Configuration carbohydrate derivative

Sulfido from oxido compounds with inversion of configuration Carbohydrate derivatives s. 17, 664 s. a. U. G. Nayak and R. L. Whistler, J. Org. Chem. 34, 97 (1969)... 

Glycosyl halides, a very important group of carbohydrate derivatives, are commonly prepared14 from per-O-acylated sugars by reaction with hydrogen halides or halides of aluminum or titanium. The selection of the method depends mainly on the anomeric configuration of the substrate, the kind of its O-acyl groups, and the stability of the product to be prepared. 

The observation of a stereoconvergent cyclization by Roy et al. [18], as shown in the third example, is of special interest from a synthetic point of view because it exploits the configurational lability of radicals in a favorable manner. The other examples, i. e. Nugent and RajanBabu s cyclization of a carbohydrate-derived epoxide [5d] and Clive s quinane synthesis [19], amply demonstrate the usefulness of the titanocene-initiated epoxide opening. 

The synthesis in Scheme 13.34 also begins with carbohydrate-derived starting material and also uses catalytic hydrogenation to establish the stereochemical relationship between the C-4 and C-6 methyl groups. As was the case in Scheme 13.33, the configuration at C-2 is not controlled in this synthesis, and separation of the diastereomeric products was necessary. 

Substitutional and Configurational Effects on Chemical Shift in Pyranoid Carbohydrate Derivatives, R. U. Lemieux and J. D. Stevens, Can. J. Chem., 43 (1965) 2059-2070. 

The application of cyclic carbohydrate derivatives as chiral dienophiles has been investigated by several groups. 2,3-Dideoxy-DL-pent-2-enopyranose-4-ulose (17) in the thermal [4+2] cycloaddition to the acyclic dienes 18a-c is reported to produce mixtures of cycloadducts 19a-c epimeric on the hemiacetal center with an all-as-configuration of the other newly formed bonds. The reaction with cyclopentadiene (10) and 1,3-cyclohexadiene (11) yields adducts 20 as the sole products31. It should be emphasized that epimerization may occur on the anomeric center during the protection step (as an equilibrium process), thus leading to a mixture of adducts, while the endolexo ratio remains the same. 

Figure 4.2.6 Two effects determine the configurations of open-chain carbohydrate derivatives (a) gauche confirmations are favored in 1,2-diols over anti conformations, and (b) 1,3-syn diaxial interactions between hydroxyl substituents are repulsive and often lead to distortions of the all-anti chain.

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