Merging total synthesis and NMR technology for deciphering the realistic structure of natural 2,6-dideoxyglycosides

The structural identification and efficient synthesis of bioactive 2,6-dideoxyglycosides are daunting challenges. Here, we report the total synthesis and structural revision of a series of 2,6-dideoxyglycosides from folk medicinal plants Ecdysanthera rosea and Chonemorpha megacalyx, which feature pregnane steroidal aglycones bearing an 18,20-lactone and glycans consisting of 2,6-dideoxy-3-O-methyl-β-pyranose residues, including ecdysosides A, B, and F and ecdysantheroside A. All the eight possible 2,6-dideoxy-3-O-methyl-β-pyranoside stereoisomers (of the proposed ecdysantheroside A) have been synthesized that testify the effective gold(I)-catalyzed glycosylation methods for the synthesis of various 2-deoxy-β-pyranosidic linkages and lays a foundation via nuclear magnetic resonance data mapping to identify these sugar units which occur promiscuously in the present and other natural glycosides. Moreover, some synthetic natural compounds and their isomers have shown promising anticancer, immunosuppressive, anti-inflammatory, and anti-Zika virus activities.

Natural product P57 induces hypothermia through targeting pyridoxal kinase

Induction of hypothermia during hibernation/torpor enables certain mammals to survive under extreme environmental conditions. However, pharmacological induction of hypothermia in most mammals remains a huge challenge. Here we show that a natural product P57 promptly induces hypothermia and decreases energy expenditure in mice. Mechanistically, P57 inhibits the kinase activity of pyridoxal kinase (PDXK), a key metabolic enzyme of vitamin B6 catalyzing phosphorylation of pyridoxal (PL), resulting in the accumulation of PL in hypothalamus to cause hypothermia. The hypothermia induced by P57 is significantly blunted in the mice with knockout of PDXK in the preoptic area (POA) of hypothalamus. We further found that P57 and PL have consistent effects on gene expression regulation in hypothalamus, and they may activate medial preoptic area (MPA) neurons in POA to induce hypothermia. Taken together, our findings demonstrate that P57 has a potential application in therapeutic hypothermia through regulation of vitamin B6 metabolism and PDXK serves as a previously unknown target of P57 in thermoregulation. In addition, P57 may serve as a chemical probe for exploring the neuron circuitry related to hypothermia state in mice.

Chemical Synthesis of Saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Chemical synthesis of saponins: An update

Saponins, as secondary metabolites in terrestrial plants and marine invertebrate, constitute one of the largest families of natural products. The long history of folk medicinal applications of saponins makes them attractive candidates for innovative drug design and development. Chemical synthesis has become a practical alternative to the availability of the natural saponins and their modified analogs, so as to facilitate SAR studies and the discovery of optimal structures for clinical applications. The recent achievements in the synthesis of these complex saponins reflect the advancements of both steroid/triterpene chemistry and carbohydrate chemistry. This chapter provides an updated review on the chemical synthesis of natural saponins, covering the literature from 2014 to 2020.

Highly Efficient Synthesis of Digoxin

Taking advantage of the reliable stereocontrol capability of DMNPA group via long-distance-participation (LDP) effect as well as the mild and efficient deprotection conditions, the first and highly efficient synthesis of digoxin was achieved through a nine-step longest linear sequence with 41% overall yield.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Effects of Hoodia Parviflora on satiety, abdominal obesity and weight in a group of overweight subjects: a randomized, blinded, placebo-controlled trial

BACKGROUND

This study aimed to assess the effects of supplementation with Hoodia Parviflora (H. Parviflora) at 9 mg+200 mg of fructo-oligosaccharides on weight loss, body composition, hydration and satiety parameters.

METHODS

A randomized blinded controlled trial was conducted in a sample of 30 overweight and obese patients (5 males and 25 females). Patients were randomly assigned in 2 groups: the intervention group, which received H. Parviflora twice a day for 4 weeks and the control group, which received a placebo.

RESULTS

After a 4-week follow-up period, the study results showed an improvement of Δ=-1.632 kg (Confidence Interval [CI]95% -2.545; -0.719) and a statistically significant decrease in waist circumference (WC) compared with the placebo group -2.080 cm ([CI]95% -4.082; -0.078). The visual analogue scale reported an improvement of satiety sensation after day 5 (P=0.001).

CONCLUSIONS

This study shows for the first time the simultaneous effect of H. Parviflora on weight loss, decreasing satiety, and improving fat mass, in particular Visceral Adipose Tissue (VAT).

C-12 vs C-3 substituted bile salts: An example of the effects of substituent position and orientation on the self-assembly of steroid surfactant isomers

Biomolecule derivatives are transversally used in nanotechnology. Deciphering their aggregation behavior is a crucial issue for the rational design of functional materials. To this end, it is necessary to build libraries of selectively functionalized analogues and infer general rules. In this work we enrich the highly applicative oriented collection of steroid derivatives, by reporting a rare example of C-12 selectively modified bile salt. While nature often exploits such position to encode functions, it is unusual and not trivial to prepare similar analogues in the laboratory. The introduction of a tert-butyl phenyl residue at C-12 provided a molecule with a self-assembly that remarkably switched from rigid pole-like structures to twisted ribbons at a biologically relevant critical temperature (∼25 °C). The system was characterized by microscopy and spectroscopy techniques and compared with the C-3 functionalized analogue. The twisted ribbons generate samples with a gel texture and a viscoelastic response. The parallel analysis of the two systems suggested that the observed thermoresponsive self-assemblies occur at similar critical temperatures and are probably dictated by the nature of the substituent, but involve aggregates with different structures depending on position and orientation of the substituent. This study highlights the self-assembly properties of two appealing thermoresponsive systems. Moreover, it adds fundamental insights hereto missing in the investigations of the relation between self-assembly and structure of synthetic steroids, which are valuable for the rational design of steroidal amphiphiles.

