Rapid spectrophotometric detection for optimized production of landomycins and characterization of their therapeutic potential.

Microbial-derived natural products remain a major source of structurally diverse bioactive compounds and chemical scaffolds that have the potential as new therapeutics to target drug-resistant pathogens and cancers. In particular, genome mining has revealed the vast number of cryptic or low-yield biosynthetic gene clusters in the genus Streptomyces. However, low natural product yields-improvements to which have been hindered by the lack of high throughput methods-have slowed the discovery and development of many potential therapeutics.

Origins of Selectivity in Glycosylation Reactions with Saccharosamine Donors.

A combination of DFT calculations and experiments is used to describe how the selection of a promoter can control the stereochemical outcome of glycosylation reactions with the deoxy sugar saccharosamine. Depending on the promoter, either α- or β-linked reactive intermediates are formed. These studies show that differential modes of activation lead to the formation of distinct intermediates that undergo highly selective reactions through an S2-like mechanism.

Rapid spectrophotometric detection for optimized production of landomycins and characterization of their therapeutic potential.

Microbial derived natural products remain a major source of structurally diverse bioactive compounds and chemical scaffolds that have potential as new therapeutics to target drug resistant pathogens and cancers. In particular, genome mining has revealed the vast number of cryptic or low yield biosynthetic gene clusters in the genus . Here, we describe our efforts to improve yields of landomycins - angucycline family polyketides under investigation as cancer therapeutics - by a genetically modified 136.

Insights into the Biological Properties of Ligands and Identity of Operator Site for LanK Protein Involved in Landomycin Production.

LanK is a TetR type regulatory protein that coordinates the late steps of the biosynthesis of the landomycin family of antitumor angucyclic polyketides and their export from the cells of Streptomyces cyanogenus S136. We recently described the structure of LanK and showed that it is the carbohydrate portion of the landomycins that is responsible for abrogating the repressing effect of LanK on landomycin production and export. The effect has been established in a series of in vitro tests using synthetic analogs of the landomycin carbohydrate chains.

Synthetic Approach to 2,4-Diamino-2,4,6-trideoxyhexoses (DATDH): Bacterial and Rare Deoxy-Amino Sugars.

A synthetic route to 2,4-diamino-2,4,6-trideoxysugar stereoisomers in 6-7 steps and 22-33% overall yield is described. A key step in this pathway is the carbonyl coupling of d- and l-threoninol or d- and l--threoninol to a phthalimido-allene mediated by chiral iridium-H-BINAP, which allows for installation of two new chiral centers in one, highly diastereoselective (>20:1 dr) step. This approach provides a more concise, diastereoselective, and versatile method to access these deoxy-amino sugars than is currently available.

Application of Redox-Active Ester Catalysis to the Synthesis of Pyranose Alkyl -Glycosides.

The direct coupling of shelf-stable, tetrachloro--hydroxyphthalimide ester (TCNHPI) glycosyl donors with a variety of alkylzinc reagents under redox catalysis is described. Alkyl -glycosides are formed directly by a decarboxylative, Negishi-type process in 31-73% yields without the need for photocatalytic activation or additional reductants. Extension of this approach to the coupling of TCNHPI donors with stereodefined α-alkoxy furan-containing alkylzinc halides enabled synthesis of methylene-linked -disaccharides via an Achmatowicz rearrangement.

Synthesis of the Branched Tetrasaccharide Fragment of Saccharomicin A.

A synthesis of the branched tetrasaccharide fragment of saccharomicin A using 1-OTBS donors to stereoselectively synthesize both α- and β-linked disaccharides is reported. The disaccharides were united using BSP/TfO to afford the tetrasaccharide fragment as a single α-anomer in 72% yield. This branched tetrasaccharide fragment can be used as donor and acceptor species to synthesize larger fragments of saccharomicin A.

New chemical processes to streamline carbohydrate synthesis.

Carbohydrates hold potential for the future of therapeutic development due to their important role in essential biological processes. However, it is still challenging to produce homogenous materials, especially for non-mammalian sugars that are considered rare. Recent developments in this field have focused on catalytic methods, including organometallic and organocatalytic approaches to regioselective functionalization.

The carbohydrate tail of landomycin A is responsible for its interaction with the repressor protein LanK.

Landomycin A (LaA) is the largest member of the landomycin group of angucyclic polyketides. Its unusual structure and strong anticancer properties have attracted great interest from chemists and biologists alike. This, in particular, has led to a detailed picture of LaA biosynthesis in Streptomyces cyanogenus S136, the only known LaA producer.

Synthesis of the α-Linked Digitoxose Trisaccharide Fragment of Kijanimicin: An Unexpected Application of Glycosyl Sulfonates.

Previously, we demonstrated that glycosyl tosylates are effective for the synthesis of β-glycosides of gluco-configured 2-deoxy sugars. Here, we show the same sulfonate system can be used for the selective synthesis of α-glycosides containing the allo-configured 2-deoxy sugar digitoxose. As with previous work, optimal selectivity is obtained through matching the donor with the appropriate arylsulfonyl chloride promoter.

The Crossroads of Glycoscience, Infection, and Immunology.

Advances in experimental capabilities in the glycosciences offer expanding opportunities for discovery in the broad areas of immunology and microbiology. These two disciplines overlap when microbial infection stimulates host immune responses and glycan structures are central in the processes that occur during all such encounters. Microbial glycans mediate host-pathogen interactions by acting as surface receptors or ligands, functioning as virulence factors, impeding host immune responses, or playing other roles in the struggle between host and microbe.

Automated, Multistep Continuous-Flow Synthesis of 2,6-Dideoxy and 3-Amino-2,3,6-trideoxy Monosaccharide Building Blocks.

