Acetonitrilium ions

A. J. Ratcliffe and B. Fraser-Reid, Generation of a-D-glucopyranosyl acetonitrilium ions. Concerning the reverse anomeric effect, J. Chem. Soc. Perkin Trans 7 747 (1990). 

A report by Jprgensen and co-workers642 about the synthesis of the first chiral tertiary alkylsilicenium ion 248 was also questioned. DFT/IGLO NMR studies showed627 that it is a silylated acetonitrilium ion. Calculations (B3LYP/6-31G level) to find a minimum energy structure for the free silicenium ion failed. Instead, the intramolecularly silylated spirocyclopropylarenium ion was found. 

Seldomycin factor 2, 131 Selectins, 358, 366, 420 Shikimate pathway, 471 (-)-Shikimic acid, 572 Showdomycin, 530 Sialyi dimeric Le 324 Sialyl glycosides, 357 p-acetonitrilium ion, 362 anomeric effect, 362 benzeneselenyl triflate, 359 biology, 366, 419, 421, 436, 445... 

Ratcliffe and Fraser-Reid found that acetonitrile was able to trap glycosyl oxocarbenium ions (e. g. 46), arising from NPGs, to give acetonitrilium ions, e. g. 84 (O Scheme 12a) [56]. The latter reacted with water to produce intermediate 85 that evolves to a-amide 86, in a Ritter-t) e reaction [57] (O Scheme 12a). 

The synthetic value of the above-mentioned transformation was considerably enhanced when a carboxylic acid, rather than water, was used to trap the pyranosylacetonitrilium ion (O Scheme 12b) [59]. Reaction of 87 with aspartic acid derivative 88, in dry acetonitrile containing NBS, led to a-imide 89 in 61% yield [60,61]. The acetonitrilium ion 90 was trapped by carboxylic acid 88, to give an imidic anhydride 91, which rearranged in situ to give the Af,Al-diacyl derivative 89. The route to 2-acetamido-l-Al-(L-aspart-4-oyl)-2-deoxy-j8-D-glycopyranosyl-amine 92, was completed by selective Ai-deacetylation of 89 with piperidine (O Scheme 12c) [59]. 

Assumed66 to be indirect via Cl2, but an exception is known0, °° Indirect in many cases,66 but exceptions have been demonstrated00 Indirect via I266 Indirect via N-iodo-acetonitrilium ion and/or N-iodo-acetamide ... 

On the other hand, it has been demonstrated that the a-D-glucopyranosyl aceto-nitrilium ions are stereospecifically generated from the corre.sponding oxocarbenium ions in dry acetonitrile [43-45]. These results indicate that the acetonitrilium ions are axially oriented by an anomeric effect. Recently, it has been concluded from the conformational analyses of glucopyranosyl-ammonium and glucopyranosyl-imidazolium derivatives that the reverse anomeric effect does not exist [46,47]. 

Therefore, it seems most plausible that the stereospecific generation of the P-acetonitrilium ion (d) of NeuSAc from the oxocarbenium ion (c) holds the key of the a-prepon-derant formation of sialyl glycosides in acetonitrile. Thus, the reactive nucleophiles, such as OH-6 of 17 have a chance to attack on other intermediates (a) (c) before the complete... 

One of the first reports [59] that challenged the existence of RAE was the stereospecific formation of a-D-glucopyranosylacetonitrilium ion 77 when a- and P-anomers of pent-4-enyl D-glucopyranoside 70 reacted with Af-bromosuccinimide in dry acetonitrile (68 % from p-anomer and 64 % from a-anomer). The anomeric configuration of 77 was determined by trapping the acetcmitrilium ion 77, in situ, with 2-chlorobenzoic acid and by subsequent conversion of the obtained a-imide 73 with sodium methoxide to a-2-chlorobenzamide 74 (Fig. 2.25). The exclusive formation of axially oriented a-acetonitrilium ion 77 is clearly in contrast to what would be predicted by the reverse anomeric effect... 

However, the indirect route involving the acetonitrilium ion adduct 64a was successful. Thus the aspartic acid derivative 61 captured nitrilium intermediate 64a (Scheme lOd) to give glycopeptide 68 (R=Ac) in 50% yield [84]. 

This scheme shows the reactions that can occur in the apparent proton-transfer equilibrium reaction between 2-butyne (PAapp = 778 kJ mol" ) and acetonitrile (PA = 787 kJ mol ). The scheme shows an equilibrium with 1,3-butadiene (PA = 787 kJ mol" ) typical of normal thermoneutral proton-transfer reactions in the gas phase with formation of complexes, 7 and 8, between acetonitrilium ion and 1,3-butadiene, and 1-methylallyl cation and acetonitrile, respectively. Formation of the ion molecule complexes 7 and 8 would be expected to be exothermic by about 60 kJ mol" as a result of charge-induced dipole and charge-dipole interactions. From the... 

Taking into account all these results the critically important factors seem to be (1) the rate of generation of the oxocarbenium ion, (2) the preferred formation of the P-acetonitrilium ion intermediate, and (3) the nucleophilicity of sugar acceptors this suggests the reaction mechanism [8c, 9] proposed in Figure 3. 

It has been demonstrated that the a-o-glucopyranosyl acetonitrilium ions are stereospecifically generated from the corresponding oxocarbenium ions in dry ace-... 

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