Nonfood Valorization of Sucrose

Sucrose, affectionately called "the royal carbohydrate,"29 is a nonreducing dis-accharide, because its component sugars, d-glucose and d-fructose, are glycosidi-cally linked through their anomeric carbon atoms. Hence, it constitutes a b-d-fructofuranosyl a-d-glucopyranoside (Figure 2.7). It is widely distributed throughout the plant kingdom, is the main carbohydrate reserve and energy source, and an indispensable dietary material for humans. For centuries, sucrose has been the world's most plentifully produced organic compound of low molecular mass (cf. Table 2.1). Due to the usual overproduction, and the potential to be

OH

HO o

HO o

Crystal

Aqueous solution

Crystal

Sucrose

P-D-Fructofuranosyl a-D-glucopyranoside fi-d-Fru/-(2<->l )-a-d-Glcp

Figure 2.7 Common structural representations of sucrose (top entries). The molecular geometry realized in the crystal is characterized by two intramolecular hydrogen bonds between the glucose and fructose portion107 (center left). In an aqueous solution, the two sugar units are similarly disposed toward each other, which is caused by insertion of a water molecule between the glucosyl-2-OH and fructosyl-1-OH,108'109—a water bridge, so to say—fixed by hydrogen bonding (center right). The bottom entries show the solvent-accessible surfaces (dotted areas) of the crystal form (left) and the form adopted in water (right).108'110

producable on an even higher scale if required, it is, together with cellulose- or starch-derived glucose, the major carbohydrate feedstock of low molecular weight, from which to elaborate organic chemicals.

The resulting chemistry of sucrose is capricious.111 The pronounced acid sen-sivity of the intersaccharidic linkage excludes any reaction that requires acidic conditions, and, featuring eight hydroxyl groups with only subtle reactivity differences, reactions with high regioselectivities for one or two of the OH-groups are few, in fact, mostly enzymatic.

2.3.4.1 Oxidation Products of Sucrose. The essentially regiospecific oxidation by Agrobacterium tumefaciens, whose dehydrogenase exclusively generates 3g-ketosucrose,112 is the prototype of an entry reaction into modified sucroses. This ready access opened the way to manifold modifications at the 3g-carbonyl function (Scheme 2.16).113 Chemical oxidation proceeds less uniformly, for

Sucrose

HOT3M

Agrobacterium (j^ROl tumefaciens O

Pt/02 water

cooe mH\

Scheme 2.16 Useful oxidation products of sucrose.

HO 1

HO 1

Scheme 2.16 Useful oxidation products of sucrose.

example agitation of an aqueous solution of pH 6.5-7.0 at 35°C with air in the presence of 0.5% Pt/C gave a 9:9:1 ratio of the 66and 1f-saccharonic acids.114 On further oxidation, particularly when using large amounts of the Pt catalyst and higher temperature (80-100°C), the preferred formation of the 6 g,6f-dicarboxylic acid has been observed,115 which may be isolated in up to a 70% yield by continuous electrodialytic removal.116

Extended catalytic oxidation finally yields the 1 6 ^-tricarboxylic acid, that is, all primary hydroxyl groups have yielded to oxidation.117 An alternate useful oxidant to the tricarboxylate is the NaOCl/TEMPO system, which, on applying high-frequency ultrasound, produces the tricarboxylate in up to a 70% yield.118

These sucrose-derived carboxylic acids have potential as the acid components of polyesters and polyamides. On amidation of the methyl ester of sucrose- 6 f,6 g -dicarboxylic acid with fat-amines, for example, surface-active dia-mides (left formula) with remarkable tensidometric properties are obtained, whereas reaction with hexamethylenediamine produces an interesting, highly hydrophilic polyamide (right):119

(n = 1-4)

2.3.4.2 Sucrose Esters. Sucrose esters have industrial interest in the area of sur-

24 120 121

factants, bleaching boosters, cosmetics, and fat substitutes. Synthetically prepared122 octa-fatty acid esters of sucrose have similar properties as the normal triglycerides, yet are not degraded by lipases, which entailed their marketing as noncaloric fat substitutes—after being approved by the U.S. Food and Drug Administration123 under the name Olestra® or Olean®.121

Less highly esterified sucroses, usually mixtures with a high proportion of either mono-, di-, or tri-esters of variable regioisomeric distribution over the 2g-, 6g-, 6-, as well as other hydroxyls (see arrows in Figure 2.8), are cosmetic emulsifiers and have favorable surfactant properties, combining low toxicity, skin compatibility, and biodegradability. Currently, they are produced at an estimated 5000 t/a level, mainly in Japan,24b yet have the potential of becoming viable alternatives to the APG biodetergents if they become more selectively producible.

2.3.4.3 Sucrose Ethers. Being next to the anomeric center and intramolecularly hydrogen-bonded, the 2g-OH of sucrose is the most acidic, which means it is deprotonated first under alkaline conditions, and thus preferentially yields to etherification. Benzylation with NaH/benzylbromide in DMF, for example, results in an 11:2:1 mixture of 2g-O-benzyl-sucrose (Figure 2.8) and its 1-O- and 3f-O -isomers. Because the former is readily accessible, it proved to be a versatile intermediate for the generation of 2g-modified sucroses, for example, the 2g-keto and 2g-deoxy derivatives as well as sucrosamine (2g-amino-2g-deoxy-sucrose),124 whose application profiles remain to be investigated.

Of higher interest industrially is the etherification of sucrose with long-chain epoxides such as 1,2-epoxydodecane125 or 1,2-epoxydodecan-3-ol,126 which are performable as one-pot reactions in dimethyl sulfoxide (DMSO) and the presence

OH

Figure 2.8 Sucrose monoesters and monoethers with useful surfactant properties.

Figure 2.8 Sucrose monoesters and monoethers with useful surfactant properties.

of a base to provide sucrose monoethers with preferred regioselectivities of the 2g-O- and 1f-O -positions. Unlike sucrose esters, the long-chain ethers are resistant to alkaline conditions, which considerably extends their potential applications as nonionic surfactants. They also have promising liquid crystalline properties, their mesophases depending on the point where the fatty chain attaches to the sucrose.

The only large-scale application of sucrose ethers appears to be to use poly-O-(hydroxylpropyl) ethers, generated by alkoxylation with propylene oxide, as the polyol component for rigid polyurethanes127—sucrose itself gives only brittle ones—which are used primarily in cushioning applications. The structures of these products, that is, the positions at which sucrose is alkoxylated and then carbamoylated with diisocyanates, and the type(s) of cross-linking involved, are not well defined though.

2.3.4.4 Sucrose Conversion to Isomaltulose. As a 6-O-(a-d-glucosyl)-d-fructose, isomaltulose is isomeric with sucrose, from which it is produced at an industrial level (cf. Table 2.1)—for food reasons, as it is hydrogenated to an equimolar mixture of

128 129

glucosyl-a(1!6)-glucitol and mannitol, the low caloric sweetener isomalt. 9 As illustrated in Scheme 2.17, the industrial process involves a glucosyl shift from the 2f-O of sucrose to the 6f-OH, effected by action of an immobilized Protaminobacter rubrum-derived a(1!6)-glucosyltransferase. Having become most readily accessible in this way, isomaltulose developed into a lucrative target for generating disaccha-ride intermediates of industrial potential. Particularly relevant in this context are oxidative conversions, hydrogen peroxide as the oxidant leading to shortening of the fructose chain by four carbons to provide the glucoside of glycolic acid (GGA) in 40% yield.130 Air oxidation in strongly alkaline solution (KOH), is less rigorous,

HO-HCL

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment