Nucleophilic catalysis of MeON-neoglycoside formation by aniline derivatives.

Neoglycosylations are increasingly being employed in the synthesis of natural products, drug candidates, glycopeptide mimics, oligosaccharide analogues, and other applications, but the efficiency of these reactions is usually limited by slow reaction times. Here, we show that aniline derivatives such as 2-amino-5-methoxybenzoic acid enhance the rate of acid-catalyzed neoglycosylation for a range of sugar substrates up to a factor of 32 relative to the uncatalyzed reaction.

Amphimedosides A-C: synthesis, chemoselective glycosylation, and biological evaluation.

The amphimedosides, discovered in 2006, are the first examples of naturally occurring glycosylated alkoxyamines. We report syntheses of amphimedosides A-C that feature a stereoselective oxyamine neoglycosylation and found that these alkaloids display modest cytotoxicity toward seven diverse human cancer cell lines, exhibiting IC(50) values ranging from 3.0 µM to greater than 100 µM.

Generation of a glycosylated asparagine residue through chemoselective acylation of a glycosylhydrazide.

Herein, we report the first selective anomeric N-acylation of a glycosylhydrazide. We show that this transformation can be harnessed to generate amino acid building blocks including FmocAsn(GlcNAc)OH (1), a residue that has been previously shown to be a competent reagent in the solid-phase peptide synthesis of N-linked glycopeptides.

Modifying the glycosidic linkage in digitoxin analogs provides selective cytotoxins.

A chemoselective reaction between oxyamines and unprotected, unactivated reducing sugars was used to construct for the first time a panel of linkage-diversified neoglycosides. This panel of digitoxin analogs included potent and selective tumor cytotoxins; cytotoxicity was dependent on the structure of the glycosidic linkage. These results validate linkage diversification through neoglycosylation as a unique and simple strategy to powerfully complement existing methods for the optimization of glycoconjugates.

'Sweetening' natural products via glycorandomization.

In an effort to explore the contribution of the sugar constituents of pharmaceutically relevant glycosylated natural products, chemists have developed glycosylation methods for the generation of 'glycorandomized' libraries. Each member of these libraries is uniquely differentiated by an attached carbohydrate. Recently, two complementary glycorandomization strategies have emerged: chemoenzymatic glycorandomization, a biocatalytic approach dependent upon the substrate promiscuity of enzymes to activate and attach sugars to natural products, and neoglycorandomization, an efficient one-step chemical sugar ligation reaction that does not require prior sugar protection or activation. These strategies are likely to have a significant impact on fundamental glycoscience and drug discovery.

Effects of alternative side chain pairings and reverse turn sequences on antiparallel sheet structure in beta-peptide hairpins.

[structure: see text] We describe a series of beta-peptide hexamers that allow us to explore relationships between sequence and hairpin folding. Different reverse turn segments are compared at the central two positions, and the outer residues allow a variety of interstrand side chain-side chain pairings. NMR analysis in methanol demonstrates that several reverse turn and side chain pairing arrangements are compatible with antiparallel beta-peptide sheet structure; however, none of the beta-peptides folds in water.

Synthesis and biological evaluation of RON-neoglycosides as tumor cytotoxins.

Cardenolides such as digitoxin have been shown to inhibit cancer cell growth, to reduce cancer metastasis, and to induce apoptosis in tumor cells. Among the most potent digitoxin-based cytotoxins identified to date are MeON-neoglycosides generated via oxyamine neoglycosylation. Here, we report our studies of oxyamine neoglycosylation aimed at facilitating the elucidation of linkage-diversified digitoxin neoglycoside structure-activity relationships. We identified conditions suitable for the convenient synthesis of digitoxin neoglycosides and found that sugar structure, rather than RON-glycosidic linkage, exerts the strongest influence on neoglycoside yield and stereochemistry. We synthesized a library of digitoxin neoglycosides and assessed their cytotoxicity against eight human cancer cell lines. Consistent with previous findings, our data show that the structure of RON-neoglycosidic linkages influences both the potency and selectivity of digitoxin neoglycosides.

A Direct Comparison of the Anticancer Activities of Digitoxin MeON-Neoglycosides and O-Glycosides: Oligosaccharide Chain Length-Dependent Induction of Caspase-9-Mediated Apoptosis.

