Trichloroacetimidate donors

Glycosyl trichloroacetimidate donors in the presence of selenoglycoside acceptors can be used with triethylsilyl triflate as promoter. 

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

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... 

Scheme 4.111 Selenide acceptors with bromide and trichloroacetimidate donors.

Immobilized 38 also proved to be an appropriate glycosyl acceptor when coupled with a full benzylated trichloroacetimidate donor (Scheme 10.8). Thus, TMSOTf mediated coupling of 38 with an excess of 48 gave resin-bound 49, which was further coupled with acceptor 38 to give trisaccharide 60 (60% overall yield based loading of the resin). 

Fig. 1. 1,6-Glycosylation with a glucose acceptor attached to a polymer support (P = solid support) and a trichloroacetimidate donor.

En route to pseudodisaccharide 96 (Scheme 13.13), the 1,2,6-0-diffcrcntiated inositol derivative 94 was used to react with the trichloroacetimidate donor 39 in the presence of TMSOTf, which supplied 95 in an 89% yield (ot/p=4/1) after full deacetylation. The mixture of anomers carried forth to undergo 4, 6 -0-benzyhdine protection, 3 -0-benzylation, and regioselective benzylidine ring opening, rendering the 04 position free. At this point, the mixture of anomers could be separated, and the pseudodisaccharide acceptor 96 was ready for the key glycosylation step. 

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