Natural products of pentacyclic triterpenoids: from discovery to heterologous biosynthesis.

Covering: up to 2022Pentacyclic triterpenoids are important natural bioactive substances that are widely present in plants and fungi. They have significant medicinal efficacy, play an important role in reducing blood glucose and protecting the liver, and have anti-inflammatory, anti-oxidation, anti-fatigue, anti-viral, and anti-cancer activities. Pentacyclic triterpenoids are derived from the isoprenoid biosynthetic pathway, which generates common precursors of triterpenes and steroids, followed by cyclization with oxidosqualene cyclases (OSCs) and decoration via cytochrome P450 monooxygenases (CYP450s) and glycosyltransferases (GTs). Many biosynthetic pathways of triterpenoid saponins have been elucidated by studying their metabolic regulation network through the use of multiomics and identifying their functional genes. Unfortunately, natural resources of pentacyclic triterpenoids are limited due to their low content in plant tissues and the long growth cycle of plants. Based on the understanding of their biosynthetic pathway and transcriptional regulation, plant bioreactors and microbial cell factories are emerging as alternative means for the synthesis of desired triterpenoid saponins. The rapid development of synthetic biology, metabolic engineering, and fermentation technology has broadened channels for the accumulation of pentacyclic triterpenoid saponins. In this review, we summarize the classification, distribution, structural characteristics, and bioactivity of pentacyclic triterpenoids. We further discuss the biosynthetic pathways of pentacyclic triterpenoids and involved transcriptional regulation. Moreover, the recent progress and characteristics of heterologous biosynthesis in plants and microbial cell factories are discussed comparatively. Finally, we propose potential strategies to improve the accumulation of triterpenoid saponins, thereby providing a guide for their future biomanufacturing.

A photocatalytic sp3 C-S, C-Se and C-B bond formation through C-C bond cleavage of cycloketone oxime esters.

The photocatalytic thiolation, selenylation and borylation of cycloketone oxime esters through iminyl radical-triggered C-C bond cleavage were described. The reactions provide a unified approach to alkyl sulfur, selenium and boron compounds tethered to a synthetically useful nitrile group.

Visible-light promoted γ-cyanoalkyl radical generation: three-component cyanopropylation/etherification of unactivated alkenes.

A photoredox approach to generate distal cyano-substituted alkyl radicals through C-C bond cleavage of cyclobutanone oximes was developed. The radicals produced by this method were applied to three-component cyanopropylation/etherification of unactivated alkenes. Their reactions with diverse radical acceptors were also demonstrated.

Synthesis of 3-Aryl-2-pyrones by Palladium-Catalyzed Cross-Coupling of Aryl Iodides with Cyclic Vinyldiazo Ester.

A palladium-catalyzed cross-coupling reaction of aryl iodides with cyclic vinyldiazo ester was developed. The reaction provides various 3-aryl-2-pyrones in good yields with high functional group tolerance. The synthetic application of the resulting 3-aryl-2-pyrones as the diene component in a Diels-Alder reaction was also described.

Visible-Light-Mediated Two-Fold Unsymmetrical C(sp3 )-H Functionalization and Double C-F Substitution.

A visible-light-mediated [3+3] annulation of tertiary amines with α-trifluoromethyl alkenes was developed. The reaction offers a direct route to fluorinated tetrahydropyridines and azabicyclo[3.m.1] frameworks under very mild conditions. This protocol presents a rare example of dual sp3 C-H functionalization of tertiary amines with the formation of two different C-C bonds (one sp3 -sp3 bond, one sp2 -sp3 bond). Moreover, two consecutive C-F substitutions in a trifluoromethyl group were achieved in one pot using visible light photoredox catalysis, which enables an unprecedented ring construction.

Parecoxib inhibits glioblastoma cell proliferation, migration and invasion by upregulating miRNA-29c.

Glioblastoma (GBM) is one of the most lethal brain cancers worldwide, and there is an urgent need for development of novel therapeutic approaches. Parecoxib is a well-known cyclooxygenase-2 (COX-2) inhibitor, and had already been developed for postoperative analgesia with high efficacy and low adverse reaction. A recent study has suggested that parecoxib potently enhances immunotherapeutic efficacy of GBM, but its effects on GBM growth, migration and invasion have not previously been studied. In the present study, MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] and BrdU (5-bromo-2-deoxyuridine) incorporation assays were used to evaluate the cell proliferation of GBM cells. Wound-healing and transwell assays were preformed to analyze GBM cell migration and invasion, respectively. The results suggested that parecoxib inhibits cell proliferation, migration and invasion of GBM cells in a dose-dependent manner. RT-qPCR (real-time quantitative PCR) analysis demonstrated that miRNA-29c can be significantly induced by parecoxib. Furthermore, our data suggests that a miRNA-29c inhibitor can significantly attenuate parecoxib's effect on proliferation, migration and invasion of GBM. In conclusion, the present study suggests that parecoxib inhibits GBM cell proliferation, migration and invasion by upregulating miRNA-29c.

Synthesis of Functionalized gem-Difluoroalkenes via a Photocatalytic Decarboxylative/Defluorinative Reaction.

A photocatalytic decarboxylative/defluorinative reaction of α-trifluoromethyl alkenes with α-keto acids and α-amino acids has been developed. The reaction occurs at room temperature under visible light irradiation, affording various γ,γ-difluoroallylic ketones and 1,1-difluorohomoallyl amines in good yields. The synthetic applications of the resulting functionalized gem-difluoroalkenes were also described.

Synthesis of Gem-Difluorinated Fused Quinolines via Visible Light-Mediated Cascade Radical Cyclization.

A facile synthesis of gem-difluorinated fused quinolines via visible light-mediated cascade radical cyclization between functionalized difluoromethyl chlorides and alkenes was developed. Various highly functionalized fused quinolines were assembled in moderate to good yields under very mild reaction conditions. The reaction extends the applications of chlorodifluoroacetic acid as the gem-difluoromethylenated building block by simple derivatization, especially in the synthesis of gem-difluorinated fused heterocyclic rings, which are difficult to access with existing methods.

Metal-free visible-light induced cross-dehydrogenative coupling of tertiary amines with diazo compounds.

The first visible-light induced cross-dehydrogenative coupling between tertiary amines and diazo compounds is described. The reaction proceeds smoothly under mild and metal-free conditions by using air or O2 as the oxidant, affording various ß-amino-α-diazo adducts in moderate to good yields with broad substrate scopes. The resulting products were successfully employed for the synthesis of 4- or 5-ester N-aryl-2,3-dihydrobenzo[d]azepines with high regioselectivity simply switched by the selection of the transition metal catalysts.

Synthesis of dihydrofuroaporphine derivatives: identification of a potent and selective serotonin 5-HT 1A receptor agonist.

A series of new aporphine analogues were synthesized and pharmacologically evaluated. 11-Allyloxy-(17), 11-propargyloxy-(20), and dihydrofuro-(19) aporphines displayed the highest affinity at the 5-HT(1A) receptor with K(i) values of 12.0, 14.0, and 6.7 nM, respectively. The high binding potential of the diastereomeric mixture of aporphine 19 was found residing in the cis-diastereomer (cis-19). [(35)S]GTP gamma S function assays on 5-HT(1A) receptor indicated that aporphines 17 and 20 were partial agonists, while trans-19 behaved as a high efficacy full antagonist and cis-19 was a full agonist. The agonistic property of cis-19 at the 5-HT(1A) receptor was further confirmed in vitro and in vivo. This compound may be useful as a potential treatment for anxiety.