benzoyl group under basic conditions gave 67, followed by demethylation with pyridine hydrochloride to afford polyhydroxy flavonol 68. Finally, cyclization of the prenyl group in 68 under acidic conditions gave the pyran ring 69, followed by acetylation, methylation at the 4′-position, and deacetylation to furnish cycloicaritin 71 (Scheme 11). This route features pre-flavonoid prenylation coupled with inexpensive Bi(OTf)₃-catalyzed ortho-rearrangement, circumventing the microwave irradiation or costly europium catalysts required in prior approaches. However, the lengthy linear sequence with multiple protection and deprotection cycles poses scalability challenges for industrial application.

Scheme 11.

Synthetic route of cycloicaritin 71.

2.3.7. Synthesis of Icaritin 77

In 2022, Sui et al. [53] reported a novel synthetic route to icaritin 77. The synthesis commenced with compound 72. Prenylation with prenyl bromide gave 73, which underwent Bi(OTf)₃-catalyzed ortho-Claisen rearrangement to afford 74. Esterification of 74 with 4-methoxybenzoyl chloride gave 75, followed by Baker–Venkataraman rearrangement under basic conditions to furnish 76. Finally, hydrogenolysis over Pd/C removed the benzyl-protecting groups to afford icaritin 77 in five steps (Scheme 12). This route features benzyl protection for facile one-pot hydrogenolytic removal and pre-flavonoid prenylation to suppress rearrangement by-products. This contrasts with the route by Liu et al. [52] (Scheme 11), which suffers from excessive methoxy protection and deprotection cycles and harsh demethylation conditions. The strategic use of benzyl groups enables milder reaction conditions and superior scalability, overcoming the industrial limitations of prior approaches.

Scheme 12.

Improved synthetic route of icaritin 77 via Baker–Venkataraman rearrangement.

The Baker–Venkataraman rearrangement offers an efficient route to flavonols from readily accessible starting materials under mild conditions, offering operational simplicity and broad substrate scope. However, the conventional protocol is limited by multi-step operations and moderate yields. Optimization, including improved catalytic systems, one-pot strategies, and streamlined protecting-group approaches, holds potential to enhance the overall efficiency of the rearrangement. With these refinements, the Baker–Venkataraman rearrangement is poised to become an increasingly valuable tool for flavonol synthesis.

2.4. Algar–Flynn–Oyamada (AFO) Reaction

The Algar–Flynn–Oyamada (AFO) reaction was independently reported by Algar, Flynn, and Oyamada in 1934 [54]. It proceeds under mild basic conditions through the oxidative cyclization of 2′-hydroxychalcone 78 with H₂O₂, affording flavonol 4 (Scheme 13). Owing to its operational simplicity, avoidance of toxic reagents, high atom economy, and environmentally benign profile, the AFO reaction has been widely adopted for flavonol synthesis.

Scheme 13.

Synthetic route of flavonol 4 via the Algar–Flynn–Oyamada reaction.

The mechanism of the AFO reaction has been the subject of considerable recent investigation. Under alkaline conditions, 2′-hydroxychalcone 78 is proposed to undergo oxidation by H₂O₂ to form the epoxide intermediate 80. If the phenoxide anion within the epoxide attacks the carbonyl β-position, dihydroflavonol 83 is generated, which is subsequently oxidized to flavonol 4. Conversely, if the carbonyl α-position is targeted, the five-membered cyclic aurone by-product 85 is produced (Scheme 14) [55,56].

Scheme 14.

The AFO reaction mechanism.

The AFO reaction is widely employed for flavonol synthesis, yet the involvement of epoxide intermediates in its mechanism continues to be debated. Although some evidence supports epoxide formation under certain conditions, particularly for 6′-substituted chalcones [57,58], no direct characterization of such species has been unequivocally verified [59]. Based on DFT calculations, Dean and Podimuang [60] proposed an alternative direct oxidative cyclization route that bypasses epoxide intermediates for unsubstituted substrates. Experimental studies further suggest that this mechanism is both kinetically and thermodynamically more favorable for unsubstituted substrates.

