C8-selective biomimetic transformation of 5,7-dihydroxylated flavonoids by an acid-catalysed phenolic Mannich reaction: Synthesis of flavonoid alkaloids with quercetin and (–)-epicatechin skeletons

Flavonoidal alkaloids: Emerging targets for drug discovery from Nature's bounty

This paper explores the potential of flavonoid alkaloids, a unique class of compounds that contain both flavonoid and alkaloid structures, as emerging targets for drug discovery. These compounds exhibit diverse biological activities, such as anti-inflammatory, anti-cancer, and anti-diabetic effects, which are attributed to the combination of different flavonoid scaffolds and alkaloid groups. Flavonoid alkaloids have attracted researchers' attention due to their diverse structures and important bio-activities. Therefore, this review summarizes recent advances in the extraction, purification, structural characterization, synthesis pathways and biological activities of flavonoid alkaloids from natural sources. Finally, the potential prospects and challenges associated with this class of compounds in pharmacological research are discussed along with details of a mechanistic investigation and future clinical applications in this research field.

Novel Flavonol Alkaloids in Green Tea: Synthesis, Detection, and Anti-Alzheimer's Disease Effect in a Transgenic Caenorhabditis elegans CL4176 Model

Novel N-ethy-2-pyrrolidinone-substituted flavonols, myricetin alkaloids A-C (1-3), quercetin alkaloids A-C (4a, 4b, and 5), and kaempferol alkaloids A and B (6 and 7), were prepared from thermal reaction products of myricetin, quercetin, kaempferol─l-theanine, respectively. We used HPLC-ESI-HRMS/MS to detect 1-7 in 14 cultivars of green tea and found that they were all present in "Shuchazao," "Longjing 43", "Fudingdabai", and "Zhongcha 108" green teas. The structures of 1-4 and 6 were determined by extensive 1D and 2D NMR spectroscopies. These flavonol alkaloids along with their skeletal flavonols were assessed for anti-Alzheimer's disease effect based on molecular docking, acetylcholinesterase inhibition, and the transgenic Caenorhabditis elegans CL4176 model. Compound 7 strongly binds to the protein amyloid β (Aβ1-42) through hydrogen bonds (BE: -9.5 kcal/mol, Ki: 114.3 nM). Compound 3 (100 μM) is the strongest one in significantly extending the mean lifespan (13.4 ± 0.5 d, 43.0% promotion), delaying the Aβ1-42-induced paralysis (PT50: 40.7 ± 1.9 h, 17.1% promotion), enhancing the locomotion (140.0% promotion at 48 h), and alleviating glutamic acid (Glu)-induced neurotoxicity (153.5% promotion at 48 h) of CL4176 worms (p < 0.0001).

Structure-Activity Relationship Prediction-Based Synthesis and Cytotoxicity Evaluation against the HEp-2 Laryngeal Carcinoma Cell of Isoflavone-Cytisine Mannich Bases

QSAR analysis of previously synthesized and nature-inspired virtual isoflavone-cytisine hybrids against the HEp-2 laryngeal carcinoma cell lines was performed using the OCHEM web platform. The validation of the models using an external test set proved that the models can be used to predict the activity of newly designed compounds such as 8-cytisinylmethyl derivatives of 5,7- and 6,7-dihydroxyisoflavones. The synthetic procedure for selective aminomethylation of 5,7-dihydroxyisoflavones with cytisine was developed. In vitro testing identified compound 7 f with cisplatin-level cytotoxicity against HEp-2 cell lines and compound 10 which was twice active than cisplatin after 72 h of incubation.

Quercetin Hybrids—Synthesis, Spectral Characterization and Radical Scavenging Potential

New quercetin-based derivatives are synthesized in an easily accessible one-pot manner. The method is based on the reaction of quercetin with in situ formed electrophilic N-alkoxycarbonylazolium ions. The position of the newly formed C-C bond and structure were spectrally characterized by 1D, 2D 1H, 13C-NMR, IR, and MS analysis. Thus, in all cases, good regioselectivity in the C-8 position for the obtained products was demonstrated. The obtained compounds were evaluated for their DPPH and ABTS free radical scavenging activity and compared to natural compounds—quercetin and rutin.

