Sulfites, reaction with carbohydrates

The oldest way to produce caramel is by heating sucrose in an open pan, a process named caramelization. Food applications require improvement in caramel properties such as tinctorial power, stability, and compatibility with food. Caramels are produced in industry by controlled heating of a rich carbohydrate source in the presence of certain reactants. Carbohydrate sources must be rich in glucose because caramelization occurs only through the monosaccharide. Several carbohydrate sources can be used glucose, sucrose, com, wheat, and tapioca hydrolysates. The carbohydrate is added to a reaction vessel at 50°C and then heated to temperatures higher than 100°C. Different reactants such as acids, alkalis, salts, ammonium salts, and sulfites can be added, depending on the type of caramel to be obtained (Table 5.2.2). 

Cyclic sulfites have been used for selective /3-glycosylation of alcohols catalyzed by YbfOTfb or Ho(OTf)3 (Equation 6) < 1995JOC6254, 1997T16391>. The stereochemistry at sulfur does not affect the /3-selectivity of the reaction (fi a = 8-13 1). Similar reactions of other carbohydrate-based cyclic sulfites with alcohols have been reported previously <1994TL7335>. 

Reaction of the carbohydrate-based cyclic sulfites with NaSCN was used for the synthesis of f -l, 2-liised oxazolidine-2-thiones (Table 4) <1995TL5347>. Seleno and oxo derivatives were obtained with KSeCN or NaOCN, respectively <2000MI397>. The stereochemistry of products can be explained by the intermediate isomerization of /3-thiocyanates into cr-isothiocyanates (Scheme 10) <1995TL5347, 2000MI397>. 

The 3-deoxyglycosuloses formed in the degradation of carbohydrates readily react further under various conditions to a variety of products. The mechanisms of the reactions catalyzed by alkali, acid, sulfite, or amines will be dealt with separately. 

Cyclic sulfites and sulfates in organic synthesis have been reviewed in a Tetrahedron report. The reactions of several carbohydrate-based sulfite and sulfate derivatives with nitrogen, carbon and sulfur nucleophiles are reported. In the area of natural products, five sulfated saikosaponins containing the trisaccharide unit 28 have been isolated as minor constituents from Bupleurum rigidum. The leaf-movement hormones, sulfated phenolic glycosides 29, have been shown to be plant specific, rather than being common to all plants. ... 

Reaction of amino groups of amino acids, peptides, and proteins with carbonyl groups of carbohydrates or with tyrosine-derived quinone groups encompass changes respectively called nonenzymatic and enzymatic browning reactions (57-75). Such changes may adversely affect the appearance, taste, nutritional quality, and safety of many foods. Until recently, sodium sulfite has been used to inhibit browning... 

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