A method for the installation of the formyl group at the anomeric position of pyranoses and furanoses starting from the corresponding lactones has been developed. The strategy involves the addition of 2-lithiothiazole to the sugar lactone, followed by the silane reduction of the acetylated resultant ketol and the unmasking of the formyl group from the thiazole ring. All steps have been studied in some details to improve chemical efficiency and stereochemical control. Hence, reversed :ß ratios of ketols were found in kinetic and thermodynamic mixtures, the former being consistent with a steric effect control of the substituents and the latter by the electronic effect of the ring oxygen. Seven sugar aldehydes with different D-pyranosidic (2,3,4,6-tetra-O-benzyl-gZuco, -galacto, and -manno, 2-azido-3,4,6-tri-0-benzyl-2-deoxy-goZacfo) and D-furanosidic moieties (5-O-benzyl-2,3-isopropylidene-nbo; 2,3,5-tri-O-benzyl-ribo; 2,3:5,6-di-0-isopropylidene-mozmo) were prepared in 52-65% isolated overall yield from the corresponding lactone.
A chiral hydrophilic environment is created at either rim of calix[4]arene by the synthesis of glycosyl derivatives such as the water‐soluble tetragalactosyl calixarene 1. This derivative is potentially an attractive synthetic receptor for chiral recognition of polar organic molecules.
A route to epimeric carbon-linked glycoglycines that exploits the
stereoselective addition of
2-lithiofuran and 2-lithiothiazole to sugar nitrones has been
described. The reaction occurs with
opposite diastereofacial selectivity depending on whether the free
nitrone or the diethyl aluminum
chloride precomplexed derivative is employed. The resulting furyl
or thiazolyl hydroxylamines
are dehydroxylated to amines by the action of titanium(III)
chloride. From these compounds the
amino acids are revealed by the oxidative cleavage of the furan ring or
by the conversion of the
thiazole into the formyl group and oxidation to carboxylic acid.
Compounds have been prepared
wherein the α-amino acid moiety is installed at C-4 and C-1 of
furanoses (ribo, manno, xylo, and
lyxo) and at C-5 and C-1 of a pyranose
(galacto).
The development of an efficient method for the stereoselective synthesis of alpha-D-(2-->1)-linked ketoside oligomers is described. The method is based on an iterative protocol composed of two key steps: a) the coupling of a thiazolylketosyl phosphite donor with an hydroxymethylketoside acceptor; and b) the introduction of the hydroxy-methyl group at the anomeric carbon atom of the resulting oligomer. To highlight its efficiency, the protocol was used in the assembly of D-galacto-2-heptulopyranose-containing oligoketosides through alpha-(2-->1) linkages up to the pentameric stage. The yield of the isolated oligomers ranged from 48 % in the first cycle to 29% in the fourth cycle. Having employed a pentenyl-substituted hydroxymethylketoside acceptor in the first cycle, all the derived oligomers contained the pentenyl group at their reducing end. This group was exploited to transform the linear oligomers into cyclic products through intramolecular glycosidation. The major product derived from the linear trisaccharide was confirmed by X-ray crystallography to be the cyclotris-(2-->1)-(alpha-D-galacto-2-heptulopyranosyl). The structure of this compound was essentially that of a [9]crown-3 ether bearing three galactopyranose rings spiroanellated in a propellerlike fashion. This arrangement of carbohydrate units linked to the crown ether created a densely alkoxylated cavity suitable for the encapsulation of alkali-metal cations (Li, Na, K, Ca, Mg).
PEG400 is described herein as a suitable medium for the condensation of various 2‐amino pyridines with α‐bromo ketones. 2‐Arylimidazo[1,2‐a]pyridines were synthetized in a short time through microwave irradiation in moderate to excellent yields. After optimization, a convenient one‐pot process enabled access to 2,3‐diarylimidazo[1,2‐a]pyridines by using a reduced amount of palladium catalyst without ligand for the C–H arylation step in the same environmentally‐sound medium.
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