Pentenyl glycoside

SCHEME 3.16 Synthesis of disaccharide 36 using a thioimidate glycosyl donor. 

SCHEME 3.18 Mechanism of glycosylation involving n-pentenyl glycosides. 

SCHEME 3.19 Glycosylation using n-pentenyl glycoside. ClAc, chloroacetyl. 

TV-Bromosuccinirnide (NBS) and NIS are the most widely used electrophiles to activate -pentenyl glycosides. Catalytic amounts of acids such as TfOH or trieth-ylsilyl triflate (TESOTf) are introduced to speed up the generation of bromonium or iodonium ions [43], The stereochemical outcome depends, as usual, on whether a participating group exists at C2. An alternative promoter is iodonium dicollidine perchlorate (IDCP), whose lower reactivity allows it to selectively activate certain disarmed n-pentenyl glycosides [44], 

Anomeric 4-pentenoate esters have also been shown to behave as glycosyl donors, liberating 4-iodomethyl-y-lactonc as the by-product [46], 

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POMCl, (i-Pr)2EtN, CH2CI2. The related pentenyl glycosides (an example is shown below), prepared by the usual methods, were used to protect the anomeric center. ... 

It was reported that selective activation of the SBox glycosyl donors over SEt and O-pentenyl glycoside acceptors can also be achieved in the presence of AgOTf... 

The synthesis of (3-D-fructofuranosides is yet another useful application of this concept [293-295]. The latter 1,2-cis-glycosidic linkage is as difficult to establish as in the case of (3-mannosides. In an elegant synthesis of a-D-fucofuranose-containing disaccharides, Plusquellec and coworkers used the IAD concept via p-methoxybenzylidene acetals in combination with a glycosylation protocol via pentenyl glycosides. Here, the intermediate NIS-adduct could be isolated (Scheme 5.108) [295]. 

Synthesis of Oligosaccharides on Solid Support Using Thioglycosides and Pentenyl Glycosides... 

Scheme 5.7 (a) Mechanism of the w-pentenyl glycoside-based glycosylation method. Electrophiles commonly employed include iodonium di-sym-collidine perchlorate (IDCP) and A-iodosuccinimide/triethylsilyl triflate (NIS/Et3SiOTf) (b) Mechanism for the activation of NIS by EtsSiOTf.55... 

Scheme 5.8 Armed/disarmed concept for chemoselective coupling of two w-pentenyl glycosides (NPGs). A 2-0-alkylated (armed) NPG 29 is much more readily activated than is a 2-0-acylated (disarmed) NPG 30.62...

Scheme 11.3 Using a C2-protecting group for arming/disarming of an n-pentenyl glycoside for oxonium ion formation.

Building on the observations of Kahne and Takahashi, Chenault and Castro relied on the selective activation of an isopropenyl glycoside donor with TMSOTf, followed by subsequent activation of a second n-pentenyl glycoside with NIS/TESOTf.12 This one-pot glycosylation offers proof of the concept that Fraser-Reid s glycosides can be incorporated into one-pot syntheses. (See Scheme 11.7.)... 

Scheme 42 N-Linked a-Glycosides Obtained through Bromonium Activation of Pentenyl Glycosides in Acetonitrilel409-410 ...

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