Chain Extensions at the Anomeric Center

There are two ways to extend the sugar skeleton at Cl. The first way, which will be summarized in this section, uses the aldehyde group, or an equivalent, for the formation of a carbon-carbon bond and results in the formation of a linear sugar. In the second way, the new bond is formed at the anomeric center, exploiting its peculiar reactivity although the heterocyclic nature of the sugar is retained. This second method, by far more investigated, will be the subject of Section 11.5. 

Methods of chain extensions generally use linear sugars with free aldehydes mostly prepared via dithioacetal formation, protection of the remaining hydroxyl groups and aldehyde recovery. This allows one to use all methods applicable to aldehydes. Among others, the condensation of ethyl diazoacetate has been studied in detail by L6pez-Herrera [207] in a new synthesis of KDO by a two carbon homologation of an open-chain mannose derivative. 

SCHEME 11.64 Reagents i (1) Ph3P=CHC02Et, benzene and (2) DIBAL ii Ti(OiPr)4, (-f-)-DET, TBHP iii (1) LiCBulsBH, NaBOj, (2) PhsP, Ij, imidazole, 70%, (3) KHMDS, TESCl, (4) CgK, ZnClj, AgOAc and (5) dimethyldioxirane. 

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C-Glycosyl compounds are important molecular targets as they occur in Nature and have interesting biological properties (for reviews on C-glycosylation, see [72-75]). Chain extensions of aldonolactones have been employed to create C-C bond formation at the anomeric center. Claisen-type reactions of aldono-1,4-lactones (e.g., 9, 55) with acetone or acetophenone (Scheme 17) generate hemi-acetals of type 56a-c [76]. Similarly, lactone 55b reacts with CHsCN/NaH to give hemiacetal 57. 

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