Publisher Summary This chapter discusses that the picture of an antiperiplanar lone pair of electrons driving off a leaving group had occurred intermittently in the literature at least as early as 1963; it is largely the initial work of Deslongchamp's group thatpresented a systematic body of work, which demanded explanation. It explores that antiperiplanar lone pair hypothesis (ALPH) requires no exceptions, whereas the explanation provided by the principle of least nuclear motion also accommodates the exceptions. Perhaps the best evidence that ALPH is too fundamental is the confusion that results when ALPH is taken as an axiom. The chapter discusses that such confusion can be illustrated by the multiplicity of otherwise unsubstantiated conformational changes required during the action of various enzymes, the successful rationalization of inadequate or incorrect data, and the calling into question of perfectly sound data, which happen not to fit an extension of the theory.

The Principle of Least Nuclear Motion and the Theory of Stereoelectronic Control

The anomerization of the tetra-O-acetyl-D-glucopyranosyl chlorides in acetonitrile is first order in chloride ion. Experiments with 36Cl-labelled chloride ion showed the reaction to involve nucleop...

The mechanism of the anomerization of the tetra-o-acetyl-d-glucopyranosyl chlorides

This review is the counterpart of a 2018 Chemical Reviews article (Adero, P. O.; Amarasekara, H.; Wen, P.; Bohe, L.; Crich, D. Chem. Rev. 2018, 118, 8242–8284) that examined the mechanisms of chemi...

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

Publisher Summary This chapter provides information on the methods for preparing affinity adsorbents having carbohydrate ligands and illustrates the use of these adsorbents for the purification of representative, macromolecular substances. Affinity chromatography is possible because of the stereospecific interactions that can occur between the functional groups of one compound and the complementary groups of another. Affinity adsorbents prepared from cellulose may exhibit nonspecific adsorption, and the purification of the desired substance could be difficult to achieve. The p -aminophenyl glycosides have been most widely used for preparing affinity adsorbents having carbohydrate ligands. Many types of affinity adsorbent have been prepared by coupling ligands containing primary amino groups to agarose activated with cyanogen bromide. Antibodies that are induced by carbohydrate antigens and that combine with carbohydrate residues of these antigens have been designated antiglycosyl antibodies. Additional studies are required to establish whether the anti-isomaltose, myeloma protein is synthesized as a homogeneous protein or in multimolecular form.

Affinity chromatography of macromolecular substances on adsorbents bearing carbohydrate ligands.

Publisher Summary This chapter discusses the methyl and phenyl glycosides of the common sugars. Several new general methods or modified procedures for glycoside synthesis have been introduced. The O -acylglycosyl halides, where the acyl group at the potential reducing group in a fully acylated reducing sugar has been replaced by halogen, are the most generally useful intermediates in the synthesis of alkyl and aryl glycosides, and the O -acetylglycosyl bromides (“acetobromosugars”) are by far the most commonly employed. They are more reactive than the chlorides and more stable than the iodides. In some instances, however, the bromide is so unstable that the chloride is preferred. Yields of O -acetylglycosyl bromides obtained from other sugars by this method were: lactose 85%, arabinose 50%, maltose 60%, cellobiose 72%, and galactose 75%. With cellobiose, some glacial acetic acid has to be added at the start to maintain solution. In general, the products are pure enough to be used without recrystallization in the next stage of glycoside synthesis. Apart from their use as starting materials for the preparation of O -acetyl-glycosyl halides, polyacetylated sugars are employed extensively in glycoside synthesis by the Helferich reaction, in which they are condensed directly with low-melting phenols.

Methyl and phenyl glycosides of the common sugars.

Publisher Summary This chapter discusses that the glycosyl halides are compounds in which the hydroxyl group at the reducing carbon atom of a sugar is replaced by halogen. As such, the glycosyl halides are rare, only a few fluorides being well authenticated. However, the fully acylated glycosyl halides, particularly the acetylated compounds, sometimes referred to as “acetohalogeno sugars” or “acetohalogenoses”, are very well known. The most familiar of these is 2,3,4,6-tetra-O-acetyl-α-D-glucosyl bromide (“acetobromo-α-D-glucose”). The poly-O-acyl derivatives of the glycosyl halides are among the most important intermediates for synthesis in carbohydrate chemistry; moreover, their chemistry has considerable intrinsic interest. Most of the methods for the preparation of glycosyl halide derivatives depend on the replacement by halogen of an acyloxyl (nearly always acetoxyl) group a t the reducing carbon atom. Hence, the methods for the synthesis of a large number of the glycosyl halide derivatives can be exemplified by description of those used for converting fully acetylated sugars into O-acetylglycosyl halides. As might be expected, treatment for prolonged periods often results in replacement of a second acetoxyl group. Other methods available for the synthesis of glycosyl halide derivatives involve the replacement of hydroxyl and alkoxyl by halogen, the opening of anhydro rings with hydrogen halides or equivalent reagents, and the addition of halogen or hydrogen halide to certain unsaturated sugar derivatives. Glycosyl iodide derivatives are usually prepared from the corresponding bromides by reaction with sodium iodide.

F.H. Newth

Papers

The glycosyl halides and their derivatives.