Bert Fraser-Reid

Iodonium promoted reactions of disarmed thioglycosides

Abstract N-Iodosuccinimide/trifluoromethanesulfonic acid, which had been shown to promote the solvolysis of disarmed n-pentenyl glycosides, has been found to induce the same reactivity with disarmed thioglycosides as substrates.

A novel reagent for the synthesis of myo-inositol phosphates: N,N-diisopropyl dibenzyl phosphoramidite

Abstract It has been found that partially protected inositols are phosphorylated in near quantitative yields by use of N , N -diisopropyl dibenzyl phosphoramidite, 1- H -tetrazole and MCPBA in CH 2 Cl 2 . Subsequent hydrogenolysis of the resulting perbenzylated products gives the corresponding free inositol phosphates in quantitative yields without tedious ion exchange chromatography.

Debenzylation of complex oligosaccharides using ferric chloride

Abstract Anhydrous FeCl 3 in CH 2 Cl 2 at room temperature and 0 °C has been used to debenzylate monosaccharides and oligosaccharides in yields generally greater than 70%. Notably, alkenes, acetates, benzoates, phthalimides, acyl amides, and sensitive glycosidic linkages are unaffected by the reaction conditions.

Evidence for cyclic bromonium ion transfer in electrophilic bromination of alkenes: Reaction of ω-alkenyl glycosides with aqueous N-bromosuccinimide

Abstract Evidence is provided to support the theory that intermolecular Br+ transfer from a cyclic bromonium ion to an alkene occurs readily and can indeed overwhelm alternative reaction pathways. In the course of a study to determine which ω-alkenyl glycosides could serve as glycosyl donors, it was found that upon treatment with N-bromosuccinimide (NBS) in aqueous acetonitrile, under conditions in which an n-pentenyl glycoside underwent oxidative hydrolysis to the corresponding hemi-acetal, allyl, butenyl, and hexenyl analogs gave bromohydrin addition products. It was further found that when pentenyl and hexenyl analogs were made to compete for an insufficient amount of NBS, the former reacted while the latter was apparently recovered unchanged. However, both reacted independently at similar rates. In addition, the phenomenon was found to be concentration dependent. These results are consistent with the intermolecular, non-degenerate transfer of Br+ from cyclic bromonium ion to alkene.

N-tetrachlorophthaloyl (TCP) for ready protection/deprotection of amino sugar glycosides☆

The tetrachlorophthaloyl (TCP) group can be utilized when imidic protection of an amine is desired and durability of the protecting group to conditions ranging from mildly basic to harshly acidic is required. Installation can be accomplished in two steps by treating the free base with the commercially available TCP anhydride, and then closing the imidic ring with acetic anhydride and pyridine. Cleavage is effected by 2-4 eq of ethylenediamine under very mild conditions under which esters and glycopeptides have been shown to be stable, and racemization of amino acid residues does not occur. Unsubstituted phthalimides, even within the same molecule, are also unaffected during TCP cleavage. TCP protecting groups serve as beta-directors on donors and can also be present on acceptor species during electrophilic couplings in oligosaccharide synthesis.

Cyclopropanation of glycals: Application to the synthesis of 2-deoxy-2-vinyl glycosides

Abstract Appropriately protected derivatives of glucal and galactal have been found to undergo efficient Cyclopropanation with ethoxycarbonyl carbene with excellent facial selectivity, while di- O -acetyl-L-rhamnal exhibits lower facial selectivity. The cyclopropano glycals derived from glucal and galactal have been transformed into 2-deoxy-2-vinyl mannosides and galactosides, which are potentially valuable branched-chain sugars.

n-Pentenyl 2-amino-2-deoxy glycosides undergo stereoselective coupling under mild, chemospecific conditions

Abstract n-Pentenyl 2-deoxy-2-phthalimido and 2-anisylimino-2-deoxy-D-glucopyranosides undergo ready iodonium ion induced coupling with a variety of sugar alcohols to give β and α disaccharides, respectively, in moderate to excellent yields. The procedure is tolerant of a wide variety of protecting groups.

N-PENTENYL FURANOSIDES : SYNTHESIS AND GLYCOSIDATION REACTIONS OF SOME GALACTO DERIVATIVES

Abstract Synthetic routes to n-pentenyl galactofuranosides and glycosidation reactions of some derived donors with alcohol and saccharide acceptors using NIS/TESOTf as the promoter are described.

The chemistry of N-pentenyl glycosides: Synthetic, theoretical, and mechanistic ramifications

A feature of N-pentenyl glycosides (NPGs) is the ability to dibrominate the pentenyl double bond and, subsequently, regenerate it by reductive elimination. This allows a given NPG to serve (a) immediately as a glycosyl donor, or (b) after dibromination, as a glycosyl acceptor and, after subsequent reductive elimination as a glycosyl donor. These properties have been exploited to develop methodology for rapid assembly of homoglycans. Based upon results from the oxidative hydrolysis of conformationally restrained NPGs, protonated axial and equatorial 2-methoxytetrahydropyran have been undertaken. The relevance of our results to the theory of stereoelectronic control (antiperiplanar lone pair hypothesis) and to the action of lysozyme is discussed. calculations to determine transition states of

n-pentenyl glycosides in the efficient assembly of the blood group substance B tetrasaccharide

Abstract The fact that n-pentenyl glycosides (nPGs) are stable to a wide variety of reaction conditions but yet can be chemospecifically activated is advantageous for the efficient, convergent assembly of oligosaccharides. The nPGs are prepared directly from the aldose or by normal glycoside-forming reactions, and by using N-iodosuccinimide/triethylsilyl triflate as the iodonium source, even “disarmed” glycosyl donors react within minutes. The four monosaccharide components of the human blood-group determinant B are prepared with full or partial protection as required. Assembly of the tetrasaccharide then requires only five steps, three to give, in sequence, the disaccharide (68%), trisaccharide (82%), and tetrasaccharide (91%), the other two steps being required to deprotect hydroxyl groups at the di- and tri-saccharide levels for the ensuing coupling reactions.