Improvements in the Regioselectivity of Alkylation Reactions of Stannylene Acetals

Stereocontrolled synthesis of the aconitine D ring from D-glucose

The synthesis of a fully oxygenated aconitine D ring precursor from (D)-(+)-glucose is described. The route features a highly diastereoselective alkynyl Grignard ketone addition and a base-mediated enelactone to 1,3-diketone rearrangement.

A Hitchhiker's Guide to Problem Selection in Carbohydrate Synthesis

Oligosaccharides are ubiquitous in molecular biology and are used for functions ranging from governing protein folding to intercellular communication. Perhaps paradoxically, the exact role of the glycan in most of these settings is not well understood. One reason for this contradiction concerns the fact that carbohydrates often appear in heterogeneous form in nature. These mixtures complicate the isolation of pure material and characterization of structure-activity relationships. As a result, a major bottleneck in glycoscience research is the synthesis and modification of pure materials. While synthetic and chemoenzymatic methods have enabled access to homogeneous tool compounds, a central problem, particularly for newer synthetic chemists, is the matter of problem selection. This outlook aims to provide an entry level overview of fundamental principles in carbohydrate chemistry with an eye toward enabling solutions to frontier challenges.

Regioselective modification of unprotected glycosides

The regioselective modification of unprotected glycosides represents shortcuts in carbohydrate chemistry and enables efficient routes to complex derivatives.

Selective 4,6-O-benzylidene formation of methyl α-D-mannopyranoside using 2,6-dimethylbenzaldehyde

While methyl α-d-glucopyranosides and α-d-galactopyranosides selectively form 4,6-O-benzylidenes when reacted with excess benzaldehyde in the presence of acid catalyst methyl α-d-mannopyranosides does not exhibit the same selectivity because of the cis-arrangement of the C2 and C3 hydroxyl groups. The selectivity for the 4,6-O-benzylidene is restored by using 2,6-dimethylbenzaldehyde instead of benzaldehyde. In addition the excess 2,6-dimethylbenzaldehyde is easily recovered from the reaction by extraction with petroleum ether and can be reused without further purification. The 2,6-dimethylbenzylidene exhibits properties similar to the unsubstituted benzylidene with regard to chemical synthesis.

Is an acyl group at O-3 in glucosyl donors able to control α-stereoselectivity of glycosylation? The role of conformational mobility and the protecting group at O-6

The stereodirecting effect of a 3-O-acetyl protecting group, which is potentially capable of the remote anchimeric participation, and other protecting groups in 2-O-benzyl glucosyl donors with flexible and rigid conformations has been investigated. To this aim, an array of N-phenyltrifluoroacetimidoyl and sulfoxide donors bearing either 3-O-acetyl or 3-O-benzyl groups in combination with 4,6-di-O-benzyl, 6-O-acyl-4-O-benzyl, or 4,6-O-benzylidene protecting groups was prepared. The conformationally flexible 3-O-acetylated glucosyl donor protected at other positions with O-benzyl groups demonstrated very low or no α-stereoselectivity upon glycosylation of primary or secondary acceptors. On the contrary, 3,6-di-O-acylated glucosyl donors proved to be highly α-stereoselective as well as the donor having a single potentially participating acetyl group at O-6. The 3,6-di-O-acylated donor was shown to be the best α-glucosylating block for the primary acceptor, whereas the best α-selectivity of glycosylation of the secondary acceptor was achieved with the 6-O-acylated donor. Glycosylation of the secondary acceptor with the conformationally constrained 3-O-acetyl-4,6-O-benzylidene-protected donor displayed under standard conditions (-35°C) even lower α-selectivity as compared to the 3-O-benzyl analogue. However, increasing the reaction temperature essentially raised the α-stereoselectivities of glycosylation with both 3-O-acetyl and 3-O-benzyl donors and made them almost equal. The stereodirecting effects of protecting groups observed for N-phenyltrifluoroacetimidoyl donors were also generally proven for sulfoxide donors.