Highly reactive 2-deoxy-2-iodo-d-allo and d-gulo pyranosyl sulfoxide donors ensure β-stereoselective glycosylations with steroidal aglycones

The preparation of well-defined d-xylo and d-ribo glycosides represents a synthetic challenge due to the limited configurational availability of starting materials and the laborious synthesis of homogeneous 2-deoxy-β-glycosidic linkages, in particular that of the sugar-steroid motif, which represents the “stereoselective determining step” of the overall synthesis. Herein we describe the use of 2-deoxy-2-iodo-glycopyranosyl sulfoxides accessible from widely available d-xylose and d-ribose monosaccharides as privileged glycosyl donors that permit activation at very low temperature. This ensures a precise kinetic control for a complete 1,2-trans stereoselective glycosylation of particularly challenging steroidal aglycones.

Highly stereoselective synthesis of challenging steroidal 2-deoxy-β-glycosides with d-xylo and d-ribo configurations enabled by low temperature activation of 2-deoxy-2-iodoglycopyranosyl sulfoxides.

Gold-catalyzed glycosylation in the synthesis of complex carbohydrate-containing natural products

Gold(i)- and gold(iii)-catalyzed glycosylation reactions and their application in the synthesis of natural glycoconjugates are reviewed.

Gold-catalyzed diversified synthesis of 3-aminosugar analogues of digitoxin and digoxin

A small library containing 3-aminosugar analogues of digitoxin and digoxin with potent anticancer activities was constructed by gold-catalyzed glycosylation.

Chemical synthesis of saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Total synthesis of periploside A, a unique pregnane hexasaccharide with potent immunosuppressive effects

Periploside A is a pregnane hexasaccharide identified from the Chinese medicinal plant Periploca sepium, which features a unique seven-membered formyl acetal bridged orthoester (FABO) motif and potent immunosuppressive activities. Here, we show the synthesis of this molecule in a total of 76 steps with the longest linear sequence of 29 steps and 9.2% overall yield. The FABO motif is constructed via a combination of Sinaÿ's and Crich's protocol for the formation of orthoester and acetal glycosides, respectively. The 2-deoxy-β-glycosidic linkages are assembled stereoselectively with judicious choice of the glycosylation methods. The epimer at the spiro-quaternary carbon in the FABO motif has also been elaborated in a stereo-controlled manner. This epimer, as well as the synthetic analogues bearing the FABO motif, retain largely the inhibitory activities of periploside A against the proliferation of T-lymphocyte, indicating the importance of the chemical connection of the FABO motif to their immunosuppressive activity.

Au(i) π-bis(tert-butyldimethylsilyl)acetylene triphenylphosphine complex, an effective pre-catalyst for Au(i)-catalyzed reactions

A [Ph₃PAu]⁺ η²-alkyne π complex is found to be shelf-stable and effective for a range of the Au(i)-catalyzed alkyne π activation reactions.

ortho-(Methyltosylaminoethynyl)benzyl glycosides as new glycosyl donors for latent-active glycosylation

A new glycosylation protocol with ortho-(methyltosylaminoethynyl)benzyl glycosides as donors is disclosed.

O-Glycosylation methods in the total synthesis of complex natural glycosides

We highlight the total syntheses of 33 complex natural O-glycosides, with a particular focus on the O-glycosylation methods that enable the connection of the saccharides and aglycones.

Molecular matchmaking between the popular weight-loss herb Hoodia gordonii and GPR119, a potential drug target for metabolic disorder

African cactiform Hoodia gordonii (Asclepiadaceae) has been used for thousands of years by Xhomani Bushmen as an anorexant during hunting trips and has been proposed as a new agent for the management of body weight. However, its in vivo targets and molecular mechanisms remain elusive. GPR119, a G protein-coupled receptor highly expressed in pancreatic β cells and intestinal L cells, has been demonstrated to facilitate glucose-stimulated insulin secretion (GSIS) and represents a novel and attractive target for the therapy of metabolic disorders. Here, we disclose that Gordonoside F (a steroid glycoside isolated from H. gordonii), but not the widely known P57, activates specifically GPR119. Successful synthesis of Gordonoside F facilitates further characterization of this compound. Gordonoside F promotes GSIS both in vitro and in vivo and reduces food intake in mice. These effects are mediated by GPR119 because GPR119 knockout prevents the therapeutic effects of Gordonoside F. Interestingly, the appetite-suppressing effect of Hoodia extract was also partially blocked by GPR119 knockout. Our results demonstrate for the first time, to our knowledge, that GPR119 is a direct target and one of the major mechanisms underlying the therapeutic effect of the popular "weight loss" herb H. gordonii. Given the long history of safe application of this herb in weight control, it is foreseeable that the novel scaffold of Gordonoside F provides a promising opportunity to develop new drugs in treating metabolic diseases.

Total synthesis of starfish saponin goniopectenoside B

Star quality: Goniopectenoside B, a minor asterosaponin from starfish Goniopecten demonstrans with antifouling activity, has been synthesized in a convergent 21 steps and in 4.3 % overall yield starting from adrenosterone. This represents the first synthesis of a complex asterosaponin, which are ubiquitous and characteristic in starfish as defense chemicals (see figure).