Total synthesis of α-1C-galactosylceramide, an immunostimulatory C-glycosphingolipid, and confirmation of the stereochemistry in the first-generation synthesis

Synthesis and medicinal chemical characterisation of antiproliferative O,N-functionalised isopulegol derivatives

Benzylation of isopulegol furnished O-benzyl-protected isopulegol, which was transformed into aminodiols via epoxidation followed by ring opening of the corresponding epoxides and subsequent hydrogenolysis. On the other hand, (-)-isopulegol was oxidised to a diol, which was then converted into dibenzyl-protected diol derivatives. The products were then transformed into aminotriols by using a similar method. The antiproliferative activity of aminodiol and aminotriol derivatives was examined. In addition, structure-activity relationships were also explored from the aspects of substituent effects and stereochemistry on the aminodiol and aminotriol systems. The drug-likeness of the compounds was assessed by in silico and experimental physicochemical characterisations, completed by kinetic aqueous solubility and in vitro intestinal-specific parallel artificial membrane permeability assay (PAMPA-GI) measurements.

Pseudo-glycoconjugates with a C-glycoside linkage

Work by the author and colleagues has been focused on the development of pseudo-glycans (pseudo-glycoconjugates), in which the O-glycosidic linkage of the natural-type glycan structure is replaced by a C-glycosidic linkage. These analogs are not degraded by cellular glycoside hydrolases and are thus expected to be useful molecular tools that may maintain the original biological activity for a long period in the cell. However, their biological potential is not yet well understood because only a few pseudo glycans have so far been synthesized. This article aims to provide a bird's-eye view of our recent studies on the creation of C-glycoside analogs of ganglioside GM3 based on the CHF-sialoside linkage, and summarizes the chemical insights acquired during our stereoselective synthesis of the C-sialoside bond, ultimately leading to pseudo-GM3. Conformational analysis of the synthesized CHF-sialoside disaccharides confirmed that the anticipated conformational control by F-atom introduction was successful, and furthermore, enhanced the biological activity. In order to improve access to C-glycoside analogs based on pseudo-GM3, it is still important to streamline the synthesis process. With this in mind, we designed and developed a direct C-glycosylation method using atom-transfer radical coupling, and employed it in syntheses of pseudo-isomaltose and pseudo-KRN7000.

Potent Neutralizing Antibodies Elicited by RBD-Fc-Based COVID-19 Vaccine Candidate Adjuvanted by the Th2-Skewing iNKT Cell Agonist

The development of a safe and effective COVID-19 vaccine is of paramount importance to terminate the current pandemic. An adjuvant is crucial for improving the efficacy of the subunit COVID19 vaccine. α-Galactosylceramide (αGC) is a classical iNKT cell agonist which causes the rapid production of Th1- and Th2-associated cytokines; we, therefore, expect that the Th1- or Th2-skewing analogues of αGC can better enhance the immunogenicity of the receptor-binding domain in the spike protein of SARS-CoV-2 fused with the Fc region of human IgG (RBD-Fc). Herein, we developed a universal synthetic route to the Th1-biasing (α-C-GC) and Th2-biasing (OCH and C20:2) analogues. Immunization of mice demonstrated that αGC-adjuvanted RBD-Fc elicited a more potent humoral response than that observed with Alum and enabled the sparing of antigens. Remarkably, at a low dose of the RBD-Fc protein (2 μg), the Th2-biasing agonist C20:2 induced a significantly higher titer of the neutralizing antibody than that of Alum.

Synthesis of CH2-linked α-galactosylceramide and its glucose analogues through glycosyl radical-mediated direct C-glycosylation

Direct C-glycosylation of a conformationally constrained and stable C1-sp³ hybridized carbohydrate donor with a carefully designed sphingosine unit afforded the CH₂-linked analogue of antitumor-active KRN7000 and its glucose congener.

The Crotylation Way to Glycosphingolipids: Synthesis of Analogues of KRN7000

A synthesis of glycosphingolipids that centers on the reaction of O- and C-glycosyl crotylstannanes and relatively simple lipid aldehydes is described. The modularity of this strategy and versatility of the crotylation products make this an attractive approach to diverse, highly substituted libraries. The methodology is applied to analogues of the potent imunostimulatory glycolipid KRN7000, including O-, methylene-, and fluoromethine-linked isosteres with diastereomeric ceramide segments and 2-amido substitutes.

Synthesis of α-C-Galactosylceramide via Diastereoselective Aziridination: The New Immunostimulant 4'-epi-C-Glycoside of KRN7000

A new immunostimulant, the 4'-epimer of α-C-GalCer, was synthesized from a C2-symmetric dienediol and α-C-allyl galactoside. The intramolecular aziridination and the following reductive ring opening provided the core of the aliphatic amino alcohol with excellent regio- and stereocontrol. The new immunostimulants 3d and 3e gave a better polarized Th1-type cytokine response in murine NKT cells than the benchmarked α-C-GalCer.

Aminodiols via stereocontrolled oxidation of methyleneaziridines

A highly diastereoselective Ru-catalyzed oxidation/reduction sequence of bicyclic methyleneaziridines provides a facile route to complex 1-amino-2,3-diol motifs. The relative anti stereochemistry between the amine and the vicinal alcohol are proposed to result from 1,3-bischelation in the transition state by the C1 and C3 heteroatoms.

Structure-activity relationship studies of novel glycosphingolipids that stimulate natural killer T-cells

KRN7000, an anticancer drug candidate developed by Kirin Brewery Co. in 1995, is an α-galactosyl ceramide. It is a ligand making a complex with CD1d protein, and it stimulates invariant natural killer T (NKT) cells, which are one of the lineages of immunocytes. NKT cells activated by recognition of the CD1d/KRN7000 complex with its invariant T-cell receptor (TCR) can induce both protective and regulatory immune responses. To determine the recognition and activation mechanisms of NKT cells and to develop drug candidates more effective than KRN7000, a large number of analogs of KRN7000 have been synthesized. Some of them show potent bioactivities and have the potential of being utilized as therapeutic agents. In this review, structure-activity relationship studies of novel glycolipids which stimulate NKT cells efficiently are summarized.

Synthesis of C-glycoside analogues of α-galactosylceramide via linear allylic C-H oxidation and allyl cyanate to isocyanate rearrangement

C-Glycoside analogues of α-galactosylceramide were synthesized in which several significant modifications known to promote Th-1 cytokine production were included. The key transformations include C-H oxidation, Sharpless asymmetric epoxidation, olefin cross metathesis, and an allyl cyanate to isocyanate rearrangement.