Bismuth(iii) triflate as a novel and efficient activator for glycosyl halides

SFox imidates as versatile glycosyl donors for chemical glycosylation

Described herein is a continuation of our studies dedicated to the development of novel classes of leaving groups based on O- and S-imidates. The main focus of the study presented herein is the synthesis of novel 3,3-difluoro-3H-indol-2-ylthio (SFox) imidates and their application as glycosyl donors in chemical glycosylation. Being thioimidates, these compounds are more stable than O-imidates albeit much more reactive than conventional alkyl/arylthio glycosides. This study demonstrates that SFox imidates can be activated either with soft thiophilic reagents (N-iodosuccinimide or transition metal salts), typical for the activation of thioglycosides or thioimidates, or hard electrophilic reagents (protic or Lewis acids) common for the activation of O-imidates. Expectedly, complete β-selectivity was obtained from SFox donors equipped with 2-O-benzoyl group. Surprisingly, complete α-selectivity was obtained from 2-O-benzylated SFox imidates in all investigated cases.

The 4K reaction

The classical Koenigs-Knorr glycosidation of bromides or chlorides promoted with Ag2O or Ag2CO3 works only with reactive substrates (ideally both donor and acceptor). This reaction was found to be practically ineffective with unreactive donors such as per-O-benzoylated mannosyl bromide. Recently, it was discovered that the addition of catalytic (Lewis) acids to a silver salt-promoted reaction has a dramatic effect on the reaction rate and yield. A tentative mechanism for this cooperatively-catalyzed glycosylation reaction has been proposed, and the improved understanding of the reaction led to more efficient protocols and broader applications to a variety of glycosidic linkages. Since Ag2O-mediated activation was introduced by German chemists Koenigs and Knorr, and "cooperatively catalyzed" is Kooperativ Katalysiert in German, we refer to this new reaction as "the 4K reaction."

Synthesis of sialyl halides with various acyl protective groups

Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.

Urea-catalyzed N-Glycosylation of Amides/Azacycles with Glycosyl Halides

The efficient synthesis of N-glycosides via direct N-glycosylation of amides/azacycles has been reported. The glycosylation of amides/azacycles with glycosyl halides in the presence of a catalytic amount of urea proceeded smoothly to provide the corresponding N-glycosylated amides or nucleosides in good to excellent yields with 1,2-trans-stereoselectivity. Moreover, by the addition of terpyridine, the 1,2-cis-stereoselectivity was achieved.

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.

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.

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.

Glycosyl Formates: Glycosylations with Neighboring-Group Participation

Protected 2-O-benzyolated glycosyl formates were synthesized in one-step from the corresponding orthoester using formic acid as the sole reagent. Glucopyranosyl, mannopyranosyl and galactopyranosyl donors were synthesized and their glycosylation properties studied using model glycosyl acceptors of varied steric bulk and reactivity. Bismuth triflate was the preferred catalyst and KPF₆ was used as an additive. The 1,2-trans-selectivities resulting from neighboring-group participation were excellent and the glycosylations were generally high-yielding.

Synthesis and Glycosidation of Anomeric Halides: Evolution from Early Studies to Modern Methods of the 21st Century

Advances in synthetic carbohydrate chemistry have dramatically improved access to common glycans. However, many novel methods still fail to adequately address challenges associated with chemical glycosylation and glycan synthesis. Since a challenge of glycosylation has remained, scientists have been frequently returning to the traditional glycosyl donors. This review is dedicated to glycosyl halides that have played crucial roles in shaping the field of glycosciences and continue to pave the way toward our understanding of chemical glycosylation.

Bismuth-Catalyzed Stereoselective 2-Deoxyglycosylation of Disarmed/Armed Glycal Donors

Bi(OTf)₃ promoted direct and highly stereoselective glycosylation of "disarmed" and "armed" glycals to synthesize 2-deoxyglycosides has been reported. The tunable and solvent-controlled chemoselective activation of deactivated glycal donors distinguishing the competitive Ferrier and 1,2-addition pathways was discovered to improve substrate scope. The practical versatility of the method has been amply demonstrated with the oligosaccharide syntheses and 2-deoxyglycosylation of high-value natural products and drugs.