Computationally Assisted Structural Revision of Flavoalkaloids with a Seven-Membered Ring: Aquiledine, Isoaquiledine, and Cheliensisine

Aquiledine and cheliensisine are flavoalkaloids isolated from Aquilegia ecalcarata and Goniothalamus cheliensis, respectively. Different structures have been proposed for these flavoalkaloids; however, their ¹H and ¹³C NMR spectroscopic data were virtually identical. In this study, the structures of aquiledine and cheliensisine were revised on the basis of the DFT calculation of NMR data including DP4+ and J-DP4 analysis, as well as specific rotations. Similarly, the structure of isoaquiledine, a regioisomer of aquiledine, was also revised. A biosynthetic pathway of these flavoalkaloids is proposed.

Results in Chemistry of Natural Organic Compounds. Synthesis of New Anticancer Vinca Alkaloids and Flavone Alkaloids

The antitumor indole–indoline alkaloids of the evergreen Catharanthus roseus—namely vinblastine and vincristine—are widely used in chemotherapy of cancer. Many efforts were made to synthesize more efficient derivatives with less side-effect. The 14,15-cyclopropane derivative of vinblastine was synthesized successfully by a five-step procedure starting from vindoline. Vincristine, vinorelbine and several derivatives condensed with a cyclopropane ring were synthesized. Various hybrid molecules were prepared by the coupling reaction of vindoline and methyl ester of tryptophan, which were conjugated by carrier peptides of octaarginine. Studying the halogenation reactions of vindoline and catharanthine some fluorine derivatives were obtained which showed promising antitumor activity on various tumor types. The synthesis of the Aspidospermane alkaloid bannucine and 5′-epibannucine were carried out using N-acyliminium intermediates. The same intermediate was also applied in the first synthesis of sessiline. The research group have synthesized of flavonoid alkaloids: dracocephins A and B. Further three flavonoid alkaloids, namely 8-(2”-pyrrolidinon-5′′-yl)quercetin, 6-(2′′-pyrrolidinon-5′′-yl)-(−)- and 8-(2′′-pyrrolidinon-5′′-yl)-(−)-epicatechin were prepared by acid-catalyzed regioselective Mannich reaction starting from the corresponding flavonoid precursor. Vindoline was also coupled to synthetic pharmacophores, such as triphenylphosphine and various N-heterocycles. Some of these hybrid molecules showed significant antitumor activity. Furthermore, 7-OH and 7-NH modified flavonoid derivatives were synthesized by a regioselective alkylation followed by Smiles rearrangement and hydrolysis.

Novel Cinnamoylated Flavoalkaloids Identified in Tea with Acetylcholinesterase Inhibition Effect

3-O-Cinnamoylepicatechin (1) was synthesized along with four flavoalkaloids, (-)-6-(5‴S)-N-ethyl-2-pyrrolidinone-3-O-cinnamoylepicatechin (2), (-)-6-(5‴R)-N-ethyl-2-pyrrolidinone-3-O-cinnamoylepicatechin (3), (-)-8-(5‴S)-N-ethyl-2-pyrrolidinone-3-O-cinnamoylepicatechin (4), and (-)-8-(5‴R)-N-ethyl-2-pyrrolidinone-3-O-cinnamoylepicatechin (5) via esterification of epicatechin followed by phenolic Mannich reaction of 1 with theanine in the presence of heat. The new compounds 1-5 were detected in leaves of three tea cultivars, Fuding-Dabai, Huangjingui, and Zimudan with the help of ultra-performance liquid chromatography hyphenated with a photodiode array detector and electrospray ionization high-resolution mass spectrometry (UPLC-PDA-ESI-HRMS), suggesting that they are naturally occurring in tea leaves. The structures of the novel natural products were characterized by one- and two-dimensional nuclear magnetic resonance (1D and 2D NMR) and mass spectroscopy. Compounds 1-5 were then evaluated for their acetylcholinesterase (AChE) inhibitory effect (IC₅₀ = 0.12-1.02 μM). The availability of the synthesized epicatechin derivatives 1-5 via a synthetic route enabled the first unequivocal identification of these derivatives as tea secondary metabolites and made it possible to determine their content in the tea material as well as the diverse bioactivities.

