Total Synthesis of Starfish Cyclic Steroid Glycosides

Starfishes have evolved with a special type of secondary metabolites, namely starfish saponins, to ward off various predators and parasites; among them, the starfish cyclic steroid glycosides stand out structurally, featuring a unique 16-membered ring formed by bridging the steroidal C3 and C6 with a trisaccharide. The rigid cyclic scaffold and the congested and vulnerable steroid-sugar etherate linkage present an unprecedented synthetic challenge. Here we report a collective total synthesis of the major starfish cyclic steroid glycosides, namely luzonicosides A (1) and D (2) and sepositoside A (3), with an innovative approach, which entails a de novo construction of the ether-linked hexopyranosyl units, use of olefinic pyranoses as sugar precursors, and a decisive ring-closing glycosylation under the mild gold(I)-catalyzed conditions.

Ni-Catalyzed Cross-Coupling of 2-Iodoglycals and 2-Iodoribals with Grignard Reagents: A Route to 2-C-Glycosides and 2'-C-Nucleosides

The synthesis of 2-C-glycals and 2-C-ribals was achieved in good yields using a nickel-catalyzed cross-coupling between 2-iodoglycals and 2-iodoribal respectively and Grignard reagents. The prepared 2-C-glycals and ribals were then transformed into 2-C-2-deoxyglycosides, 2-C-diglycosides and 2'-C-2'-deoxynucleosides. The developed method was applied to the synthesis of a 2-chloroadenine 2'-deoxyribonucleoside - a structural analogue of cladribine (Mavenclad®, Leustatin®) and clofarabine (Clolar®, Evoltra®), two compounds used in the treatment of relapsing-remitting multiple sclerosis and hairy cell leukemia.

Biocatalytic Alkylation Chemistry: Building Molecular Complexity with High Selectivity

Biocatalysis has traditionally been viewed as a field that primarily enables access to chiral centers. This includes the synthesis of chiral alcohols, amines and carbonyl compounds, often through functional group interconversion via hydrolytic or oxidation-reduction reactions. This limitation is partly being overcome by the design and evolution of new enzymes. Here, we provide an overview of a recently thriving research field that we summarize as biocatalytic alkylation chemistry. In the past 3-4 years, numerous new enzymes have been developed that catalyze sp3 C-C/N/O/S bond formations. These enzymes utilize different mechanisms to generate molecular complexity by coupling simple fragments with high activity and selectivity. In many cases, the engineered enzymes perform reactions that are difficult or impossible to achieve with current small-molecule catalysts such as organocatalysts and transition-metal complexes. This review further highlights that the design of new enzyme function is particularly successful when off-the-shelf synthetic reagents are utilized to access non-natural reactive intermediates. This underscores how biocatalysis is gradually moving to a field that build molecules through selective bond forming reactions.

Helical Multi-Coordination Anion-Binding Catalysts for the Highly Enantioselective Dearomatization of Pyrylium Derivatives

A general and highly enantioselective synthesis of oxygen heterocycles from readily available in situ generated pyrylium derivatives has been realized by embracing a multi-coordination approach with helical anion-binding tetrakistriazole catalysts. The high activity of the tetrakistriazole (TetraTri) catalysts, with distinct confined anion-binding pockets, allows for remarkably low catalyst loadings (down to 0.05 mol %), while providing a simple access to chiral chromanones and dihydropyrones in high enantioselectivities (up to 98:2 e.r.). Moreover, experimental and theoretical studies provide new insights into the hydrogen-donor ability and key binding interactions of the TetraTri catalysts and its host:guest complexes, suggesting the formation of a 1:3 species.

Recognition and site-selective transformation of monosaccharides by using copper(II) catalysis

We demonstrate copper(II)-catalyzed acylation and tosylation of monosaccharides. Various carbohydrate derivatives, including glucopyranosides and ribofuranosides, are obtained in high yields and regioselectivities. Using this versatile strategy, the site of acylation can be switched by choice of ligand. Preliminary mechanistic studies support nucleophilic addition of a copper-sugar complex to the acyl chloride to be turnover limiting.

The winding pathway to erythropoietin along the chemistry-biology frontier: a success at last

The total synthesis of a homogeneous erythropoietin (EPO), possessing the native amino acid sequence and chitobiose glycans at each of the three wild-type sites of N glycosylation, has been accomplished in our laboratory. We provide herein an account of our decade-long research effort en route to this formidable target compound. The optimization of the synergy of the two bedrock sciences we now call biology and chemistry was central to the success of the synthesis of EPO.

An advance in the chemical synthesis of homogeneous N-linked glycopolypeptides by convergent aspartylation

We describe a useful advance in glycopeptide synthesis. We have developed a one-flask aspartylation/deprotection method, wherein long peptide fragments, bearing proximal pseudoproline functionality are merged with complex glycan domains. Following aspartylation, acidmediated global deprotection reveals the elaborated glycopeptide. The temporary pseudoproline functionality serves to suppress formation of aspartimide side products during solid phase peptide synthesis and aspartylation.

Toward Homogeneous Erythropoietin: Application of Metal Free Dethiylation in the Chemical Synthesis of the Ala79-Arg166 Glycopeptide Domain

We describe herein the assembly of hEPO(79-166), a key glycopeptide segment en route to erythropoietin, in minimally protected form. Key to the success of this synthetic endeavor was the application of our two-step cysteine-free native chemical ligation strategy, by which we achieved formal ligation at alanine and proline residues through the use of an N-terminal amino acid surrogate presenting a readily removable thiol functionality.