Overcoming Challenges in Chemical Glycosylation to Achieve Innovative Vaccine Adjuvants Possessing Enhanced TLR4 Activity.

Lipopolysaccharide (LPS) mimicry leading to toll-like receptor 4 (TLR4) active compounds has been so far based mainly on reproducing the lipid A portion of LPS. Our work led to a series of structurally simplified synthetic TLR4 agonists in preclinical development as vaccine adjuvants called FPs. FPs bind MD2/TLR4 similarly to lipid A, inserting the lipid chains in the MD2 lipophilic cavity. A strategy to improve FPs' target affinity is introducing a monosaccharide unit in C6, mimicking the first sugar of the LPS core. We therefore designed a panel of FP derivatives bearing different monosaccharides in C6. We report here the synthesis and optimization of FPs' C6 glycosylation, which presented unique challenges and limitations. The biological activity of glycosylated FP compounds was preliminarily assessed in vitro in HEK-Blue cells. The new molecules showed a higher potency in stimulating TLR4 activation when compared to the parent molecule while maintaining TLR4 selectivity.

Chemical Synthesis of Human Milk Oligosaccharides: para-Lacto-N-hexaose and para-Lacto-N-neohexaose.

Human milk oligosaccharides (HMO) have emerged as a very active area of research in glycoscience and nutrition. HMO are involved in the early development of infants and may help to prevent certain diseases. The development of chemical methods for obtaining individual HMO aids the global effort dedicated to understanding the roles of these biomolecules. Reported herein is the chemical synthesis of two common core hexasaccharides found in human milk, i. e. para-lacto-N-hexaose (pLNH) and para-lacto-N-neohexaose (pLNnH). After screening multiple leaving groups and temporary protecting group combinations, a 3+3 convergent coupling strategy was found to work best for obtaining these linear glycans.

Expanding the scope of stereoselective α-galactosylation using glycosyl chlorides.

Recently, we reported that silver(I) oxide mediated Koenigs-Knorr glycosylation reaction can be dramatically accelerated in the presence of catalytic acid additives. We have also investigated how well this reaction works in application to differentially protected galactosyl bromides. Reported herein is the stereoselective synthesis of α-galactosides with galactosyl chlorides as glycosyl donors. Chlorides are easily accessible, stable, and can be efficiently activated for glycosylation. In this application, the most favorable reactions conditions comprised cooperative Ag2SO4 and Bi(OTf)3 promoter system.

Stereocontrolled α-Galactosylation under Cooperative Catalysis.

A recent discovery of a cooperative catalysis comprising a silver salt and an acid led to a dramatic improvement in the way glycosyl halides are glycosidated. Excellent yields have been achieved, but the stereoselectivity achieved with 2-O-benzylated donors was poor. Reported herein is our first attempt to refine the stereoselectivity of the cooperatively catalyzed galactosylation reaction. Careful optimization of the reaction conditions along with studying effects of the remote protecting groups led to excellent stereocontrol of α-galactosylation of a variety of glycosyl acceptors with differentially protected galactosyl donors.

Comparative Study on the Effects of Picoloyl Groups in Sialylations Based on Their Substitution Pattern.

A novel 8-O-picoloylated sialyl donor has been developed, and the performance of various picoloylated sialyl donors in glycosylations with primary glycosyl acceptors has been evaluated. 8-O-Picoloyl and 4,9-di-O-picoloyl sialyl donors produced moderate to excellent yields of disaccharides with complete α-stereoselectivities. Synergistic effects between picoloyl and the accompanying O-protecting groups (benzoyl vs acetyl) were evaluated, as well as the effects of triflic acid concentration on the 8-O-picoloyl donor. 1H NMR analysis was also carried out to assess differences in the hydrogen-bonding net between sialyl donors.

Facile and robust methods for the regioselective acylation of N-acetylneuraminic acid.

The stereoselective synthesis of sialic acid glycoconjugates is still a challenge in the field. Surprisingly, little is known on the regioselective O-substitution of sialic acids. Consequently, the effect of O-protecting groups and/or regioselectively protected building blocks in sialylations, remains practically unexplored. O-Picoloyl protecting groups have emerged as novel substituents that have a profound effect on sialylations. Recently, high stereoselectivities were obtained by introducing picoloyl groups at the C-4 and C-7/C-8 positions. However, to understand the relationship between the position of the picoloyl group and its exact effect in sialylations, a convenient access to a wider range of regioselectively picoloylated building blocks is needed. Reported herein is a new method that provides an accessible route to a wide array of regioselectively acylated building blocks. The regioselective introduction of picoloyl groups at various O-positions was achieved either by controlled direct picoloylation or by applying a modified ReSET methodology.