The synthesis of enantiopure N-benzylidene nitrones of N-hydroxy-alpha-amino acids and their incorporation using standard Fmoc-based peptide chemistry into solid-supported peptide chains is described. Deprotection and resin cleavage affords N-terminal peptide hydroxylamines, which are the key substrates for chemoselective ligations with C-terminal peptide alpha-ketoacids. This general route is applicable to a variety of different N-terminal residues and provides a general approach to the solid phase synthesis of peptide hydroxylamines.
Palladium(II) in combination with a monodentate phosphine ligand enables the unprecedented direct and α‐stereoselective catalytic synthesis of deoxyglycosides from glycals. Initial mechanistic studies suggest that in the presence of N‐phenyl‐2‐(di‐tert‐butylphosphino)pyrrole as the ligand, the reaction proceeds via an alkoxy palladium intermediate that increases the proton acidity and oxygen nucleophilicity of the alcohol. The method is demonstrated with a wide range of glycal donors and acceptors, including substrates bearing alkene functionalities.
A general and practical route to carbohydrate-aryl ethers by nucleophilic aromatic substitution (SNAr) is reported. Upon treatment with KHMDS, C-O bond formation occurs between carbohydrate alcohols and a diverse range of fluorinated (hetero)aromatics to provide the targets in good to excellent yields. Commercially available arylating agents, high atom economy, and high regioselectivity are notable features of the protocol. The aryl ether products have potential for wide-ranging applications as exemplified by the synthesis of a novel chiral P,N-ligand.
The
use of a bifunctional cinchona/thiourea organocatalyst for
the direct and α-stereoselective glycosylation of 2-nitrogalactals
is demonstrated for the first time. The conditions are mild, practical,
and applicable to a wide range of glycoside acceptors with products
being isolated in good to excellent yields. The method is exemplified
in the synthesis of mucin type Core 6 and 7 glycopeptides.
Hoveyda-Grubbs' catalyst is able to catalyze crossed [2 + 2 + 2] cyclotrimerizations of diynes with nitriles. Pyridines are obtained with excellent yields when using activated nitriles and low to moderate yields with nonactivated nitriles which are unreactive under previously described ruthenium catalyzed reaction conditions. Both terminal and internal alkynes give the reaction using this experimental procedure.
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