Efforts toward the first total synthesis of a mixed-ligand siderophore: Gobichelin-A.

The synthesis of Gobichelin-A, a naturally occurring mixed-ligand siderophore isolated from Streptomyces sp. NRRL F-4415, is described. The target molecule was planned to be synthesized by a convergent process involving the combination of two halves, Gob-A 1st half and Gob-A 2nd half, at the prefinal stage of the synthetic route. By adopting this method, fully protected Gobichelin-A was synthesized in excellent yield.

Synthesis of Hexenuloses and a Library of Disaccharides Possessing 3-oxo-glycal Unit.

An expeditious method for the synthesis of monosaccharides and disaccharides possessing 3-oxo-glycal units is revealed. Several monosaccharides and disaccharide-derived glycals are converted to the corresponding hexenuloses in three steps involving halo-alkoxylation, dehydrohalogenation, and ketalyzation reactions. A number of 3-oxo-glycals are synthesized to show the methodology's importance and generality. Further, the protocol is successfully applied to synthesize a rare-sugar disaccharide donor unit present as part of the trisaccharide moiety in the reported natural product, versipelostatin.

The first total synthesis of gobichelin B: a mixed-ligand siderophore of Streptomyces sp. NRRL F-4415.

The first total synthesis of a structurally diverse mixed ligand siderophore, gobichelin B produced by Streptomyces sp. NRRL F-4415, is reported. The systematic assembly of the building blocks to synthesize Gob-B 1st half and Gob-B 2nd half and subsequent coupling of these two fragments followed by global deprotection using Pearlman's catalyst led to the isolation of gobichelin B in excellent yield and purity.

Stereoselective Synthesis of 2-(ß-C-Glycosyl)glycals: Access to Unusual ß-C-Glycosides from 3-Deoxyglycals.

A novel method for the highly stereoselective synthesis of ß-(1→2)-C-saccharides employing 3-deoxy- and 3-C-branched glycals as hermaphroditic substrates is revealed. The generality of the C-C bond formation reaction between the two sugar units is evaluated. The developed methodology is successfully applied to the synthesis of biologically significant subunits that are present in various natural products, which include mixed C-disaccharides with adjacent THP-THF rings, C-aryl glycosides, and highly functionalized ß-C-glycosides.

The first stereoselective total synthesis of neosemburin and isoneosemburin.

The Claisen rearrangement of sugar derived allyl vinyl ether, Wittig olefination and intramolecular acetalization reactions were used as key steps in the first stereoselective total synthesis of neosemburin and isoneosemburin through the formation of a 3-C-branched sugar precursor as a key intermediate.

A one-pot septanoside formation and glycosylation of acyclic dithioacetals derived from 1,2-cyclopropanated sugars.

Ring opening of 3-oxo-1,2-cyclopropanated sugars with thiols leads to the serendipitous discovery of the synthesis of sugar based homologated acyclic dithioacetals. These acyclic dithioacetals were found to undergo one-pot septanoside formation followed by stereoselective glycosylation in the presence of glycosyl acceptors under glycosylation reaction conditions.

Ring-contraction vs ring-expansion reactions of spiro-cyclopropanecarboxylated sugars.

Electrophilic ring opening of spiro-cyclopropanecarboxylated sugars followed by reaction with DBU revealed interesting ring-contraction and ring-expansion reactions depending on the substrate and the kind of hydroxyl protective group present adjacent to the spiro center. A stereoselective method for accessing a new class of carbon chain extended keto-furanoses and C-glycosylated bicyclic compounds is reported.

A ring expansion-glycosylation strategy toward the synthesis of septano-oligosaccharides.

A one-pot ring-expansion-glycosylation reaction was performed using 1,2-cyclopropanated sugars as glycosyl donors and carbohydrate O-nucleophiles as acceptors to provide septanohexose mimics of pyranose and furanose derivatives. The methodology was successfully extended to the synthesis of septano-oligosaccharides by adopting a divergent strategy as well as an iterative protocol.

Stereoselective synthesis of sugar fused ß-disubstituted γ-butyro-lactones: C-spiro-glycosides from 1,2-cyclopropanecarboxylated sugars.

A stereoselective methodology was developed for the construction of C-spiro-glycosides in two steps involving bromonium ion activated solvolytic ring opening of sugar derived 1,2-cyclopropanecarboxylates followed by a one-pot dehydrohalogenation, intramolecular hetero-Michael addition (IHMA) and ester hydrolysis. The obtained spirocyclic lactols were further converted to enantiomerically pure spirolactones.

In vitro bacterial polysaccharide biosynthesis: defining the functions of Wzy and Wzz.

