Propargyl 1,2-orthoesters of arabinose are exploited for the synthesis of 1,2-trans furanosides; easily accessible 1,2-trans ribofuranosides are converted to challenging 1,2-cis-arabinofuranosides by oxidoreduction. Utility of these protocols was demonstrated by the successful synthesis of major structural motifs present in the cell surface of Mycobacterium tuberculosis. Key furanosylations were carried out under gold-catalyzed glycosidation conditions.
An orthogonal activation strategy with propargyl and n-pentenyl glycosides has been identified. According to this methodology, n-pentenyl glycosides can be selectively activated with NIS/TMSOTf in the presence of either armed or disarmed propargyl O-glycosides. In addition, we report herein that propargyl 1,2-orthoesters can be selectively activated with AuBr(3) in CH(2)Cl(2) at room temperature in the presence of n-pentenyl glycosides. Similarly, pentenyl 1,2-orthoesters can be selectively activated with NIS/Yb(OTf)(3) in the presence of propargyl glycosides.
We report the development of a new class of nucleic acid ligands that is comprised of Janus bases and the MPγPNA backbone and is capable of binding rCAG repeats in a sequence-specific and selective manner via, inference, bivalent H-bonding interactions. Individually, the interactions between ligands and RNA are weak and transient. However, upon the installation of a C-terminal thioester and an N-terminal cystine and the reduction of disulfide bond, they undergo template-directed native chemical ligation to form concatenated oligomeric products that bind tightly to the RNA template. In the absence of an RNA target, they self-deactivate by undergoing an intramolecular reaction to form cyclic products, rendering them inactive for further binding. The work has implications for the design of ultrashort nucleic acid ligands for targeting rCAG-repeat expansion associated with Huntington’s disease and a number of other related neuromuscular and neurodegenerative disorders.
An impressive array of antigene approaches has been developed for recognition of double helical DNA over the past three decades; however, few have exploited the 'Watson-Crick' base-pairing rules for establishing sequence-specific recognition. One approach employs peptide nucleic acid as a molecular reagent and strand invasion as a binding mode. However, even with integration of the latest conformationally-preorganized backbone design, such an approach is generally confined to sub-physiological conditions due to the lack of binding energy. Here we report the use of a class of shape-selective, bifacial nucleic acid recognition elements, namely Janus bases, for targeting double helical DNA or RNA. Binding occurs in a highly sequence-specific manner under physiologically relevant conditions. The work may provide a foundation for the design of oligonucleotides for targeting the secondary and tertiary structures of nucleic acid biopolymers.
Emergence of multidrug-resistant and extreme-drug-resistant strains of Mycobacterium tuberculosis (MTb) can cause serious socioeconomic burdens. Arabinogalactan present on the cellular envelope of MTb is unique and is required for its survival; access to arabinogalactan is essential for understanding the biosynthetic machinery that assembles it. Isolation from Nature is a herculean task and, as a result, chemical synthesis is the most sought after technique. Here we report a convergent synthesis of branched heneicosafuranosyl arabinogalactan (HAG) of MTb. Key furanosylations are performed using [Au]/[Ag] catalysts. The synthesis of HAG is achieved by the repetitive use of three reactions namely 1,2-trans furanoside synthesis by propargyl 1,2-orthoester donors, unmasking of silyl ether, and conversion of n-pentenyl furanosides into 1,2-orthoesters. Synthesis of HAG is achieved in 47 steps (with an overall yield of 0.09%) of which 21 are installation of furanosidic linkages in a stereoselective manner.
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