2.4.1. Synthesis of Quercetagetin 90

In the AFO reaction, quantities of homogeneous base catalysts such as NaOH, KOH, or pyrrolidine are typically employed. However, chalcone substrates are sensitive to strongly basic environments. In 2021, Bulut et al. [61] described a streamlined and efficient synthesis of quercetagetin 90 using modified AFO conditions. The synthesis began with the aldol condensation of 3,4,5-trimethoxyacetophenone 86 and 3,4-dimethoxybenzaldehyde 87 in the presence of NaOH to afford 2′-hydroxychalcones 88. Subsequently, the AFO reaction was successfully carried out using H₂O₂ as the oxidant and Na₂CO₃ as the base to achieve the oxidative cyclization of 88 to flavonol 89. Finally, demethylation with BBr₃ provided the target product, quercetagetin 90 (Scheme 15). In this improved protocol, the conventional strong base NaOH was replaced with Na₂CO₃, significantly optimizing the reaction process. Compared to NaOH, Na₂CO₃ provides milder conditions, and exhibits improved stability across a range of solvents. These modifications help suppress side reactions commonly promoted by strong alkalis, thereby improving the overall synthetic yield.

Scheme 15.

Synthetic route of quercetagetin 90.

2.4.2. One-Pot Synthesis of Polyhydroxy Flavonols

To reduce by-product formation in the AFO reaction and overcome limitations of conventional basic media, Tamaddon et al. [62] developed a bio-based catalyst, alkaline amylopectin (AAp). The catalyst has a high base capacity of 7.3 mmol HO⁻/g and exhibits significant catalytic activity in both aldol condensation and AFO reactions. The synthetic sequence involves aldol condensation between 2′-hydroxyacetophenone 91 and aromatic aldehydes 92 to afford 2′-hydroxychalcones 93, followed by oxidative cyclization with H₂O₂ in the presence of AAp to furnish various substituted flavonols 94 in 94–98% yield (Scheme 16). Compared with earlier AFO protocols, this one-pot procedure achieves excellent yields without aurone formation using low catalyst loading. The key innovation is the use of recoverable AAp as a bio-based heterogeneous catalyst, which can be recycled five times. However, the catalyst preparation is relatively complex, and the protocol still requires H₂O₂ as the oxidant.

Scheme 16.

One-pot synthetic route of flavonols 94.

The AFO reaction involves three distinct steps, condensation to form the chalcone, cyclization to generate the dihydroflavonol, and oxidation to yield the flavonol. Yan et al. [63] analyzed the reaction conditions and identified commonalities, enabling the development of an optimized one-pot AFO procedure. 2′-hydroxyacetophenone 95 and substituted benzaldehyde 96 were combined in ethanol containing NaOH and refluxed at 80 °C for 3 h. After cooling to room temperature, 35% H₂O₂ was added, and the mixture was stirred for an additional 3 h. This afforded substituted flavonols 97 (Scheme 17). Compared with traditional stepwise conditions, this one-pot protocol achieved higher yields. Response surface methodology optimized base loading, solvent volume, and water content. However, the protocol requires a large excess of NaOH and a relatively complex optimization process, which may limit its utility for rapid reaction development.

Scheme 17.

One-pot synthetic route of flavonols 97.

Aligned with green chemistry principles, Kumar et al. [64] developed a one-pot method for flavonol synthesis by optimizing the traditional AFO reaction. The method utilizes an “on-water” strategy, a contemporary approach that minimizes waste while affording an environmentally benign reaction medium. The process is further enhanced by sonochemistry, which improves reaction homogeneity, kinetics, and yield through ultrasound activation. Starting from the substituted 2′-hydroxyacetophenone 98 and the corresponding substituted benzaldehyde 99, this reaction employs water as the sustainable solvent, with LiOH·H₂O as the mild base and H₂O₂ as the oxidant. This design enables direct one-pot conversion to flavonols without isolating the intermediate chalcone, affording flavonols 100 in 62–76% yield (Scheme 18). This route achieves moderate to good yields with operational simplicity and low cost, using water as the sole solvent and ultrasound to enhance kinetics. The key innovation lies in combining on-water conditions with sonochemistry, eliminating the need for intermediate isolation and external heating. However, the yields are modest compared to some other one-pot AFO protocols, and ultrasound reliance may limit scalability.

Scheme 18.

One-pot synthetic route of flavonols 100.