An automated continuous flow system capable of producing protected deoxy-sugar donors from commercial material is described. Four 2,6-dideoxy and two 3-amino-2,3,6-trideoxy sugars with orthogonal protecting groups were synthesized in 11-32 % overall yields in 74-131.5 minutes of total reaction time.

Glycosyl Sulfonates Beyond Triflates.

While glycosyl triflates are frequently invoked as intermediates in many chemical glycosylation reactions, the chemistry of other glycosyl sulfonates remains comparatively underexplored. Given the reactivity of sulfonates can span several orders of magnitude, this represents an untapped resource for the development of stereoselective glycosylation reactions. This personal account describes our laboratories efforts to take advantage of this reactivity to develop β-specific glycosylation reactions.

Modular continuous flow synthesis of orthogonally protected 6-deoxy glucose glycals.

An efficient, modular continuous flow process towards accessing two orthogonally protected glycals is described with the development of reaction conditions for several common protecting group additions in flow, including the addition of benzyl, naphthylmethyl and tert-butyldimethylsilyl ethers. The process affords the desired target compounds in 57-74% overall yield in just 21-37 minutes of flow time. Furthermore, unlike batch conditions, the flow processes avoided the need for active cooling to prevent unwanted exotherms and required shorter reaction times.

Reagent-Controlled α-Selective Dehydrative Glycosylation of 2,6-Dideoxy Sugars: Construction of the Arugomycin Tetrasaccharide.

The first synthesis of the tetrasaccharide fragment of the anthracycline natural product Arugomycin is described. A reagent controlled dehydrative glycosylation method involving cyclopropenium activation was utilized to synthesize the α-linkages with complete anomeric selectivity. The synthesis was completed in 20 total steps, and in 2.

Versatile Glycosyl Sulfonates in β-Selective C-Glycosylation.

C-Glycosides are both a common motif in many bioactive natural products and important glycoside mimetics. We demonstrate that activating a hemiacetal with a sulfonyl chloride, followed by treating the resultant glycosyl sulfonate with an enolate results in the stereospecific construction of β-linked C-glycosides. This reaction tolerates a range of acceptors and donors, including disaccharides.

Rapid Preparation of 2,6-Dideoxy Sugar Libraries through Gold-Catalyzed Homopropargyl Orthoester Cyclization.

A flexible route capable of producing libraries of 2,6-dideoxy sugars is described. We have found that Au(JackiePhos)SbFMeCN promotes the conversion of homopropargyl orthoesters into functionalized 2,3-dihydro-4-pyran-4-ones in good to excellent yields (71-90%). These latter compounds can be easily converted into a number of otherwise difficult to access 2,6-dideoxy sugars.

Matching Glycosyl Donor Reactivity to Sulfonate Leaving Group Ability Permits S2 Glycosylations.

Here we demonstrate that highly β-selective glycosylation reactions can be achieved when the electronics of a sulfonyl chloride activator and the reactivity of a glycosyl donor hemiacetal are matched. While these reactions are compatible with the acid- and base-sensitive protecting groups that are commonly used in oligosaccharide synthesis, these protecting groups are not relied upon to control selectivity. Instead, β-selectivity arises from the stereoinversion of an α-glycosyl arylsulfonate in an S2-like mechanism.

Recent Developments in Stereoselective Chemical Glycosylation.

Because of their pivotal biological functions, attention to sugars and glycobiology has grown rapidly in recent decades, leading to increased demand for homogeneous oligosaccharides. The stereoselective preparation of oligosaccharides by chemical means remains challenging and continues to be a vivid research area for organic chemists. In the past decade, new approaches and reinvestigated traditional methods have transformed the field.

Reagent Controlled Direct Dehydrative Glycosylation with 2-Deoxy Sugars: Construction of the Saquayamycin Z Pentasaccharide.

The first synthesis of the pentasaccharide fragment of the angucycline antibiotic saquayamycin Z is described. By using our sulfonyl chloride mediated reagent controlled dehydrative glycosylation, we are able to assemble the glycosidic linkages with high levels of anomeric selectivity. The total synthesis was completed in 25 total steps, and in 2.

Synthesis of the Hexasaccharide Fragment of Landomycin A Using a Mild, Reagent-Controlled Approach.

The synthesis of the hexasaccharide fragment of landomycin A is reported. Using p-toluenesulfonyl chloride mediated dehydrative glycosylation, we constructed the deoxy-sugar linkages in a stereoselective fashion without the need for temporary prosthetic groups to control selectivity. Through this approach, the hexasaccharide was obtained in 28 steps and 8.

Synthesis of the Non-Reducing Hexasaccharide Fragment of Saccharomicin B.

A synthesis of the nonreducing end hexasaccharide of saccharomicin B, α-l-Eva-(1→4)-α-l-Eva-(1→4)-α-l-Dig-(1→4)-α-l-Eva-(1→4)-α-l-Dig-(1→4)-β-d-Fuc, has been developed. Selective glycosylations of l-digitoxose (l-Dig) using AgPF/TTBP-mediated thioether activation and l-4-e pi-vancosamine (l-Eva) using TfO/DTBMP-mediated sulfoxide activation produced the hexasaccharide as a single diastereomer in very good yield. This hexasaccharide is properly functionalized to serve as a glycosyl donor for the total synthesis of saccharomicin B.

Stereospecific Synthesis of the Saccharosamine-Rhamnose-Fucose Fragment Present in Saccharomicin B.

A synthetic route has been developed for constructing the d-saccharosamine-l-rhamnose-d-fucose (Sac-Rha-Fuc) trisaccharide fragment present in the antibacterial natural product saccharomicin B. The Sac monosaccharide was synthesized through a modified nine step procedure starting from d-rhamnal in 23% overall yield. 1- O-TBS Sac donors were used to construct the β-linked Sac-Rha disaccharide.