A first total synthesis of a hybrid-type ganglioside associated with amyotrophic lateral sclerosis-like disorder

The hybrid ganglioside X1, which was identified in the bovine brain, was synthesized for the first time. Ganglioside X1 is believed to be involved in the development of amyotrophic lateral sclerosis-like disorders in patients with neurological disorders after treatment with bovine brain gangliosides. A convergent approach using two branched glycan units, the GM2-core trisaccharide and the lacto-ganglio tetrasaccharide, efficiently provided the highly branched heptasaccharide part of ganglioside X1, which was conjugated with the ceramide part to produce the protected ganglioside X1. Global deprotection delivered homogenous ganglioside X1, with which serum from the patient was reacted.

Ganglioside GQ1b: efficient total synthesis and the expansion to synthetic derivatives to elucidate its biological roles

The convergent total synthesis of ganglioside GQ1b based on the "cassette approach" between the nonreducing end GQ1b-core heptasaccharide and glucosylceramide building blocks was accomplished in high overall yield. The use of a sialylalpha(2-->8)sialylalpha(2-->3)galactose sequence as the key building block enhanced the efficiency of the glycan assembly and led to preparative-scale synthesis readily applicable for large-scale preparation. In addition, a judicious choice of p-methoxybenzyl protecting groups on glucosylceramide provided a solution to the previous synthetic problems, including a decrease in the yield of the deprotection steps, and led to elevation of the total yield. Furthermore, unnatural-type GQ1b derivatives were synthesized systematically in good yields by capitalizing on a similar approach in order to elucidate their biological roles.

Synthesis and Antigenic Analysis of the BclA Glycoprotein Oligosaccharide from theBacillus anthracis Exosporium

The glycoprotein BclA is an important constituent of the exosporium of Bacillus anthracis spores. This glycoprotein is substituted with an oligosaccharide composed of a beta-L-rhamnoside substituted with the previously unknown terminal saccharide, 2-O-methyl-4-(3-hydroxy-3-methylbutanamido)-4,6-dideoxy-D-glucopyranose, also referred to as anthrose. Anthrose has not been found in spores of B. cereus and B. thuringiensis, making it a potential species-specific marker for B. anthracis. In order to study the antigenicity of anthrose, efficient syntheses of an anthrose-containing trisaccharide and a series of structurally related analogues were developed. The analogues lacked either the methyl ether at C-2 or contained modified C-4 amino functionalities of anthrose. The synthetic compounds were equipped with an aminopropyl spacer to facilitate conjugation to the carrier proteins mariculture Keyhole Limpet Hemocyanin (mcKLH) and bovine serum albumin (BSA). Serum antibodies of rabbits immunized with live or irradiated spores of B. anthracis Sterne 34F(2) were able to recognize the synthetic trisaccharide-mcKLH conjugate. The specificity of the interaction was confirmed by competitive inhibition with the free- and BSA-conjugated trisaccharides. Inhibition using the trisaccharide analogues demonstrated that the isovaleric acid moiety of anthrose is an important structural motif for antibody recognition. These data demonstrate that 1) anthrose is a specific antigenic determinant of the B. anthracis Sterne spore; 2) this antigen is presented to the immune system of rabbits receiving the anthrax live-spore vaccine; 3) synthetic analogues of the oligosaccharide retain the antigenic structure; and 4) the antigenic region is localized to specific terminal groups of the oligosaccharide. Collectively these data provide an important proof-of-concept step in the synthesis and development of spore-specific reagents for detection and targeting of non-protein structures in B. anthracis.

Synthesis of a d-rhamnose branched tetrasaccharide, repeating unit of the O-chain from Pseudomonas syringae pv. Syringae (cerasi) 435

The first synthesis of a d-rhamnose branched tetrasaccharide, corresponding to the repeating unit of the O-chain from Pseudomonas syringae pv. cerasi 435, as methyl glycoside is reported. The approach used is based on the synthesis of an opportune building-block, that is the methyl 3-O-allyl-4-O-benzoyl-alpha-D-rhamnopyranoside, which was then converted into both a glycosyl acceptor and two different protected glycosyl trichloroacetimidate donors. Successive couplings of these three compounds afforded the target oligosaccharide. The reported synthesis is also useful to perform the oligomerization of the repeating unit.