The conversion of 2′-hydroxyacetophenone and benzaldehyde to flavonols via the AFO reaction requires both alkaline conditions and an oxidant. Expanding on the AFO transformation, Xiong et al. [65] developed an innovative catalytic approach using pyrrolidine. Under aerobic conditions in water, pyrrolidine acts as an organocatalytic base to promote the direct, one-pot synthesis of flavonols 104 from 2′-hydroxyacetophenones 101 and benzaldehydes 102 (Scheme 19). This route simplifies the synthetic sequence, employs water as a green solvent, and tolerates diverse substrates, highlighting its promise for practical application.

Scheme 19.

One-pot synthetic route of flavonols 104.

2.4.3. Synthesis of Icaritin 77

In 2022, Zhang et al. [66] developed a four-step total synthesis of icaritin 77 from 2,4,6-trihydroxyacetophenone 36. Magnesium succinate-promoted C-prenylation of 36 afforded 105, which upon selective MOM protection gave 106. Attempted flavonol construction under classical AFO conditions using NaOH/H₂O₂ failed due to prenyl group incompatibility. To overcome this, a pyrrolidine/air oxidation system was employed as a mild alternative, enabling 106 to condense with 4-methoxybenzaldehyde in methanol/water (1:3) to furnish 107. Final acid deprotection provided icaritin 77 in 33% overall yield (Scheme 20). This approach resolves both regioselectivity and functional group compatibility challenges. However, the moderate yield and solvent sensitivity in the flavonol construction step remain as limitations, warranting further optimization of oxidative cyclization conditions.

Scheme 20.

Synthetic route of icaritin 77 via AFO reaction.

2.4.4. Synthesis of Morin 44

In 2023, Gurzadyan et al. [67] reported an improved AFO reaction as part of a streamlined five-step synthesis of morin 44. The synthesis commenced with Friedel–Crafts acylation of phloroglucinol 7, affording 36. The hydroxyl groups were then protected by methylation using TsOMe/Na₂CO₃ to give 37. Subsequent treatment of 37 with 2,4-dimethoxybenzaldehyde in the presence of Ba(OH)₂·8H₂O afforded chalcone 108. Oxidative cyclization of 108 with TBHP/NaOH under AFO conditions then furnished flavonol 43. Finally, demethylation with 48% HBr in acetic acid yielded morin 44 (Scheme 21). This route offers practical improvements through five linear steps with chromatography-free purification and scalability to multigram-scale production, though the modest overall yield of 18.3% reflects significant by-product formation in the oxidative cyclization step. Addressing these competing pathways represents a key challenge for enhancing the overall efficiency and generality of this scalable approach.

Scheme 21.

Synthetic route of morin 44.

2.4.5. Synthesis of Fisetin 114

Fisetin, a polyhydroxy flavonol, has been widely studied as a potential therapeutic agent for multiple diseases. In 2021, Chu et al. [68] described a concise total synthesis of fisetin 114. The route began with Friedel–Crafts acylation of resorcinol 109 to give 2,4-dihydroxyacetophenone 110, which was methylated to afford 111, improving its stability under basic conditions. Condensation of 111 with benzaldehyde yielded 2′-hydroxychalcone 112. Initial attempts at the AFO reaction using 0.5 M NaOH and 35% H₂O₂ in water were unsuccessful due to the poor solubility of the methylated chalcone in the aqueous base. Switching to an ethanolic solution of NaOH and H₂O₂ afforded the flavonol product in only 20% yield. Further optimization revealed that gradual, simultaneous addition of aqueous NaOH and H₂O₂ enabled efficient formation of 113. Finally, demethylation with pyridine hydrochloride delivered fisetin 114 (Scheme 22). This five-step route achieves approximately 40% overall yield with chromatography-free purification, though the AFO oxidation requires precise control of reagent addition to avoid yields as low as 20% under unoptimized conditions. The key innovation lies in the continuous, simultaneous addition strategy that overcomes the poor aqueous solubility of methylated chalcones, a critical challenge for scalable flavonol synthesis.

Scheme 22.

Synthetic route of fisetin 114.

The AFO reaction offers several practical advantages, including mild conditions, broad substrate scope, and operational simplicity. Recent advances have focused on green chemistry principles, with the development of bio-based catalysts, on-water strategies, ultrasound assistance, and chromatography-free purification. These improvements have significantly enhanced the efficiency, selectivity, and scalability of the AFO transformation. However, challenges remain, including aurone by-products and moderate yields for certain substituted substrates. Future optimization of catalytic systems and reaction conditions will further broaden the applicability of this reliable strategy for flavonol synthesis.

