Enol ether-type glycosides

Enol Ether-Type Glycosides 3.1 Early Contributions... 

Boons and co-workers introduced stable allyl glycosides (e. g. 323, O Scheme 53), which are converted to the enol ether-type glycosides 324, prior to glycosylation [176],... 

Of the three types of oxepines shown in Fig. 7, those of the enol ether type (136) seem to be the most versatile in terms of their ability to form septanose glycosides. 

One of the most widely used methods for the formation of C-glycosides involves electrophilic species derived from the sugar component. Some representative examples, shown in Figure 2.0.1, include oxonium compounds, lactones, enones, carbenes, and enol ethers. Once formed, nucleophiles can be induced to react with these reactive species thus providing tremendous diversity in the types of C-glycosides accessible. This chapter focuses on various approaches made use of in the formation of C-glycosides via the addition of nucleophiles to carbohydrate-derived electrophiles. 

Type-Ill cyclizations are initiated by conversion of an acetal, ketal or enol ether into an oxonium ion. Such an activated carbonyl group attacks an olefin intramolecularly in an exo reaction. Mono- and trialkylated double bonds form new C-C bonds regioselectively at the least hindered terminus according to Markovnikov s rule, and in this case endo cyclization products can be produced25. It is worth noting that acetals are also used as initiators in cation olefin cyclizations (see Section 1.5.5.3.)26. Use of activated acetals in Lewis acid catalyzed C-glycoside synthesis is also known 12c 12e. 

In previous investigations we could show that carbohydrate enones with the electrophilic terminus of the unsaturated carbonyl system at the anomeric carbon react with silyl enol ethers under catalysis of titanium tetrachloride, i. e. in a Mukaiyama-type reaction, to give the corresponding C-glycosides in high diastereoselectivity. The high stereocontrol observed could be explained in terms of a strongly adjusted cycloaddition-like transition state. This... 

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