Carbohydrates Aldehyde reactions

A reaction of practical importance is the oxidation of a carbohydrate aldehyde group to a carboxyl group. This is the basis for a process converting glucose to calcium gluconate, a substance of pharmaceutical interest. The oxidation reaction occurs at graphite electrodes in the presence of the Brj/Br" redox system. Calcium salt is added to the solution to prevent further oxidation of free gluconic acid. 

Several reviews have already been published on the subject, for example, the acetala-tion of alditols [4], of aldoses and aldosides [5,6], and of ketoses [7]. Some aspects of the stereochemistry of cyclic acetals have been discussed in a review dealing with cyclic derivatives of carbohydrates [8], also in a general article [9] and, more recently, in a chapter of a monograph devoted to the stereochemistry and the conformational analysis of sugars [10], Aspects on predicting reactions patterns of alditol-aldehyde reactions are reviewed within a general series of books on carbohydrates [11]. The formation and migration of cyclic acetals of carbohydrates have also been reviewed [12,13],... 

Dioxolanes are stable to bases. They are hydrolysed by dilute acids even at room temperature in a reversal of their formation. The conversion of aldehydes and ketones into 1,3-dioxolanes is one of the most important methods for the protection of the carbonyl function in multistep syntheses. It is also the standard method for blocking two vicinal cw-positioned hydroxy groups in a carbohydrate, via reaction with acetone, e.g. ... 

The formation of hemiacetals and hemiketals from aldehydes and ketones and their subsequent conversions to acetals and ketals are reversible reactions. The position of equilibrium depends on the specific compounds and the reaction conditions. Both forward and reverse reactions are catalyzed by acids or enzymes. The conversions of acetals and ketals to aldehydes and ketones is an important reaction in biological systems, occurring in the digestion of carbohydrates. The reaction is referred to as hydrolysis since water is a reactant. 

Oxidative esterification, Japanese chemists have reported a novel oxidation discovered in the carbohydrate field. Reaction of the tri-n-butylstannyl alkoxide (D with 1 equiv. of NBS in CCU at 20° gives the ester (2) and succinimide (3) in high yield. The reaction involves oxidation of (1) to an aldehyde, which then reacts with (1) to form the ester (2). 

In addition to sugars and their degradation products and the degradation products of amino acids (amines, ammonia and aldehydes), these reactions also include carbonyl compounds already present in foods as primary substances (e.g. aldehydes and ketones occurring in essential oils), ascorbic acid and carbonyl compounds formed in foods from precursors other than carbohydrates (aldehydes resulting from fat oxidation), thereby the reactions become even more complex. 

X = SnMe3 (19) X = SnMe2l reaction with a carbohydrate aldehyde. ... 

Knoevenagel reactions have been carried out on 2,3 4,5-di-O-isopropylidene-aIdehydo-D-xylose with methyl acetoacetate and pentane-2,4-dione and products such as (48) have been obtained. Reactions of Wittig reagents with carbohydrate aldehydes have been used to obtain access to cyclopentanes (Chapter 18) and an isomer of Compound (47) (Chapter 24). 

This chapter is divided into two parts The first and major portion is devoted to carbohydrate structure You will see how the principles of stereochemistry and confer matronal analysis combine to aid our understanding of this complex subject The remain der of the chapter describes chemical reactions of carbohydrates Most of these reactions are simply extensions of what you have already learned concerning alcohols aldehydes ketones and acetals... 

The reaction of aldoses with nitric acid leads to the formation of aldaric acids by oxidation of both the aldehyde and the terminal primary alcohol function to carboxylic acid groups Aldaric acids are also known as saccharic acids and are named by substi tutmg aric acid for the ose ending of the corresponding carbohydrate... 

Cleavage reactions of carbohydrates also occur on treatment with aqueous base for prolonged periods as a consequence of base catalyzed retro aldol reactions As pointed out m Section 18 9 aldol addition is a reversible process and (3 hydroxy carbonyl com pounds can be cleaved to an enolate and either an aldehyde or a ketone... 

Sections Carbohydrates undergo chemical reactions characteristic of aldehydes and... 

The basic carbohydrate molecule possesses an aldehyde or ketone group and a hydroxyl group on every carbon atom except the one involved in the carbonyl group. As a result, carbohydrates are defined as aldehyde or ketone derivatives of polyhydroxy alcohols and their reaction products. A look at the formula for glucose shows that it contains hydrogen and oxygen atoms in the ratio in which they are found in water. The name carbohydrate... 

Acetalation. As polyhydroxy compounds, carbohydrates react with aldehydes and ketones to form cycHc acetals (1,13). Examples are the reaction of D-glucose with acetone and a protic or Lewis acid catalyst to form l,2 5,6-di-0-isoprop5lidene-a-D-glucofuranose [582-52-5] and its reaction with benzaldehyde to form 4,6-0-benzyhdene-D-glucopyranose [25152-90-3]. The 4,6-0-(l-carboxyethyhdine) group (related to pymvic acid) occurs naturally in some polysaccharides. 

Sections Carbohydrates undergo chemical reactions characteristic of aldehydes and 25.17-25.24 ketones, alcohols, diols, and other classes of compounds, depending on their structure. A review of the reactions described in this chapter is presented in Table 25.2. Although some of the reactions have synthetic value, many of them are used in analysis and structure deter-mination. 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

Acetal and hemiacetal groups are particularly common in carbohydrate chemistry. Glucose, for instance, is a polyhydroxy aldehyde that undergoes an internal nucleophilic addition reaction and exists primarily as a cyclic hemiacetal. 

Attack of the OH radical on carbohydrates of low molecular mass gives rise to a variety of products. Indeed, the reaction of radiolytically-generated OH radical with lower hexose sugars produces lower saccharides (for di- and higher saccharide species), uronic and aldonic acids, and 3-, 2- and 1-carbon aldehydic fragments, e.g. 

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