2.5. DMDO Oxidative Reaction

DMDO is a mild and versatile oxidizing agent widely used for the stereoselective epoxidation of double bonds. The oxidation proceeds through a sequence in which oxone oxidizes acetone to form peroxyacetone 115. At room temperature, DMDO reacts with the double bond via electrophilic addition to generate epoxide 116. Subsequent treatment with TsOH opens the epoxide ring to afford the enol form, flavonol 4 (Scheme 23). This method is characterized by high stereoselectivity, minimal by-product formation, and operational simplicity [69,70].

Scheme 23.

Synthetic route of flavonol 4 via the DMDO oxidative reaction.

2.5.1. Synthesis of Kaempferol 3-O-Neohesperidoside 123

In 2010, Yamasaki et al. [71] reported the synthesis of kaempferol 3-O-neohesperidoside 123 from 2,4,6-trihydroxyacetophenone 36. The synthesis commenced with selective benzylation to afford 30, followed by aldol condensation with 4-(benzyloxy)benzaldehyde under basic conditions to give chalcone 117. Oxidative cyclization of 117 using I₂/DMSO at 120 °C furnished flavone 118. Oxidation with DMDO selectively introduced a hydroxyl group at the C3 position to yield flavonol 119. Due to intramolecular hydrogen bonding between the 3-OH and the carbonyl group that interfered with glycosylation, the 5-O-benzyl group was removed by heating in aqueous acetic acid to give 120. Finally, glycosylation of 120 with neohesperidosyl bromide under phase-transfer conditions, then sequential removal of the acetyl and benzyl protecting groups, furnished the target kaempferol 3-O-neohesperidoside 123 (Scheme 24). This route represents the first total synthesis of kaempferol 3-O-neohesperidoside 123. It introduces DMDO as a mild reagent for selective C3 hydroxylation of flavones under neutral, metal-free conditions, while the I₂/DMSO cyclization results in high yield. However, the linear sequence suffers from a very low overall yield of 7% due to multiple protection/deprotection steps and is not readily adaptable to other flavonoid scaffolds.

Scheme 24.

Synthetic route of kaempferol 3-O-neohesperidoside 123.

2.5.2. Synthesis of Icaritin 77

In 2014, Nguyen et al. [72] reported the total synthesis of icaritin 77 via microwave-assisted Claisen rearrangement. Starting from 2,4,6-trihydroxyacetophenone 36, selective benzylation afforded 30. Reaction with 4-methoxybenzoyl chloride gave the aryl ester, which underwent Baker–Venkataraman rearrangement and dehydrative cyclization to afford flavone 124. Oxidation of flavone 124 with DMDO at low temperature, followed by acid-catalyzed rearrangement, furnished flavonol 125. Hydrogenolysis of 125 over Pd/C removed the benzyl groups to give 126. Selective O-methoxymethylation of 126 with MOMCl in dry acetone gave 127. O-prenylation of the free 5-hydroxyl group of 127 using 3,3-dimethylallyl bromide as the electrophile gave 128. Subsequently, microwave-assisted Claisen rearrangement of 128 in N,N-diethylaniline regioselectively afforded the p-prenylated product 130, whereas conventional heating gave the ortho-isomer. Finally, acid-catalyzed deprotection of the MOM groups in 130 furnished icaritin 77 (Scheme 25). In this route, DMDO oxidation converted flavone to flavonol in 75% yield with complete C3 regioselectivity. Notably, DMDO serves as a mild, metal-free oxidant under neutral conditions, avoiding traditional harsh methods. However, the in situ generation of DMDO from oxone under controlled low temperature adds operational complexity and limits scalability.

Scheme 25.

Synthetic route of icaritin 77 via DMDO oxidative reaction.

