Trichloroacetimidates, synthesis

These results encouraged the authors to attempt the synthesis of more-complex oligosaccharides, such as the Lex and Lea derivatives, 65 and 66, respectively. After formation of the (1 — 3)-fucosyl linkage, as promoted by HC104-Si02, in a stereo-and regio-selective manner, the peracetylated galactosyl trichloroacetimidate was added to afford the desired Lex derivative 65 in 62% yield. The same method was applied to obtain the Lea derivative 66 in 59% (Scheme 14).86... 

The on-column synthesis approach was also explored, followed by in situ purification of the products for the glycosylation reactions using trichloroacetimidates. The authors primed the top of a standard silica chromatography column with perchloric acid immobilized on silica. After charging both reactants, dissolved in dry CH2C12, onto the... 

SCHEME 15. Synthesis of an Avermectin B1 analogue by glycosidation of a trichloroacetimidate donor catalyzed by HC104-Si02. 

This review covers the recent advances in the use of O-glycosyl imidates in oligosaccharide and glycoconjugate synthesis, with emphasis on literature published between 1999 and 2006. However, because of the large volume of work in this area, only the most representative applications will be presented. One can refer to the similar preceding review [381] published in 2000 and another quite comprehensive review [375] on trichloroacetimidate method published in 1994 for earlier application of glycosyl trichloroacetimidates. Trifluoroacetimidate method will be discussed separately in this review in the light of its less popularity in carbohydrate chemistry. 

The trichloroacetimidate method has also found wide applications in the synthesis of various complex oligosaccharides. In the course of the development of chemically defined glycoconjugate vaccines against shigellosis, a decasaccharide, corresponding to two consecutive repeating units of the O-specifk polysaccharide of Shigella... 

In addition, the fulvestrant could be glycosylated effectively at its 17-OH position with pivaloylated glycosyl trichloroacetimidates, which suppressed the competing transacylation side reaction and led to improved yields of the desired glycosides (Scheme 3.48d) [503]. In this synthesis, the inverse procedure (i.e. addition of a trichloroacetimidate donor to a mixture of an acceptor and a promoter) was found to be superior for glycosylations. Very recently, a stepwise synthesis of branched... 

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