Glycosidation trichloroacetimidates

The selective activation of glycoside trichloroacetimidates over phenyl selenoglycosides was also demonstrated by glycosidation of phenyl seleno-glycoside acceptors 168 and 169 with glycosyl trichloroacetimidate 172. In the presence of catalytic amounts of triethylsilyl trifluoromethanesulfonate at -78°C, the disaccharide derivatives 170 and 171 were obtained in respectively 84 and 90% yields. 

Trichloroacetonitrile reacts with glycosidic hydroxy groups of protected sugars to form glycosyl trichloroacetimidates (R. R. Schmidt, 1980, 1984,1985,1986 B. Wegmann, 1988). The imidate is substituted by alcohols in the presence of trimethylsilyl trifluoromethanesulfonate... 

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

Deoxy-P-glycosides are important structural components of many natural products. Recently, 2-deoxy-2-iodoglycosyl trichloroacetimidates [469] have proved to be... 

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... 

PTFAs are generally less reactive than the corresponding trichloroacetimidate donors presumably because of the lower N-basicity or the presence of an N-sub-stituent. Although most trichloroacetimidate activators could also be used to promote PTFA glycosidations, such as TMSOTf [384], BF3Et20 [384,547], TBSOTf [548], Yb(OTf)3 [549,550] and acid-washed molecular sieves [551], the activation of PTFA usually requires more forceful conditions. Several representative Lewis-acid-catalyzed PTFA glycosidation reactions are listed m Scheme 3.58. It is worth... 

ANOMERIC-OXYGEN ACTIVATION FOR GLYCOSIDE SYNTHESIS THE TRICHLOROACETIMIDATE METHOD... 

The trichloroacetimidate method for glycoside synthesis extended its versatility right at the outset (51,52a) by exhibiting an especially smooth reaction of 0-(glycosyl)trichloroacetimidates with Bransted acids. Without the addition of any catalyst, simple Bransted acids are able to substitute the trichloroacetimidate group at room temperature in high yields, as shown (17) for la-a in Scheme 6. Because of anomerization of possible / products... 

The synthesis and application of 0-glucosyl trichloroacetimidates is focused on 0-benzyl- and 0-acetyl protected derivatives (1,52a) because these two protective groups have proven to be the most valuable in glycoside synthesis. Representative examples of trichloroacetimidate formation are collected in Table I (la- Id). As already outlined (Section III. 1), the glucosyl trichloroacetimidates are obtained in high yields and the diastereoselectivi-... 

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