Digitoxin is a cardiac glycoside currently being investigated for potential use in oncology. While a number of structure-activity relationship studies have been conducted, an investigation of anticancer activity as a function of oligosaccharide chain length has not yet been performed. We generated mono-, di-, and tri-O-digitoxoside derivatives of digitoxin and compared their activity to the corresponding MeON-neoglycosides. Both classes of cardenolide derivatives display comparable oligosaccharide chain length-dependent cytotoxicity toward human cancer cell lines. Further investigation revealed that both classes of compounds induce caspase-9-mediated apoptosis in non-small cell lung cancer cells (NCI-H460). Since O-glycosides and MeON-neoglycosides share a similar mode of action, the convenience of MeON-neoglycosylation could be exploited in future SAR work to rapidly survey large numbers of carbohydrates to prioritize selected O-glycoside candidates for traditional synthesis.

Neoglycorandomization and chemoenzymatic glycorandomization: two complementary tools for natural product diversification.

In an effort to explore the contribution of the sugar constituents of pharmaceutically relevant glycosylated natural products, chemists have developed glycosylation methods that are amenable to the generation of libraries of analogues with a broad array of glycosidic attachments. Recently, two complementary glycorandomization strategies have been described, namely, neoglycorandomization, a chemical approach based on a one-step sugar ligation reaction that does not require any prior sugar protection or activation, and chemoenzymatic glycorandomization, a biocatalytic approach that relies on the substrate promiscuity of enzymes to activate and attach sugars to natural products. Since both methods require reducing sugars, this review first highlights recent advances in monosaccharide generation and then follows with an overview of recent progress in the development of neoglycorandomization and chemoenzymatic glycorandomization.

Enhancing the anticancer properties of cardiac glycosides by neoglycorandomization.

Glycosylated natural products are reliable platforms for the development of many front-line drugs, yet our understanding of the relationship between attached sugars and biological activity is limited by the availability of convenient glycosylation methods. When a universal chemical glycosylation method that employs reducing sugars and requires no protection or activation is used, the glycorandomization of digitoxin leads to analogs that display significantly enhanced potency and tumor specificity and suggests a divergent mechanistic relationship between cardiac glycoside-induced cytotoxicity and Na+/K+-ATPase inhibition. This report highlights the remarkable advantages of glycorandomization as a powerful tool in glycobiology and drug discovery.

The correct space group of NaPF6 x H2O.

The structure of sodium hexafluorophosphate monohydrate, NaPF(6) x H(2)O, has been inadvertently redetermined, revealing that the previously reported space group, Imma, was assigned incorrectly, with the a and b axes interchanged. The correct space group is Pnna. The program PLATON [Spek (2003). J. Appl. Cryst. 36, 7-13] suggested both Imma and Pmma as possible space groups, but only Pnna is consistent with the systematic absences. The inter-ionic and hydrogen-bonding interactions in the lattice form a three-dimensional network.

Preparation of protected syn-alpha,beta-dialkyl beta-amino acids that contain polar side chain functionality.

We report the synthesis of syn-alpha,beta-dialkyl beta-amino acid derivatives suitably protected for solid-phase synthesis that give rise to residues containing positively charged lysine-like side chains. These amino acids, as well as syn-alpha,beta-dialkyl beta-amino acids that contain diverse hydrophobic side chains, are prepared in good de and ee. The key step in this route involves Davies's protocol for the conjugate addition of a chiral lithium amide to alpha,beta-unsaturated tert-butyl esters (Davies, S. G.; Ichihara, O.; Walters, I. A. S. J. Chem. Soc., Perkin Trans. 1 1994, 9, 1141). syn-alpha,beta-Dialkyl beta-amino acids are interesting building blocks because of their sheet-forming propensity and because of their presence in bioactive compounds.

Combinatorial chemoenzymatic strategies for in vitro glycorandomization: Efforts toward antibiotic optimization.

Extract: The natural product pool, which includes many glycosylated secondary metabolites, is the source of over half of the world's drug leads. For example, the antibiotics vancomycin and erythromycin, the antitumor compounds bleomycin and doxorubicin, and the antifungal agents amphotericin and nystatin all contain essential sugar attachments. Carbohydrate groups of natural product-based drugs have long been known to generally influence pharmacokinetic properties and there is an increasing recognition that these carbohydrate appendages also play a key role in drug-target interactions. These findings suggest that the alteration of glycosylation patterns on secondary metabolites is a potential strategy for the generation of novel therapeutics. We postulate that combinatorial chemoenzymatic strategies toward glycorandomized natural products will be able to overcome the limitations associated with total synthesis (time, protecting group manipulations, regio- and stereo-selectivity problems) and with in vivo pathway engineering (structural complexity, pathway bias, toxicity) by combining the advantages of enzymatic catalysis (efficiency, possibility of fermentation-based scale-up) with the power of chemical synthesis (unlimited diversity). We have recently demonstrated the power of in vitro glycorandomization (IVG) using vancomycin as the model.