Branched tetrahydrofuran α,α-disubstituted-δ-sugar amino acid scaffolds from branched sugar lactones: a new family of foldamers?

α/β-Chimera peptide synthesis with cyclic β-sugar amino acids: the efficient coupling protocol

The synthesis of α/β-chimeras comprises peptide bond formation from α- to β-, from β- to β-, and from β- to α-amino acid residues. The fine-tuned solid phase synthesis of -GXXG- chimera peptides containing the simplest achiral α-amino acid glycine and two cyclic SAAs of different ring size [X denoting cyclic β-Sugar Amino Acids (β-SAA)] is reported, variants containing Fmoc-RibAFU(ip)-OH a furanoid-, and Fmoc-GlcAPU(Me)-OH a pyranoid-type structural "Lego-element". Systematic search for the best coupling strategy with both H-β-SAA-OHs is described, including the comparison of the different coupling reagents and conditions. Selecting the optimal reagent (from commonly used PyBOP, HATU and HOBt) was assisted by time-resolved ¹H-NMR: formation and stability of the Fmoc protected active esters were compared. We found that PyBOP is the best choice for successfully coupling both H-β-SAA-OH prototypes. The present comparative results open a reasonable route for building efficiently various -β-SAA- containing homo- and heterooligomers.

Origin of problems related to Staudinger reduction in carbopeptoid syntheses

We report the solid phase synthesis of -GG-X-GG- type α/β-carbopeptoids incorporating RibAFU(ip) (1a, tX) or XylAFU(ip) (2a, cX) sugar amino acids. Though coupling efficacy is moderate, both the lengthier synthetic route using Fmoc derivative (e.g., Fmoc-RibAFU(ip)-OH) and the azido derivative (e.g., N₃-RibAFU(ip)-OH) via Staudinger reaction with nBu₃P can be successfully applied. Both X-ray diffraction, ¹H- and ³¹P-NMR, and theoretical (QM) data support and explain why the application of Ph₃P as Staudinger reagent is "ineffective" in the case of a cis stereoisomer, if cX is attached to the preceding residue with a peptide (-CONH-) bond. The failure of the polypeptide chain elongation with N₃-cX originates from the "coincidence" of a steric crowdedness and an electronic effect disabling the mandatory nucleophilic attack during the hydrolysis of a quasi penta-coordinated triphenylphosphinimine. Nevertheless, the synthesis of the above α/β-chimera peptides as completed now by a new pathway via 1,2-O-isopropylidene-3-azido-3-deoxy-ribo- and -xylo-furanuronic acid (H-RibAFU(ip)-OH 1a and H-XylAFU(ip)-OH 2a) coupled with N-protected α-amino acids on solid phase could serve as useful examples and starting points of further synthetic efforts.

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

5-O-Acetyl-d-ribono-1,4-lactone

The title compound, C₇H₁₀O₆, was obtained from a regioselective enzyme-catalysed acyl­ation of d-ribono-1,4-lactone. The five-membered ring of the acyl­ated sugar shows an envelope conformation. In the crystal, the mol­ecules are linked by inter­molecular O—H⋯O hydrogen-bonds, forming a one-dimensional polymeric structure parallel to [010]. In addition, packing analysis shows stacking along the b axis.

C-3 branched δ-3,5-cis- and trans-THF sugar amino acids: synthesis of the first generation of branched homooligomers

This article describes the efficient synthesis of the first generation of branched sugar amino acid (SAA) oligomers in solution phase via two main routes: by the use of a standard coupling reagent and via the use of active ester intermediates. Benzyl-protected dimeric carbopeptoid and methyl-protected dimeric and tetrameric, hexameric and octameric carbopeptoids were obtained from a branched δ-3,5-trans-tetrahydrofuran (THF) SAA and methyl-protected dimeric and tetrameric carbopeptoids were synthesised from a branched δ-3,5-cis-THF SAA. These systems are of interest because of their potential to display foldameric properties reminiscent of those observed in α-peptides and proteins. Amongst their many uses, foldamers provide simpler models in the study of the factors which induce the folding and unfolding of proteins and, ultimately, potential insights into their functioning.

An efficient route towards a new branched tetrahydrofurane δ-sugar amino acid from a pyrolysis product of cellulose

(1R,5S)-1-hydroxy-3,6-dioxa-bicyclo[3.2.1]octan-2-one, is a bicyclic lactone obtained in gram-scale by catalytic pyrolysis of the renewable source cellulose. Now it has been used as a chiral building block in the preparation of the new δ-sugar amino acid, (3R,5S)-5-(aminoethyl)-3-hydroxytetrahydrofurane-3-carboxylic acid, by an efficient synthesis in five steps with a 67% overall yield. The structure of this tetrahydrofurane amino acid, isolated in protonated form, was assigned by extensive mono- and bidimensional (1)H- and (13)C-NMR analysis and mass spectrometry, including measurements by electrospray and matrix-assisted laser desorption ionization techniques, the latter one for high-resolution experiments. This amino acid is an isoster of dipeptide glycine-alanine (H-Gly-Ala-OH), with a potential use in the access of new peptidomimetics with conformationally restricted structures due to the presence of tetrahydrofurane ring. As a preliminary study in order to disclose this effect, density functional theory calculation performed in water using polar continuum model was applied to the new amino acid and H-Gly-Ala-OH dipeptide, so that to evaluate and compare the relative torsional angles for the energy-minimized structures.

Classification of conformation for sugar amino acid systems using chiroptical spectroscopy

A number of structurally related sugar amino acid systems have been examined by chiroptical methods to aid interpretation of their conformational preference. The use of circular dichroism, in addition to NMR and solution IR, has enabled classification of the conformations adopted by sugar amino acid systems as hydrogen-bonded regular, non-hydrogen-bonded regular, and non-hydrogen-bonded irregular. A set of tetrameric SAAs are examined and the effect of change in primary structure related to conformation.