Sugar chemistry without protecting groups: A regioselective addition of the primary hydroxyl of monosaccharides to alkylisocyanates

Selective Anomeric Acetylation of Unprotected Sugars with Acetic Anhydride in Water

Unprotected sugars are selectively acetylated simply by stirring in aqueous solution in the presence of acetic anhydride and a weak base such as sodium carbonate. The reaction is selective for acetylation of the anomeric hydroxyl group of mannose, 2-acetamido, and 2-deoxy sugars and can be performed on a large scale. Competitive intramolecular migration of the 1-O-acetate to the 2-hydroxyl group when these two substituents are cis causes over-reaction and the formation of product mixtures.

2- O- N-Benzylcarbamoyl as a Protecting Group To Promote β-Selective Glycosylation and Its Applications in the Stereoselective Synthesis of Oligosaccharides

This study examines the utility of the N-benzylcarbamoyl (BnCar) protecting group in glycosylation reactions of the parent O-2 protected carbohydrate donor. It was found that the BnCar group imparted exclusively β-selectivity with primary and secondary alcohols. A mechanistic study revealed the activated intermediate to be the glycosyl triflate in a skew conformation, which results in β-selective glycosylation via an S_N2-like pathway. The BnCar group can be readily cleaved using tetrabutylammonium nitrite, without affecting ester and ether protecting groups. Taken together, these results show BnCar to be useful for the synthesis of complex oligosaccharides, an undertaking that requires delicate chemical differentiation of various protecting groups.

Protecting-group-free one-pot synthesis of glycoconjugates directly from reducing sugars

The conversion of sugars into glycomimetics typically involves multiple protecting-group manipulations. The development of methodology allowing the direct aqueous conversion of free sugars into glycosides, and mimics of oligosaccharides and glycoconjugates in a high-yielding and stereoselective process is highly desirable. The combined use of 2-azido-1,3-dimethylimidazolinium hexafluorophosphate and the Cu-catalyzed Huisgen cycloaddition allowed the synthesis of a range of glycoconjugates in a one-step reaction directly from reducing sugars under aqueous conditions. The reaction, which is completely stereoselective, may be applied to the convergent synthesis of triazole-linked glycosides, oligosaccharides, and glycopeptides. The procedure provides a method for the one-pot aqueous ligation of oligosaccharides and peptides bearing alkyne side chains.

Synthesis of N-unsubstituted, mono- and disubstituted carbohydrate-1-O-carbamates and their behaviour in glycoside syntheses

The syntheses of 44 1-carbamates from six different 1-O-unprotected carbohydrate derivatives (compounds 1-6), representing typical protecting pattern in glycoside synthesis, are described. The carbamate function is N-unsubstituted (compounds 1b-6b), mono- (compounds a: N-trichloroacetyl, c: N-monochloroacetyl, d: N-acetyl, e: N-ethyl, f: N-allyl, g: N-phenyl) or disubstituted (compounds h: imidazolyl, i: N-diethyl, j: N-diphenyl). Additionally, three N-chlorosulfonyl carbamates are synthesized and used as intermediates for the synthesis of N-unsubstituted compounds b. The accessibility of these compounds is described and compared. Some of the carbamates (1, 4, 5a-j) are used as model compounds for systematic investigations in glycoside syntheses. Selected experimental data (reaction conditions, anomeric ratios, rotation values, selected NMR data) are tabulated.

Porphyrin sensitizers in tumour phototherapy. Novel sensitizers of the chlorin and bacteriochlorin class with amphiphilic properties

The requirements for activity in a tumour-photosensitizing drug are outlined. A series of metallo tetrasulphonatophthalocyanines are shown to be inactive in an in vivo assay of tumour photosensitization; however, some less water-soluble compounds (hydroxylated derivatives of octaethylchlorin and octaethylbacteriochlorin) are shown to possess promising activity.