An Approach to the Synthesis of Electron-Rich and Hindered Esters and Its Application to the Synthesis of Acteoside

High-level sustainable production of complex phenylethanoid glycosides from glucose through engineered yeast cell factories

Complex phenylethanoid glycosides (PhGs), such as verbascoside and echinacoside, comprise a vital family of natural products with renowned nutraceutical and pharmaceutical significance. Despite the high demand for these compounds across various industries, traditional plant extraction methods yield insufficient quantities, highlighting the need for alternative production methods. Therefore, this paper reports the successful engineering of Saccharomyces cerevisiae cell factories for the efficient production of complex PhGs from glucose. First, key pathway enzymes with enhanced catalytic activities in yeast were primarily screened from various verbascoside-producing plants. Second, intermediate osmanthuside B was produced with a titer of 21.5 ± 1.5 mg/L from glucose by overexpressing several enzymes, including glucosyltransferase RrUGT33 from Rhdiola rosea, acyltransferase SiAT, and 1,3-rhamnosyltransferase SiRT from Sesamum indicum, UDP-L-rhamnose synthase AtRHM2, and 4-coumarate: coenzyme A ligase At4CL1 from Arabidopsis thaliana in a p-coumaric acid-overproducing S. cerevisiae strain. Third, the production of osmanthuside B was further enhanced by increasing the copy number of SiAT and AtRHM2 in genome and diverting L-tyrosine into tyrosol biosynthesis by introducing an aromatic aldehyde synthase PcAAS from Petroselinum crispum with a titer of 320.6 ± 59.3 mg/L. Fourth, the biosynthesis of verbascoside was accomplished by integrating genes CYP98A20 and AtCPR1 into the chromosomes of the osmanthuside B-producing strain, the titer reached 184.7 ± 5.7 mg/L. Furthermore, the overexpression of the glucose-6-phosphate dehydrogenase (ZWF1) led to significantly enhanced verbascoside production to 230.6 ± 11.8 mg/L. The strains were further engineered to produce echinacoside with a titer of 184.2 ± 11.2 mg/L. Finally, the fed-batch fermentation in a 5-L bioreactor yielded 4497.9 ± 285.2 mg/L of verbascoside or 3617.4 ± 117.4 mg/L of echinacoside. This work provides a crucial foundation for the green, industrial, and sustainable production of verbascoside and echinacoside and sets an initial point for the microbial production of other complex PhG derivatives.

Reliable Chemical Synthesis of UDP-3-O-[(R)-3-Hydroxydecanoyl]-GlcNAc: The Native Substrate of Pseudomonas aeruginosa LpxC

Herein, we describe a reliable and efficient approach for the first chemical synthesis of biologically significant and complex 3-O-(R-3-hydroxydecanoyl) modified uridine diphosphate N-acetylglucosamine that is the native substrate of LpxC involved in the biosynthesis of the cell wall of Pseudomonas aeruginosa. The synthetic protocol provides a successful example for the reliable preparation of modified nucleoside diphosphate sugar, which features judiciously selected protecting groups, the formation of pyrophosphate linkage with 5'-phosphate nucleoside as nucleophile, and the straightforward purification process.

Synthesis and pharmacological evaluation of nature-inspired phenacyl glycosides

Phenylethanoid glycosides are a well-studied class of bioactive compounds found throughout the plant kingdom. In contrast, research on the synthesis and pharmacological activity of phenacyl glycosides, a specific subgroup of phenylethanoid glycosides with a ketone functionality at the alpha position of the phenol ring, has been limited. In this study, we report the synthesis, cytotoxic, antiviral, and anti-inflammatory evaluation of a series of 18 4'-hydroxyphenacyl glycosides. These compounds consist of six different sugar residues (β-d-glucose, β-d-galactose, α-l-arabinose, β-d-xylose, α-l-rhamnose, and β-d-glucuronic acid) and display three distinct methoxylation patterns at the phenacyl ring, similar to the substitution motifs of anthocyanins. We obtained the target phenacyl glycosides in high yield and stereoselectivity through the coupling of benzoyl-protected trichloroacetimidate glycosyl donors and corresponding acetophenones. Our work represents the first total synthesis of the natural products 4'-hydroxyphenacyl-β-d-glucopyranoside (1) and 4'-hydroxy-3'-methoxyphenacyl-β-d-glucopyranoside (2). None of the phenacyl glycosides showed cytotoxicity against the tested cell lines. Notably, several of the synthesized compounds exhibited antiviral activity, with natural product 2 being the most active against herpes simplex virus type 1, while phenacyl arabinoside 9 and natural product 2 were the most active against human coronavirus OC43. Natural product 2 significantly inhibited the production of interleukin-6 in lipopolysaccharide-stimulated microglia cells. Overall, our findings highlight the importance of the sugar residue and phenacyl ring substitution pattern in modulating the antiviral activity of phenacyl glycosides. Natural product 2 and phenacyl arabinoside 9 emerge as promising leads for the development of antiviral agents.

Esterification and Etherification of Aliphatic Alcohols Enabled by Catalytic Strain-Release of Donor-Acceptor Cyclopropane

We herein disclose a highly efficient protocol for the esterification and etherification of alcohols, leveraging a Sc(OTf)3-catalyzed ring-strain release event in the meticulously designed, chromatographically stable mixed anhydrides or benzyl esters that incorporate an intramolecular donor-acceptor cyclopropane (DAC). This versatile method facilitates the straightforward functionalization of sugar, terpene, and steroid alcohols under mild acidic conditions, as showcased by the single-catalyst-driven, dual protection of sugar diol.

Complete biosynthesis of the phenylethanoid glycoside verbascoside

Verbascoside, which was first discovered in 1963, is a well-known phenylethanoid glycoside (PhG) that exhibits antioxidant, anti-inflammatory, antimicrobial, and neuroprotective activities and contributes to the therapeutic effects of many medicinal plants. However, the biosynthetic pathway of verbascoside remains to be fully elucidated. Here, we report the identification of two missing enzymes in the verbascoside biosynthesis pathway by transcriptome mining and in vitro enzymatic assays. Specifically, a BAHD acyltransferase (hydroxycinnamoyl-CoA:salidroside hydroxycinnamoyltransferase [SHCT]) was shown to catalyze the regioselective acylation of salidroside to form osmanthuside A, and a CYP98 hydroxylase (osmanthuside B 3,3'-hydroxylase [OBH]) was shown to catalyze meta-hydroxylations of the p-coumaroyl and tyrosol moieties of osmanthuside B to complete the biosynthesis of verbascoside. Because SHCTs and OBHs are found in many Lamiales species that produce verbascoside, this pathway may be general. The findings from the study provide novel insights into the formation of caffeoyl and hydroxytyrosol moieties in natural product biosynthetic pathways. In addition, with the newly acquired enzymes, we achieved heterologous production of osmanthuside B, verbascoside, and ligupurpuroside B in Escherichia coli; this work lays a foundation for sustainable production of verbascoside and other PhGs in micro-organisms.