Novel "reverse Kahne-type glycosylation": access to O-, N-, and C-linked epipodophyllotoxin conjugates

Overview of the current procedures in synthesis of heparin saccharides

Natural heparin, a glycosaminoglycan consisting of repeating hexuronic acid and glucosamine linked by 1 → 4 glycosidic bonds, is the most widely used anticoagulant. To subvert the dependence on animal sourced heparin, alternative methods to produce heparin saccharides, i.e., either heterogenous sugar chains similar to natural heparin, or structurally defined oligosaccharides, are becoming hot subjects. Although the success by chemical synthesis of the pentasaccharide, fondaparinux, encourages to proceed through a chemical approach generating homogenous product, synthesizing larger oligos is still cumbersome and beyond reach so far. Alternatively, the chemoenzymatic pathway exhibited exquisite stereoselectivity of glycosylation and regioselectivity of modification, with the advantage to skip the tedious protection steps unavoidable in chemical synthesis. However, to a scale of drug production needed today is still not in sight. In comparison, a procedure of de novo biosynthesis in an organism could be an ultimate goal. The main purpose of this review is to summarize the current available/developing strategies and techniques, which is expected to provide a comprehensive picture for production of heparin saccharides to replenish or eventually to replace the animal derived products. In chemical and chemoenzymatic approaches, the methodologies are discussed according to the synthesis procedures: building block preparation, chain elongation, and backbone modification.

A Strain-Promoted Divergent Chemical Steroidation Unveils Potent Anti-Inflammatory Pseudo-Steroidal Glycosides

The development of novel agents with immunoregulatory effects is a keen way to combat the growing threat of inflammatory storms to global health. To synthesize pseudo-steroidal glycosides tethered by ether bonds with promising immunomodulatory potential, we develop herein a highly effective deoxygenative functionalization of a novel steroidal donor (steroidation) facilitated by strain-release, leveraging cost-effective and readily available Sc(OTf)3 catalysis. This transformation produces a transient steroid-3-yl carbocation which readily reacts with O-, C-, N-, S-, and P-nucleophiles to generate structurally diverse steroid derivatives. DFT calculations were performed to shed light on the mechanistic details of the regioselectivity, underlying an acceptor-dependent steroidation mode. This approach can be readily extended to the etherification of sugar alcohols to enable the achievement of a diversity-oriented, pipeline-like synthesis of pseudo-steroidal glycosides in good to excellent yields with complete stereo- and regiospecific control for anti-inflammatory agent discovery. Immunological studies have demonstrated that a meticulously designed cholesteryl disaccharide can significantly suppress interleukin-6 secretion in macrophages, exhibiting up to 99% inhibition rates compared to the negative control. These findings affirm the potential of pseudo-steroidal glycosides as a prospective category of lead agents for the development of novel anti-inflammatory drugs.

2, 4, 5-Trideoxyhexopyranosides Derivatives of 4'- Demethylepipodophyllotoxin: De novo Synthesis and Anticancer Activity

BACKGROUND

Podophyllotoxin is a natural lignan which possesses anticancer and antiviral activities. Etoposide and teniposide are semisynthetic glycoside derivatives of podophyllotoxin and are increasingly used in cancer medicine.

OBJECTIVE

The present work was aimed to design and synthesize a series of 2, 4, 5-trideoxyhexopyranosides derivatives of 4'-demethylepipodophyllotoxin as novel anticancer agents.

METHODS

A divergent de novo synthesis of 2, 4, 5-trideoxyhexopyranosides derivatives of 4'-demethylepipodophyllotoxin has been established via palladium-catalyzed glycosylation. The abilities of synthesized glycosides to inhibit the growth of A549, HepG2, SH-SY5Y, KB/VCR and HeLa cancer cells were investigated by MTT assay. Flow cytometric analysis of cell cycle with propidium iodide DNA staining was employed to observe the effect of compound 5b on cancer cell cycle.

RESULTS

Twelve D and L monosaccharides derivatives 5a-5l have been efficiently synthesized in three steps from various pyranone building blocks employing de novo glycosylation strategy. D-monosaccharide 5b showed highest cytotoxicity on five cancer cell lines with the IC50 values from 0.9 to 6.7 mM. It caused HepG2 cycle arrest at G2/M phase in a concentration-dependent manner.

CONCLUSION

The present work leads to the development of novel 2, 4, 5-trideoxyhexopyranosides derivatives of 4'- demethylepipodophyllotoxin. The biological results suggested that the replacement of the glucosyl moiety of etoposide with 2, 4, 5-trideoxyhexopyranosyl is favorable to their cytotoxicity. D-monosaccharide 5b caused HepG2 cycle arrest at G2/M phase in a concentration-dependent manner.

Enantioselective total synthesis of (+)-galbulin via organocatalytic domino Michael-Michael-aldol condensation

A concise and practical enantioselective synthesis of (+)-galbulin has been achieved using organocatalytic domino Michael-Michael-aldol condensation and organocatalytic kinetic resolution as the key steps.

Studies on the stereoselective synthesis of C-allyl glycosides

An investigation was carried out to explore the use of sulfoxide donors as common precursors to stereoisomeric C-glycoconjugates of glycoprotein and glycolipid tumor antigens. A study focusing on the effects of reaction conditions and substrate structure on the stereoselectivity of allylation was carried out. Although conditions were realized to selectively afford alpha-allylation products in good to excellent yields, the search for conditions favoring beta-selectivity proved less successful.

Podophyllotoxin: distribution, sources, applications and new cytotoxic derivatives

Several podophyllotoxin derivatives modified in the A, B, C, D and E rings were prepared from podophyllotoxin and methyl isoxazopodophyllic acid and evaluated for their cytotoxicity on several neoplastic cell lines. Chemical transformations performed on these compounds have yielded derivatives more potent and more selective that the parent compound. Most of the compounds maintained their cytotoxicity at the microM level. Distribution, biosynthesis, production, biotechnology, applications and synthesis have also been reviewed.

A concise stereocontrolled formal total synthesis of (+/-)-podophyllotoxin using sulfoxide chemistry

A short stereoselective formal total synthesis of (+/-)-podophyllotoxin has been carried out from a sulfoxide, using a one-pot tandem conjugate addition/aldol/electrophilic aromatic substitution reaction to form a tetralin, which was converted into picropodophyllin in two steps.

Efficient synthesis of glycosylated phenazine natural products and analogs with DISAL (methyl 3,5-dinitrosalicylate) glycosyl donors

Inspired by the occurrence and function of phenazines in natural products, new glycosylated analogs were designed and synthesized. DISAL (methyl 3,5-dinitrosalicylate) glycosyl donors were used in an efficient and easily-handled glycosylation protocol compatible with combinatorial chemistry. Benzoylated D-glucose, D-galactose and L-quinovose DISAL glycosyl donors were synthesized in high yields and used under mild conditions to glycosylate methyl saphenate and 2-hydroxyphenazine. The glycosides were screened for biological activity and one compound showed inhibitory activity towards topoisomerase II.