Ketohexose nucleosides

Significant stability of ketohexose nucleosides in various media has facilitated their direct transformation, leading to important structures of biological interest, such as branched-chain and rare-sugar nucleosides (for example, amino-, epimino-, thio-, and trideoxy-nucleosides). 

With ketohexose nucleosides, removal of protecting groups in acidic... 

The use of ketonucleosides as synthetic intermediates is based on their stability, particularly in alkaline media. Unlike ketopentosylpyrimi-dines, which are rapidly decomposed, ketohexose nucleosides appear to be convenient intermediates for syntheses of branched-chain, deoxy, and rare sugar nucleosides. 

Several studies on the action of alkalis on ketohexose nucleosides have shown that ketohexosyl-purines or -pyrimidines react slowly, and often without concomitant glycosylic cleavage. 

Significant stability has also been observed when ketohexose nucleosides,57 as well as ketopentosylpyrimidines,2,25 were treated with sodium borohydride (see Section V). 

The reduction of ketopentose-, as well as ketohexose-, nucleosides with metal hydrides has been used to obtain biologically important nucleosides and rare sugar nucleosides, by epimerization of a chiral center. Moreover, reduction of unsaturated ketonucleosides with borohydride provides new and direct routes to unsaturated and deoxy-nucleosides. 

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