2.5.3. Synthesis of Houttuynoid A 138

In 2018, Jian et al. [73] reported the first total synthesis of Houttuynoid A 138, the most potent antiviral member of the Houttuynoid family. The synthesis commenced with a Claisen–Schmidt condensation between 30 and benzofuran aldehyde, using NaOH as a base, affording chalcone 131. Subsequent I₂-catalyzed oxa-Michael addition and oxidative cyclization of 131 in DMSO at 110 °C furnished 132. Methylation of the C2′′ hydroxycarbonyl group in 132 afforded ester 133, which was then subjected to hydroxylation at C3 using in situ generated DMDO to deliver flavonol 134. Selective removal of the C5 benzyl group under acidic conditions allowed glycosylation with a galactosyl donor to yield 136. Finally, deprotection and reduction afforded Houttuynoid A 138 (Scheme 26). The DMDO oxidation represents a key breakthrough, enabling C3 hydroxylation in 62% yield and overcoming a major obstacle in previous Houttuynoid B synthesis. The reaction features mild conditions, high selectivity, and good protecting-group compatibility. However, it suffers from poor atom economy, a prolonged reaction time of 32 h, and suboptimal yield, making it the rate-limiting step of the nine-step sequence. Despite these limitations, the method remains viable for laboratory-scale preparation.

Scheme 26.

Synthetic route of Houttuynoid A 138.

Beyond this specific application, the broader utility of DMDO in flavonoid chemistry is constrained by its instability, limited storage time, and a demanding preparation [74]. To address these challenges, Wu et al. [75] introduced an intermittent addition of oxone aqueous solution to carefully control reaction temperature, addition rate, and duration. Through parameter optimization, the oxidation of flavones to flavonols resulted in over 95% yield with minimal by-product formation, aligning with green chemistry principles. Despite these advances, the process remains operationally cumbersome due to the thermal and chemical instability of the epoxidation products. Furthermore, DMDO-mediated oxidations display marked selectivity toward flavonoid substituent patterns, often leading to mixtures of isomers or side products that limit substrate scope and synthetic utility [76].

To provide a clear comparison of the total synthesis methods for the five polyhydroxy flavonols and their derivatives described in this review, Table 2 summarizes the key reaction conditions, protection/deprotection requirements, yields, scalability, advantages and limitations for each method.

Table 2.

Comparison of total synthesis methods for polyhydroxy flavonols.

Synthesis Methods	Key Reaction Conditions	Protection/Deprotection Requirements	Yields (Key Step)	Scalability	Advantages	Limitations
Auwers reaction	(1)alkaline rearrangement.	-	low	Not applicable.	(1)Applicable to flavonol synthesis.	(1)Stringent structural requirements for reactants. (2)Difficult preparation of dibromide intermediates with low yields.
Allan–Robinson reaction	(1)Aromatic anhydride to provide B-ring. (2)Base conditions for cyclization.	Protecting groups: CH3O-. Deprotection methods: BBr3.	about 53%	Moderate scalability for improved method.	(1)Streamlined one-pot operation. (2)Moderate yields.	(1)Formation of 3-aryloxyflavone by-products. (2)Poor atom economy.
Baker–Venkataraman rearrangement reaction	(1)Base conditions for rearrangement. (2)acid for cyclization.	Protecting groups: THPO-, MOMO-, CH3O-, BnO-, AcO-. Deprotection methods: TsOH/MeOH, HCl/MeOH, Py·HCl or BBr3, Pd/C, K2CO3/MeOH.	20–74%	Conventional: limited scalability. improved: scalable.	(1)Readily accessible starting materials. (2)Mild reaction conditions. (3)Broad substrate scope.	(1)Tedious reaction process. (2)Moderate yields.
AFO reaction	(1)Mild basic conditions. (2)H2O2 or air as oxidant.		30–98%	Wide range of compatible substrates.	(1)Mild reaction conditions. (2)Broad substrate scope. (3)Streamlined one-pot operation. (4)Green chemistry compatibility. (5)Good scalability.	(1)Formation of aurone by-products. (2)Moderate yields for certain substituted substrates.
DMDO oxidative reaction	(1)DMDO as oxidant.		62–75%	Limited by DMDO instability and demanding preparation	(1)Good yields under optimized conditions. (2)Minimal by-product formation. (3)High regional selectivity at the C3 position. (4)Green chemistry compatibility.	(1)Instability and limited storage time of DMDO. (2)Demanding preparation of DMDO. (3)Operationally cumbersome process. (4)Limited substrate scope and synthetic utility.

3. Biological Activity

Flavonols are important plant secondary metabolites that exhibit a broad spectrum of biological activities, including antioxidant, anticancer, anti-inflammatory, antiviral, antidiabetic, and hepatoprotective effects. Detailed biological activities, mechanisms of action, and key references are summarized in Table 3.

