Bromination carbohydrates

Derivatives of aldoses in which the terminal aldehyde function is oxidized to a car boxylic acid are called aldonic acids Aldonic acids are named by replacing the ose ending of the aldose by omc acid Oxidation of aldoses with bromine is the most com monly used method for the preparation of aldonic acids and involves the furanose or pyranose form of the carbohydrate... 

Branched chain carbohydrate (Section 25 12) Carbohydrate in which the main carbon chain bears a carbon substituent in place of a hydrogen or hydroxyl group Bromohydrin (Section 6 17) A halohydnn in which the halo gen IS bromine (see halohydnn)... 

Radical-mediated brominations, at ring positions of carbohydrates, 49, 37-92... 

When the configuration at (at least) one chiral unit is known in a nonracemic product, then the relative configuration needs to be determined. This situation occurs in substrate-induced reactions with many classical examples in steroid, terpene, and carbohydrate chemistry, such as the reduction of a carbonyl group (diastereoface-differentiating reaction), the transformation —CH2------> —CHBr— by radical bromination (diastereotopos-differentiating reaction), or dif-... 

Certain polysaccharides are normally hydrolyzed with mineral acid, usually sulfuric acid, either by direct refluxing with dilute acid or by preliminary dissolution in concentrated acid. Typical procedures have been described, and the associated problems discussed.22,23 Although prior solution of the polysaccharide in 72% sulfuric acid is a standard procedure,24 it has been shown that part of the carbohydrate may become sulfated, leading to erroneous results.23 When noncrystalline polysaccharides are being hydrolyzed, the treatment with 72% acid may be slightly modified.26 In special situations, oxidative hydrolysis, for example, of carrageenan, may be achieved by using sulfuric acid in the presence of bromine.27... 

Several halogenated carbohydrates have been studied. One example is methyl 6-bromo-6-deoxy-a-D-galactopyranoside.48 Bromine... 

A procedure for bromination of 2-phenyl-l,3-dioxolanes with N,N-dibromobenzenesulfonamide (C6H5S02NBr2) has been described,184 but it has not yet been applied in the carbohydrate field. The behavior of this reagent is essentially identical to that of N-bromosuccinimide, but some differences were noted, and a suggested mechanism was discussed.184... 

Some elements found in hody tissues have no apparent physiological role, hut have not been shown in he toxic. Examples arc rubidium, strontium, titanium, niobium, germanium, and lanthanum. Other elements are toxic when found in greater than trace amounts, and sometimes in trace amounts. These taller elements include arsenic, mercury, lead, cadmium, silver, zirconium, beryllium, and thallium. Numerous irlhcr elements are used in medicine in non-nulrieni roles. These include lithium, bismuth, antimony, bromine, platinum, and gold. The interactions of mineral nutrients with carbohydrates, fats, and proteins, minerals with vitamins (qv). and mineral nutrients with toxic elements are areas of active investigation. 

RADICAL-MEDIATED BROMINATIONS AT RING POSITIONS OF CARBOHYDRATES... 

The aim of the present article is to survey a radical-mediated reaction by which bromine atoms may be substituted directly into some carbohydrate derivatives at ring positions by the following general mechanism. 

The survey is intended to cover the published literature on the subject as fully as possible. Although the four halogens are well known as constituents of a wide range of carbohydrate compounds, radical-mediated halogenations have, in most cases, been realized only with bromine. There are, however, some examples of chlorinations to the best of our knowledge, there are no reported fluorinations or iodinations which occur by direct, radical-mediated processes. The radical-mediated reactions by which bromodeoxy carbohydrates are obtainable from benzylidene acetals17-19 are not considered. 

Radical-mediated brominations of carbohydrate derivatives have usually been conducted in refluxing carbon tetrachloride, under a tungsten (150— 250 W) or heat (250-450 W) lamp, with either JV-bromosuccinimide (1.2— 5 molar equivalents) or bromine (2-5 equivalents) as the source of halogen. Addition of bromotrichloromethane to the carbon tetrachloride can be advantageous.25 Substrate concentrations have ranged from 0.02 to 0.2 mol.l-1 and have depended to some degree on solubility factors, and ordinary laboratory glassware has most often been used. 

Previous studies had indicated that, in general, although some differences were sometimes observable, the products formed by photobromination of carbohydrate derivatives using bromine or yV-bromosuccinimide were similar. When compound 31 or 32, however, was treated with the latter reagent, a major difference was found, and the main product (74% isolated) was the orthoamide 35, formed, it was concluded, by way of the bromides 33 and 34 and, thence, a cyclic 3,4-benzoxonium ion.35 Support for this route was obtained by observing that treatment of a mixture of the bromides with yV-bromosuccinimide in refluxing carbon tetrachloride without irradiation resulted in their complete conversion into the orthoamide 35. yV-Bromoacet-... 

The synthetic procedures available to the carbohydrate chemist have been largely dominated by standard reactions proceeding by heterolytic processes within a chiral matrix. The preparative utility of radical-mediated reactions has, however, been amply demonstrated in recent years. The chapter contributed here by L. Somsak (Debrecen) and R. J. Ferrier (Wellington), on bromination reactions of carbohydrates proceeding by radical processes integrates the literature related to Ferrier s pioneering work in this area and underscores its excellent potential in synthesis. 

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