Synthesis of 3-C-(6-O-acetyl-2,3,4-tri-O-benzyl-alpha-D-mannopyranosyl)-1-propene: a caveat

C-Mannosyl Lysine for Solid Phase Assembly of Mannosylated Peptide Conjugate Cancer Vaccines

Dendritic cells (DCs) are armed with a multitude of Pattern Recognition Receptors (PRRs) to recognize pathogens and initiate pathogen-tailored T cell responses. In these responses, the maturation of DCs is key, as well as the production of cytokines that help to accomplish T cell responses. DC-SIGN is a frequently exploited PRR that can effectively be targeted with mannosylated antigens to enhance the induction of antigen-specific T cells. The natural O-mannosidic linkage is susceptible to enzymatic degradation, and its chemical sensitivity complicates the synthesis of mannosylated antigens. For this reason, (oligo)mannosides are generally introduced in a late stage of the antigen synthesis, requiring orthogonal conjugation handles for their attachment. To increase the stability of the mannosides and streamline the synthesis of mannosylated peptide antigens, we here describe the development of an acid-stable C-mannosyl lysine, which allows for the inline introduction of mannosides during solid-phase peptide synthesis (SPPS). The developed amino acid has been successfully used for the assembly of both small ligands and peptide antigen conjugates comprising an epitope of the gp100 melanoma-associated antigen and a TLR7 agonist for DC activation. The ligands showed similar internalization capacities and binding affinities as the O-mannosyl analogs. Moreover, the antigen conjugates were capable of inducing maturation, stimulating the secretion of pro-inflammatory cytokines, and providing enhanced gp100 presentation to CD8⁺ and CD4⁺ T cells, similar to their O-mannosyl counterparts. Our results demonstrate that the C-mannose lysine is a valuable building block for the generation of anticancer peptide-conjugate vaccine modalities.

Acceptor-influenced and donor-tuned base-promoted glycosylation

Base-promoted glycosylation is a recently established stereoselective and regioselective approach for the assembly of di- and oligosaccharides by using partially protected acceptors and glycosyl halide donors. Initial studies were performed on partially methylated acceptor and donor moieties as a model system in order to analyze the key principles of oxyanion reactivities. In this work, extended studies on base-promoted glycosylation are presented by using benzyl protective groups in view of preparative applications. Emphases are placed on the influence of the acceptor anomeric configuration and donor reactivities.

Synthesis of 7-O-galloyl-d-sedoheptulose

A facile synthetic approach to 7-O-galloyl-D-sedoheptulose (1), a natural product with notable immunosuppressant activity, was developed. The starting material, 2,7-anhydro-d-sedoheptulose (2), was converted in three steps into 1,3,4,5-tetra-O-benzyl-d-sedoheptulose (5), a key intermediate that allows specific functionalization at C-7 of the sedoheptulpyranose. After regioselective esterification of 5 with 3,4,5-tri-O-benzyl galloyl acid, followed by catalytic debenzylation (Pd-C), 1 was obtained in an overall yield of 60%. The spectroscopic data and TLC behavior of 1 were found to be identical to that of the natural product.

Facile oxidative cleavage of 4-O-benzyl ethers with dichlorodicyanoquinone in rhamno- and mannopyranosides

On exposure to dichlorodicyanoquinone in wet dichloromethane at room temperature, equatorial 4-O-benzyl ethers are removed with moderate selectivity in the presence of other benzyl ethers in glycopyranosides and glycothiopyranosides.

StereoselectiveC-Glycosylation Reactions of Pyranoses: The Conformational Preference and Reactions of the Mannosyl Cation

A systematic study of C-glycosylations of acetals related to mannose and other pyranoses was conducted. The C-5 alkoxyalkyl group provides only a modest influence on stereoselectivity. On the other hand, studies of pentopyranoses bearing alkoxy groups at C-2, C-3, and C-4 showed that the alkoxy groups exerted powerful influences on selectivity. In the case of mannose, the high alpha selectivity observed with C-mannosylation was reversed to high beta selectivity if the C-5 alkoxyalkyl group were removed. An analysis of the conformational preferences of the intermediate oxocarbenium ions, including the mannosyl cation, as well as consideration of steric effects that develop in the transition states for nucleophilic attack provide explanations for these phenomena.