Table 3.

Summary of the biological activities of polyhydroxy flavonols.

Activity		Compounds	Chemical Structure	Mechanism of Action	Refs.
Antioxidant		Fisetin		Activates Nrf2/ARE pathway. Upregulates HO-1.	[11,12,77]
		kaempferol
Anticancer	Breast Cancer	Myricetin		Induces ROS-dependent mitochondrial apoptosis. Activates the BRCA1-GADD45 pathway.	[18,78]
		Quercetin		Upregulates Bax and Caspase-3. Downregulates Bcl2.	[79]
	Ovarian Cancer	Quercetin		Inhibits CDK/cyclin expression. Activates Bax/Bcl-2-caspase pathway.	[80,81]
		Morin		Inhibits NF-κB pathway.	[82]
	Skin Cancer	Fisetin		Inhibits PI3K pathway. Downregulates TNF-α, IL-1β, IL-2.	[83]
Anti-inflammatory		Gossypetin		Inhibits MKK3/6-p38 MAPK signaling pathway. Downregulates IL-1α, IL-1β, TNF-α expression.	[84,85]
		Galangin		Inhibits NF-κB signaling pathway. Inhibits iNOS, COX-2 expression.	[86,87]
Antiviral		Myricetin		Inhibit 3CLpro/M pro. Inhibits RIPK1/NF-κB signaling pathway.	[88,89]

Flavonols are a widely distributed class of natural bioactive compounds in plants. Their biological activity is closely related to specific structural features, including the number and substitution pattern of hydroxyl groups, the presence of conjugated double bonds, and the overall molecular planarity. These structural attributes are key determinants of their pharmacological properties. Moreover, targeted structural modifications can profoundly influence both activity and bioavailability. Investigating these SARs provides fundamental insights into their mechanisms of action and enables the rational design of analogs with improved therapeutic profiles. This review systematically summarizes the current understanding of SARs of flavonols, offering a valuable framework to guide future research and potential applications (Figure 2). According to SAR analysis, the 6-OH group of ring A may reduce anticancer potential and anti-inflammatory activity by causing steric hindrance or affecting molecular planarity. Conversely, the simultaneous presence of hydroxyl groups at the C5 and C7 positions confers strong anti-inflammatory activity [90]. For example, gossypetin and galangin exhibit potent anti-inflammatory activity. Notably, the 7-OH group is essential for antiviral activity. Introducing alkyl chains of varying lengths at this position not only enhances the inhibitory activity against SARS-CoV-2 3CLpro but also improves cell membrane permeability [24]. The ortho-dihydroxyl group on the B ring is a key structural feature for anticancer and antioxidant activity [91]. For example, quercetin and fisetin contain a 3′,4′-ortho-dihydroxyl structure on the B ring and exhibit strong antioxidant activity due to the formation of stable semiquinone radicals following the reaction of the ortho-dihydroxyl group with free radicals. If a 5′-OH group is present in the B ring, antioxidant activity is reduced. For example, myricetin, which contains a 5′-OH group, exhibits lower activity than quercetin, which lacks a 5′-OH group. Additionally, the 4′-OH group is a key moiety for antiviral activity, as it acts as a hydrogen bond donor, forming hydrogen bonds with critical amino acid residues at the active site of viral proteases to enhance binding affinity [92]. For example, myricetin exhibits broad-spectrum antiviral activity against various viruses, including HIV, SARS-CoV-2, and EBOV. The C2=C3 double bond and the C4 carbonyl group in the C ring are essential structural features for biological activity [90]. Most active compounds retain these two functional groups, which jointly maintain the molecular conjugated system and planarity, thereby facilitating binding to the target. Furthermore, the 3-OH group on the C ring can form a conjugated system with the C2=C3 double bond to stabilize free radicals, thereby enhancing antioxidant activity and exerting a synergistic effect on anticancer activity [93].

Figure 2.

Structure–activity relationship diagram of polyhydroxy flavonols.

