Hypervalent iodine and carbohydrates : a new liaison

Various elimination procedures conducted on appropriate pyranoid and furanoid carbohydrate derivatives, especially on O-protected glycosyl halides afford cyclic vinyl ethers which Fischer (inappropriately) named glycals. These are used extensively in general organic synthesis and for the preparation of non-carbohydrate natural products as well as biologically important complex carbohydrates and glycoconjugates. The best known member, tri-O-acetyl-D-glucal, is normally made from tetra-O-acetyl-alpha-D-glucopyranosyl bromide, is commercially available, and is used very frequently in this chapter to represent the family in examples of the reactions under discussion.



Because of the pronounced region- and stereoselectivities with which their addition reactions can be conducted, glycal derivatives are of major importance in synthesis. They also, however, take part in rearrangement processes that, likewise, have proved useful for synthesis. The principal one involves nucleophilic substitution of the allylic group with allylic rearrangement and results in products having double bonds in the 2, 3 positions and new substituents at the anomeric centers. By far the simplest and most commonly used way to this conversion involves the removal of the allylic substituent of the glycal and the generation of highly resonance-stabilized oxocarbenium ion intermediate. This may then react with nucleophiles at the anomeric center to give products as mixtures of diastereomers. Many examples and variations of this theme are described and form the major part of this chapter, but other ways are also considered



Almost no formal mechanistic studies have been carried out on the reactions in this chapter. Categorization of mechanism required for the treatment of this topic has been done on the basis of conditions used, product identification and largely, chemical intuition.


Keywords:

glycals;
transformation;
oxocarbenium ions;
regioselectivity;
diasterioselectivity;
nucleophilic substitutions;
homoallylic center;
addition-elimination reactions;
palladium;
leaving groups;
electrocyclic reactions;
unsaturated compounds;
free sugars;
glycosyl peroxides;
glycosyl caroxylates;
S-glycosides;
glycosyl halides;
glycosyl azides;
glycosyl phosphonates;
glycosyl hydrides;
furanoid glycols;
intramolecular applications;
reverse reaction;
reaction variations;
scope;
limitations;
configuration;
experimental procedures;
other methods

Transformation of Glycals into 2,3‐Unsaturated Glycosyl Derivatives

Synthesis of 2,3-dideoxy-d-hex-2-enopyranosyl cyanides

The reaction of 3-azido-2,3-dideoxypyranose and 3-azido-2,3-dideoxy-2-halohexopyranose compounds with (diacetoxyiodo)benzene and iodine generated 2-azido-1,2-dideoxy-1-iodoalditols and 2-azido-1,2-dideoxy-1-halo-1-iodoalditols, respectively. These beta-iodo azides could be transformed by chemoselective dehydroiodination into 2-azido-1,2-dideoxy-4- O-formyl-pent-1-enitols and (Z, E)-2-azido-1,2-dideoxy-1-halo-4- O-formyl-pent-1-enitols in good yields. Thermolysis and photochemical studies of these vinyl azides and 1-halovinyl azides for the synthesis of polyhydroxylated 3-alkyl-2 H-azirines and the hitherto unknown 2-halo-3-alkyl-2 H-azirines have also been accomplished.

Synthesis of polyhydroxylated 2H-azirines and 2-halo-2H-azirines from 3-azido-2,3-dideoxyhexopyranoses by alkoxyl radical fragmentation.

A one-step method for the preparation of 2-pyridyl 2-deoxy-1-thiohexopyranosides from glycal esters

The uncatalysed displacement of allylic benzoyloxy groups with azide in unsaturated sugars has been studied by using hexamethylphosphoramide as solvent. Tri-O-benzoylglycals and 4,6-O-benzylidene-3-O-benzoylglycals were investigated. The former gave rise to mixtures of 3-azido-glycals and hex-2-enopyranosyl azides, whereas the latter gave only glycal products. The mechanism of the reactions is considered in some detail. Displacements on allylic trimethylsilyl ethers were not successful.

R. W. Irvine

Papers

Allylic nucleophilic substitution reactions in sugars. III. Uncatalysed displacements in hexamethylphosphoramide

Allylic nucleophilic substitution reactions in sugars. iii. uncatalyzed displacements in hexamethylphosphoramide