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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.

The influence of acid on reactions of glycals and their acetates is discussed. 3,4,6-Tri-O-acetyl-~-glucalreacts near 180" with methanol and ethanol to give methyl and ethyl 4,6-di-O-acetyl-2,3-didehydro-2,3-dideoxy-a, p-uerythro-hexoside, respectively. The ethyl a-glycoside is readily isolated in crystalline form. Methyl 4,6-O-benzylidene-2-deoxy-3-0-[ (methylthio) thiocarbonyll-a-D-glucopyranoside (VIII) takes part in the Chugaev reaction to yield mainly the 2,3-unsaturated glycoside (IX), whilst the isomeric 3,4unsaturated compound (XI) is the major product of the elimination which occurs on treatment of methyl 4,6-O-benzylidene-2-deoxy-3-oxo-a-~-evythvohexoside toluene-p-sulphonylhydrazone (X ; R = :N*NH*SO,*C,H,*Me) with base.2,3-DIHYDROPYRAN, in the presence of an acidic catalyst, reacts with water,2 alcohols 2 a 3 3 and phenols * to give 2-hydroxy-, 2-alkoxy-, and 2-aryloxy-tetrahydropyran. The glycals give 2-deoxyaldoses and 2-deoxyglycosides in analogous reactions. Thus, for example, methyl 2-deoxy-a-~-galactopyranoside can be prepared readily from D-galactal and this, in our experience, represents the most satisfactory synthesis of this glycoside. The O-acetylated glycals also take part in addition reactions with alcohols and phenols, and this same deoxy-glycoside has been prepared by treatment of 3,4,6-tri-O-acetyl-Dgalactal with methanol in the presence of hydrochloric acid, followed by removal of the ester groups.6b Replacement of the methanol by phenol and of the hydrochloric acid by toluene-$-sulphonic acid gave phenyl 3,4,6-tri-O-acetyl-2-deoxy-a-~-galactopyranos~de.~ The mechanism of these additions involves, most probably, attack by the hydroxynucleophile on the stabilised carbonium ion (11) formed by p-protonation of the vinyl ether (I). Nitrogen analogues (the enamines) also protonates and react with other electrophiles at the p-carbon atom.Under neutral conditions addition to the double bond does not occur. Glucal is stable in boiling water,lO but its 3,4,6-tri-O-acetyl derivative suffers a displacement reaction to give the well-known 4,6-di-0-acetyl-2,3-didehydro-2,3-dideoxy-~-erythro-hexose l1 (4,6-di-O-acetyl-#-glucal; 111; R = OH). Similar reactions occur with phenols and with mercaptoacetic acid: l2 the aryl glycosides [III; R = OPh (a-isomer); R = O*C6H,*N0, (a-and p-isomers)] and the thio-ester [III; R = SAC (a-isomer)] are the products which (a)

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