Trichloroacetimidates reaction

Benzyl groups are usually introduced by the Williamson reaction (Section 3.2.3). They can also be prepared under nonbasic conditions if necessary. Benzyl alcohols are converted to trichloroacetimidates by reaction with trichloroacetonitrile. These then react with an alcohol to transfer the benzyl group.183... 

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... 

Conventionally, the use of NaH or Cs2C03 as a base for the reaction of glycosyl hemiacetals with C13CCN often yields the thermodynamically favored a-glycosyl trichloroacetimidates, whereas the use of K2C03 often yields kinetically controlled P-glycosyl trichloroacetimidates (Scheme 3.26). The use of l,8-diazabicyclo[5.4.0] undec-7-ene (DBU) often provides a/p mixtures, mostly favoring a-products. 

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... 

Scheme 3.52), whereas the corresponding acetyl-protected donor gave little product under the same reaction conditions [512], Additionally, the trichloroacetimidate method has also been applied to synthesize other natural products, such as bupres-tin A and B [517], macrophylloside D [518] and neomycin mimetics [519]. 

In addition, a novel fluorous support has been developed recently as an alternative to traditional polymer supports and applied successfully to oligosaccharide synthesis in combination with the trichloroacetimidate method [541]. Each intermediate in the fluorous oligosaccharide synthesis [542,543] could be obtained by simple fluorous-organic solvent extraction, and the reactions could be monitored by TLC, NMR and MS, in contrast to solid-phase reactions. Moreover, the new liquid-phase technique is anticipated to be easily applicable to the large-scale synthesis. 

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... 

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... 

Accordingly, phosphoric acid mono- and di-esters permit uncatalyzed glycosyl transfer from 0-(glycosyl)trichloroacetimidates (52a,55-57,58a,58b). The reaction is thus very useful in the synthesis of glycophos-pholipids (1,55), which are important constituents of cell membranes (1). Commonly, direct phosphorylation at the anomeric hydroxyl group leads to... 

Scheme 7.—Reaction of a- and / -Trichloroacetimidates with Dibenzyl Hydrogenphosphate.

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