4. Conclusions

Flavonols have attracted considerable research interest owing to their distinctive chemical structures and extensive biological activities. These compounds exhibit a broad spectrum of pharmacological effects, including antioxidant, anti-inflammatory, anticancer, and antiviral properties, making them highly valuable for medicine and functional food applications. In recent years, classical synthetic methods such as the Baker–Venkataraman rearrangement and the AFO reaction have been widely employed. Advances in reaction design, including one-pot protocols and green catalysts, have significantly improved synthetic efficiency. Nevertheless, the synthesis of flavonols still faces challenges such as multistep sequences, moderate yields, limited regioselectivity, and issues related to atom economy and rate-limiting steps.

Flavonols can be obtained through either natural extraction or chemical synthesis, with each method complementing the other in terms of application. Natural extraction is simple and cost-effective, suitable for obtaining high-content and simple-structured flavonols from plants. However, it is limited by factors such as plant resources, seasonality, and geographical constraints. It also faces sustainability challenges and struggles to produce rare monomers and flavonol derivatives. In contrast, chemical synthesis allows for precise control over product structure, and offers high yields and strong scalability, making it suitable for the synthesis of novel flavonols and SAR studies. However, for flavonols with multiple hydroxyl groups or complex glycosidic modifications, the chemical synthesis process is cumbersome and costly, making natural extraction more appropriate.

In addition to traditional preparation methods, biosynthesis represents a cutting-edge direction for the green production of flavonols. This method utilizes genetically engineered microorganisms to synthesize target products in fermenters, with heterologous biosynthesis serving as the mainstream technology. It features renewable resources, mild reaction conditions, high regioselectivity and stereoselectivity, and environmental friendliness, effectively addressing the shortcomings of chemical synthesis, such as high pollution and numerous side reactions. However, this technology currently faces challenges including long production cycles and insufficient yields for industrial applications. Looking ahead, a combined chemical–enzymatic approach could be adopted, utilizing chemical synthesis to construct the core skeleton of flavonols, followed by enzyme-catalyzed modification at specific sites. This would strike a balance between synthetic efficiency and regioselectivity, representing a promising future direction for this field.

Future efforts should focus on developing more efficient and selective synthetic methodologies, optimizing synthesis processes, and conducting deeper investigations into structure–activity relationships. Such studies will enhance the clinical translatability and practical utility of flavonols.

Abbreviations

The following abbreviations are used in this manuscript:

EA	Ethyl Acetate
TEA	Triethylamine
BBr3	Boron Tribromide
LiHMDS	Lithium bis(trimethylsilyl)amide
DBU	1,8-Diazabicyclo [5.4.0]undec-7-ene
p-TsOH	p-Toluenesulfonic Acid
TFA	Trifluoroacetic acid
PPA	Polyphosphoric Acid
MOM	methoxymethyl ether
DHP	3,4-Dihydro-2H-pyran
PPTS	Pyridinium p-Toluenesulfonate
EDC	1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide (hydrochloride)
DMAP	4-Dimethylaminopyridine
TBAB	Tetrabutylammonium bromide
DMF	N, N-Dimethylformamide
DEG	Diethylene glycol
LDA	Lithium diisopropylamide
TMSCl	Chlorotrimethylsilane
THF	Tetrahydrofuran
m-CPBA	meta-Chloroperoxybenzoic acid
BF3·OEt2	Boron trifluoride etherate
PTT	phenyltrimethylammonium tribromide
Pd(PPh3)4	Tetrakis(triphenylphosphine)palladium
i-PrOH	Isopropyl Alcohol
Py	pyridine
DEP	diethyl phosphite
DMS	dimethyl sulfate
Bi(OTf)3	Bismuth(III) trifluoromethanesulfonate
NMP	N-Methyl-2-pyrrolidone
TsOMe	Methyl p-toluenesulfonate
TBHP	Tert-Butyl hydroperoxide
DMDO	Dimethyldioxirane
Oxone	potassium peroxymonosulfate
DFT	density functional theory
DMSO	Dimethyl Sulfoxide
MOMCl	Chloromethyl methyl ether
PhNEt2	N, N-Diethylaniline
DIBAL-H	Diisobutylaluminium hydride
SAR	Structure–activity relationship

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding Statement

This work was funded by the National Natural Science Foundation of China (82074122, 82204766), Natural Science Foundation of Hunan Province (2025JJ80077, 2023JJ40490), Scientific research project of Hunan Provincial Education Department (21A0239) and Undergraduate Research and Innovation Foundation of Hunan University of Chinese Medicine (2025BKS182).