Flavoalkaloids with a Pyrrolidinone Ring from Chinese Ancient Cultivated Tea Xi-Gui

Chinese Xi-Gui tea is one ancient cultivated variety of Camellia sinensis var. assamica. At present, it is used for producing expensive and elite tea in China. Five new flavoalkaloids, (-)-6-(5''' S)- N-ethyl-2-pyrrolidinone-epicatechin-3- O-gallate (ester-type catechins pyrrolidinone E, etc-pyrrolidinone E, 1), (-)-6-(5''' R)- N-ethyl-2-pyrrolidinone-epicatechin-3- O-gallate (etc-pyrrolidinone F, 2) (-)-8-(5''' S)- N-ethyl-2-pyrrolidinone-epicatechin-3- O-gallate (etc-pyrrolidinone G, 3a), (-)-8-(5''' S)- N-ethyl-2-pyrrolidinone-catechin-3- O-gallate (etc-pyrrolidinone I, 4a), (-)-8-(5''' R)- N-ethyl-2-pyrrolidinone-catechin-3- O-gallate (etc-pyrrolidinone J, 4b), and one new naturally occurring natural product (-)-8-(5''' R)- N-ethyl-2-pyrrolidinone-epicatechin-3- O-gallate (etc-pyrrolidinone H, 3b) together with the known flavoalkaloids etc-pyrrolidinones A-D (5, 6, 7a, and 7b) were detected and isolated from Xi-Gui green tea. Their structures were identified by comprehensive NMR spectroscopic analyses. Absolute configurations of 1-3 were established by comparison of the CD analyses with epicatechin-3- O-gallate (ECG). Compounds 1-4 were evaluated for their protection against high glucose induced cell senescence on human umbilical vein endothelia cells (HUVECs) and showed significant protection effects ( p < 0.01) at both 1.0 and 10 μM. A discussion on the possible evolution of tea plants divergent from related food plants on the basis of phytochemical view is also provided.

Novel Flavoalkaloids from White Tea with Inhibitory Activity against the Formation of Advanced Glycation End Products

Two novel flavoalkaloids, (-)-6-(5'''- S)- N-ethyl-2-pyrrolidinone-epigallocatechin- O-gallate (ester-type catechin pyrrolidinone A, etc-pyrrolidinone A, 1), (-)-6-(5'''- R)- N-ethyl-2-pyrrolidinone-epigallocatechin- O-gallate (etc-pyrrolidinone B, 2), and new naturally occurring flavoalkaloids, (-)-8- N-ethyl-2-pyrrolidinone-epigallocatechin- O-gallate (etc-pyrrolidinone C, 3a, and etc- pyrrolidinone D, 3b), were isolated from white tea ( Camellia sinensis). Their structures were identified by extensive nuclear magnetic resonance spectra. The absolute configuration of compounds 1 and 2 was decided by comprehensive circular dichroism spectroscopic analyses. The isolated flavoalkaloids together with (-)-epigallocatechin- O-gallate (EGCG) were evaluated for their inhibition against the formation of advanced glycation end products, with IC₅₀ values ranging from 10.3 to 25.3 μM. Ultra performance liquid chromatography coupled with diode array detection and electrospray ionization mass spectrometry detected these flavoalkaloids in both white tea and fresh tea leaves, which demonstrated the existence of a corresponding biosynthetic pathway in tea plants. Therefore, a possible pathway was proposed to involve deamination, decarboxylation, and spontaneously cyclization of l-theanine and then attachment of the product to EGCG to form the flavoalkaloids.

Primary aminomethyl derivatives of kaempferol: hydrogen bond-assisted synthesis, anticancer activity and spectral properties

Primary aminomethyl derivatives of kaempferol with anticancer activity were synthesized by a combination strategy involving a hydrogen bond-assisted process.