Branched-chain sugars types

Geminal Doubly Branched-Chain Sugars (Type IV)... 

Variations on this basic theme have been reported to reach more or less complex Type I branched-chain sugars (Scheme 2). A particular emphasis has been given on the use of... 

The synthesis of type III branched-chain sugars is based mainly on the use of ketosugars treated under Wittig-type conditions (see path a, Scheme 4) [13]. Several other methods, such as aldolization-crotonization or direct alkylidenation at the a-position of the carbonyl group of a keto sugar have been developed (path b). 

Spiro epoxides are also valuable intermediates for the synthesis of type I branched-chain sugars. These spiro epoxides are formed from ketosugars using diazomethane addition or sulfonium chemistry. Further ring opening of the epoxide allows the introduction of various nucleophiles [1], Chloro spiroepoxide 15 (Scheme 9) has been prepared recently from ketosugar 1 dichloromethyllithium [28]. 

The synthesis of type II branched-chain sugars should seem a more difficult task because it needs activation of a carbon atom on the sugar template and a control of the stereochemical course of the carbon-carbon bond formation. Probably because of this apparent difficulty several methods have been devised in the last decade, in particular, in the field of organo-metallic and free radical reactions. 

Other methods are available to prepare type II branched-chain sugars. In particular the classic reduction of the double bond of type III branched-chain sugars (see path g,... 

The synthesis of doubly branched-chain sugars should, in principle, follow the same principles through two successive formations of carbon-carbon bonds as described in the foregoing. However, additional methods especially designed to introduce two carbon chains on the same carbon (type IV) or on two vicinal carbons (type V) are now available. 

A well-developed concept to achieve high stereocontrol in the formation of a quaternary chiral center has been introduced by Fraser-Reid, using the Claisen rearrangement along two lines. In the first approach a type III branched-chain sugar is prepared by Wittig... 

Some saccharides are branched and these types are found as constituents of various natural products. For example, D-apiose occurs widely in plant polysaccharides. Antibiotics produced by the microorganism Streptomyces are another rich source of branched chain sugars. 

Diels-Alder reaction, 233, 239, 252 dithiane, 214, 216, 219, 242, 244 doubly branched-chain sugars, 235 enolate type anions, 214, 242, 248... 

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