Polysaccharides constitute a major component of bacterial cell surfaces and play critical roles in bacteria-host interactions. The biosynthesis of such molecules, however, has mainly been characterized through in vivo genetic studies, thus precluding discernment of the details of this pathway. Accordingly, we present a chemical approach that enabled reconstitution of the E. coli O-polysaccharide biosynthetic pathway in vitro. Starting with chemically prepared undecaprenyl-diphospho-N-acetyl-D-galactosamine, the E. coli O86 oligosaccharide repeating unit was assembled by means of sequential enzymatic glycosylation. Successful expression of the putative polymerase Wzy using a chaperone coexpression system then allowed demonstration of polymerization in vitro using this substrate. Analysis of more substrates revealed a defined mode of recognition for Wzy toward the lipid moiety. Specific polysaccharide chain length modality was furthermore demonstrated to result from the action of Wzz. Collectively, polysaccharide biosynthesis was chemically reconstituted in vitro, providing a well defined system for further underpinning molecular details of this biosynthetic pathway.

Novel chalcogenides of thymidine and uridine: synthesis, properties and applications.

A facile and efficient methodology has been developed for the synthesis of dithymidine and di-uridine derived disulfides using benzyltriethylammonium tetrathiomolybdate as a sulfur transfer reagent. However, a similar reaction of thymidine derivative with tetraethylammonium tetraselenotungstate as a selenium transfer reagent resulted in the formation of an unexpected cyclic diselenide. The disulfide derivatives of nucleosides have been used as precursors in a tandem disulfide cleavage-Michael addition/ring opening reactions to construct aminoacid and carbocyclic derivatives of nucleosides.

Efficient synthesis of fused perhydrofuro[2,3-b]pyrans (and furans) by ring opening of 1,2-cyclopropanated sugar derivatives.

[structure: see text]. An efficient method has been developed for the construction of fused perhydrofuro[2,3-b]pyrans by diastereoselective ring opening of 1,2-cyclopropanated sugar derivatives. The methodology has been successfully applied to the synthesis of fused perhydrofuro[2,3-b]pyrano-gamma-butyrolactone derivatives.

Structure-based design of novel HIV-1 protease inhibitors to combat drug resistance.

Structure-based design and synthesis of novel HIV protease inhibitors are described. The inhibitors are designed specifically to interact with the backbone of HIV protease active site to combat drug resistance. Inhibitor 3 has exhibited exceedingly potent enzyme inhibitory and antiviral potency. Furthermore, this inhibitor maintains impressive potency against a wide spectrum of HIV including a variety of multi-PI-resistant clinical strains. The inhibitors incorporated a stereochemically defined 5-hexahydrocyclopenta[b]furanyl urethane as the P2-ligand into the (R)-(hydroxyethylamino)sulfonamide isostere. Optically active (3aS,5R,6aR)-5-hydroxy-hexahydrocyclopenta[b]furan was prepared by an enzymatic asymmetrization of meso-diacetate with acetyl cholinesterase, radical cyclization, and Lewis acid-catalyzed anomeric reduction as the key steps. A protein-ligand X-ray crystal structure of inhibitor 3-bound HIV-1 protease (1.35 A resolution) revealed extensive interactions in the HIV protease active site including strong hydrogen bonding interactions with the backbone. This design strategy may lead to novel inhibitors that can combat drug resistance.

Efficient methodology for the synthesis of 2-C-branched glyco-amino acids by ring opening of 1,2-cyclopropanecarboxylated sugars.

An efficient methodology for the synthesis of 2-C-branched glyco-amino acid derivatives by diastereoselective ring opening of 1,2-cyclopropanecarboxylated sugars in good yields is reported. [reaction--see text]

Selective reduction of anomeric azides to amines with tetrathiomolybdate: synthesis of beta-D-glycosylamines.

A number of beta-d-glycosyl azide derivatives undergo reduction on treatment with tetrathiomolybdate to produce the corresponding beta-d-glycosylamines exclusively without anomerization under very mild and neutral reaction conditions. Acetyl, allyl, benzoyl, and benzyl protective groups are left untouched under the reaction conditions. An exclusive selectivity in the reduction of anomeric azides is observed, while the C-2 and C-6 azides are left untouched.

Tetrathiomolybdate assisted epoxide ring opening with masked thiolates and selenoates: multistep reactions in one pot.

Tetrathiomolybdate provides an easy access to beta-hydroxy disulfides, beta-hydroxy sulfides, and selenides from epoxides in a tandem, multistep process in one pot. This strategy has been utilized effectively in the construction of thiabicylo[3.2.2]nonane derivative 24.

Propargyloxycarbonyl (Poc) as a protective group for the hydroxyl function in carbohydrate synthesis.

[reaction: see text] The propargyloxycarbonyl (Poc) group can be used for the selective protection of the hydroxyl function in carbohydrates and can be removed under neutral conditions using tetrathiomolybdate MoS(4)(2-) (1) in CH(3)CN at room temperature. Under the conditions of deprotection benzylidine acetals, benzyl ethers, acetyl and levulinoyl esters, and allyl and benzyl carbonates are left untouched. It has also been shown that the new protective group (Poc) is compatible with acidic, basic, and also glycosylation conditions.