Carbohydrate transformations derivatives

DHAP (A) is used in nature as a C3-methylene component for the efficient and stereoselective formation of carbohydrates (Scheme 1). In this pathway DHAP reacts with D-glyceraldehyde-3-phosphate (1) via an enzyme-catalyzed aldol reaction to form 2. After dephosphonyla-tion the aldol adduct 3 is then transformed to various carbohydrates and derivatives (Calvin 1962). 

The principal steps in the mechanism of polyisoprene formation in plants are known and should help to improve the natural production of hydrocarbons. Mevalonic acid, a key intermediate derived from plant carbohydrate via acetylcoen2yme A, is transformed into isopentenyl pyrophosphate (IPP) via phosphorylation, dehydration, and decarboxylation (see Alkaloids). IPP then rearranges to dimethylaHyl pyrophosphate (DMAPP). DMAPP and... 

A similar synthetic approach has also been applied by Rees et al. <1999J(P1)2937>. These authors transformed a carbohydrate-substituted analogue of 284 (R = Me, R1 = /3-Z)-ribofuranosyl) and carried out the ring closure to a product related to 285 (R = Me) in high yield. The same authors found that treatment of the free hydrazine 286 with DMF and acetic acid in refluxing conditions yields the triazolotriazinone derivative 287 in medium yield (55%). 

In the last twenty years, nitro sugars became powerful chemical tools on account of their usefulness for the construction of carbon-carbon bonds prior to the transformation of the nitro group into a variety of other chemical functionalities. As a result, a diverse range of funcionalized carbohydrates and other derivatives as carbasugars, cyclitols and heterocycles have been prepared. 

More recently, a similar transformation was used for the preparation of the nitro sugar derivative 32, a precursor of a carbohydrate mimic of 2-deoxystreptamine (Scheme 12).33... 

The term glycal is used to define sugar derivatives having a double bond between C-1 and C-2. Accordingly, C-1 glycals are A1,2 unsaturated carbohydrate derivatives with a carbon substituent at the anomeric position. These compounds are versatile synthetic intermediates, owing to the variety of transformations associated with their enol ether functionality, and have found ample use in the preparation of C-glycosyl compounds,1 carbohydrate mimics,2 and natural products.3... 

Furan Derivatives Catalytic processes used to obtain furan derivatives from carbohydrates and the catalytic routes from furan intermediates to chemicals and polymers have been reviewed by Moreau et al. [27]. Some of the main reactions are summarized in Fig. 3.2. From fructose or carbohydrates based on fructose (sucrose, inulin), the first transformation step is dehydration to 5-hydroxy methylfur-fural (HMF). Fructose dehydration at 165 °C was performed in the presence of... 

By treating Nafion (NR-50), a perfluorinated acidic ion exchanger based on sulfonic acid groups, with scandium(III) chloride hexahydrate Kobayashi et al. generated a solid scandium-derived catalyst (29) (Nafion-Sc) that proved to be effective in al-lylation reactions of carbonyl compounds with tetraallyltin (Scheme 4.15). Since the catalyst is stable in both organic solvents and water, even unprotected carbohydrates could be transformed directly in aqueous solvents. The resulting homo-allylic alcohols were separated by simple filtration [97]. 

An important question with regard to the scope of this method for complex carbohydrate synthesis was whether an unstabilized carbohydrate phosphorane might be compatible with a suitably blocked nucleoside aldehyde. Condensation of the ylide derived from XXI with uridine derivative XXIII proceeded under the usual conditions to give a 25% yield of XXIV, from which the N-benzoyl group could be readily removed with methanolic ammonia. The yield from this condensation reaction was not optimized, but its success clearly demonstrated the feasibility of such a transformation, if design considerations dictate the use of a nucleoside directly. 

Optically pure 7-oxanorbom-5-en-2-yl derivatives ("naked sugars") are readily available. Substitution of their centers C(3), C(5) and C(6) can be done with high stereo- and regioselectivity in a predictable fashion. The polysubstituted 7-oxanor-boman-2-ones so-obtained can be transformed into D- or L-carbohydrate derivatives, C-nucleoside precursors or polysubstituted cyclohex-2-enones and cyclohexenols (Scheme 12). Stereoselective rearrangements of the 7-oxanorbom-2-yl systems into polyhydroxylated cyclopentyl derivatives are also possible. 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

Enzymatic reduction of carbonyl compounds and enzymatic enantioselective transformation of racemic or meso alcohols (25,43.) are two methodologies that have proven to be beneficial in the preparation of optically active hydroxyl compounds, key chiral building blocks used in carbohydrate and natural product syntheses (44-45. Our interest in this area is to develop enzymatic routes to optically active glycerol and furan derivatives, and hydroxyaldehydes. 

R. G. Edwards, L. Hough, and A. C. Richardson, Transformations of cellobiose derivatives into analogues of lactose, Carbohydr. Res., 55 (1977) 129-148. 

To facilitate accesses to suitably functionalized sialic acid derivatives and complex sialyloligosaccharides for other usehil neoglycoconjugates, phase transfer catalysis (PTC) has been exploited extensively [for reviews see 42]. This process provided a wide range of carbohydrate derivatives under essentially clean Sn2 transformations. In the case of acetochloroneuraminic acid 1, the PTC reactions always provided inverted a-sialic acid derivatives [43]. para-Formylphenyl sialoside 7 [44], together with many other sialoside derivatives such as 8-10 [43], including thioacetate 12 [45] and azide 14 [46], were thus obtained (Scheme 1). Aldehyde 7 and similar glycosides are of particular interest since they could be directly conjugated to protein by reductive amina-tion after suitable deprotection [44]. 

---