A Hitchhiker's Guide to Problem Selection in Carbohydrate Synthesis

Oligosaccharides are ubiquitous in molecular biology and are used for functions ranging from governing protein folding to intercellular communication. Perhaps paradoxically, the exact role of the glycan in most of these settings is not well understood. One reason for this contradiction concerns the fact that carbohydrates often appear in heterogeneous form in nature. These mixtures complicate the isolation of pure material and characterization of structure-activity relationships. As a result, a major bottleneck in glycoscience research is the synthesis and modification of pure materials. While synthetic and chemoenzymatic methods have enabled access to homogeneous tool compounds, a central problem, particularly for newer synthetic chemists, is the matter of problem selection. This outlook aims to provide an entry level overview of fundamental principles in carbohydrate chemistry with an eye toward enabling solutions to frontier challenges.

Palladium catalysed carbonylation of 2-iodoglycals for the synthesis of C-2 carboxylic acids and aldehydes taking formic acid as a carbonyl source

Pd catalyzed carbonylative reaction of 2-iodo-glycals has been developed taking formic acid as a carbonyl source for the synthesis of 2-carboxylic acids of sugars by the hydroxycarbonylation strategy. The methodology was successfully extended to the synthesis of 2-formyl glycals by using a reductive carbonylation approach. Both ester and ether protected glycals undergo the reaction and furnished sugar acids in good yield which is otherwise not possible by literature methods. The C-2 sugar acids were successfully utilized for the construction of 2-amido glycals, 2-dipeptido-glycal by Ugi reaction and C-1 and C-2 branched glycosyl esters.

Cross dehydrogenative coupling of sugar enol ethers with terminal alkenes in the synthesis of pseudo-disaccharides, chiral oxadecalins and a conjugated triene

An efficient strategy for the synthesis of C-2 and C-3 branched sugar dienes via cross dehydrogenative coupling of sugar enol ethers with terminal alkenes was developed.

Synthesis of glucopyranos-6′-yl purine and pyrimidine isonucleosides as potential cholinesterase inhibitors. Access to pyrimidine-linked pseudodisaccharides through Mitsunobu reaction

The synthesis of new isonucleosides comprising purine and pyrimidine-derived systems linked to methyl glucopyranosidyl units at C-6 and evaluation of their cholinesterase inhibitory profiles is reported. Their access was based on the Mitsunobu coupling of partially acetylated and benzylated methyl glucopyranosides with purine and pyrimidine derivatives. While the reactions with purines and theobromine proceeded with complete regioselectivity, affording exclusively N9- or N1-linked 6′-isonucleosides, respectively, the use of pyrimidine nucleobases led to N1 and/or N3-glucopyranosid-6′-yl pyrimidines and/or to N1,N3/2-O,4-O-pyrimidine-linked pseudodisaccharides through bis-coupling, depending on the substitution pattern of the sugar precursor and on the nature of the nucleobase. From this series of compounds, four were shown to be effective and selective inhibitors of acetylcholinesterase with inhibition constants in the micromolar concentration range. A tri-O-acetylated N1-glucopyranosid-6′-yl theobromine and a benzylated N1,N3-bis-glucopyranosid-6-yl thymine were the most active molecules with Ki values of 4 μM. A tri-O-benzylated glucopyranosid-6′-yl uracil displayed good and selective inhibition of butyrylcholinesterase (Ki=8.4±1.0 μM), similar to that exhibited by the standard galantamine. Molecular docking simulations, performed with the two most effective acetylcholinesterase inhibitors, showed interactions with key amino acid residues located at the enzyme’s active site gorge, which explain the competitive component of their inhibitory activities.

Regioselective modification of unprotected glycosides

The regioselective modification of unprotected glycosides represents shortcuts in carbohydrate chemistry and enables efficient routes to complex derivatives.

Diastereoselective synthesis of highly functionalized cis-1-oxadecalines via 6-endo-tet-cyclizations of 2-C-branched sugars

Diastereoselective synthesis of highly functionalized cis-1-oxadecalines via 6-endo-tet-cyclizations of 2-C-branched sugars.