Carbonylonium ions: the onium ions of the carbonyl group

• Daniel Blanco-Ania and
• Floris P. J. T. Rutjes

Beilstein J. Org. Chem. 2018, 14, 2568–2571, doi:10.3762/bjoc.14.233

• aldehyde- and ketone-based intermediates, respectively. Keywords: carboxonium ion; glycosylium ion; oxacarbenium ion; oxocarbenium ion; oxycarbenium ion; Introduction There is much confusion in the literature over the name of the intermediates R1C(=O+R3)R2 (R1, R2, R3 = H or organyl [1], 1; Figure 1). In

• replacement of carbon atoms by the heteroatoms oxygen, nitrogen and sulfur, respectively. Thus, “oxacarbenium ion” would denote a carbenium ion whose carbon atom is replaced by an oxygen atom, that is, an oxonium ion (3; Figure 2). Although if a coherent structure by formal subtraction of hydride from the

Intramolecular glycosylation

• Xiao G. Jia and
• Alexei V. Demchenko

Beilstein J. Org. Chem. 2017, 13, 2028–2048, doi:10.3762/bjoc.13.201

• reacted with 1,2-anhydro donor 80. It was assumed that this reaction proceeds via the oxacarbenium ion tethered to a tetracoordinated boronate ester. The subsequent glycosylation then proceeds regioselectively from the less-hindered boron–oxygen bond (see intermediate A). In this case, where gluco

• glycosyl acceptors 84–86 were reacted with 1,2-anhydromannosyl donor 87 (Scheme 20). The tethered oxacarbenium ion intermediate then directs the nucleophilic attack intramolecularly to the β-face of the mannosyl donor. As a result, disaccharides 88–90 were obtained in 83–99% yields and exclusive β-manno

• orientation of the picolinyl/picoloyl group [114]. The stereoselectivity was explained by the occurrence of the hydrogen bonding between the hydroxy group of glycosyl acceptor (NuH) and the nitrogen atom of the picolinyl/picoloyl group. Subsequently, the glycosyl acceptor is delivered towards the oxacarbenium

Synthesis of D-fructose-derived spirocyclic 2-substituted-2-oxazoline ribosides

• Madhuri Vangala and
• Ganesh P. Shinde

Beilstein J. Org. Chem. 2015, 11, 2289–2296, doi:10.3762/bjoc.11.249

• in toluene through nucleophilic addition of electron-rich nitriles to the oxacarbenium ion intermediate of 1,2;3,4-di-O-isopropylidene-β-D-psicofuranose derivatives with concomitant intramolecular trapping of the C2 hydroxymethyl group on the electrophilic nitrilium carbon. These carbohydrate-derived

• oxacarbenium-ion intermediate by a nitrile and subsequent intramolecular nucleophilic attack of the vicinal C2 ether or a free hydroxy group [32][33][34]. Such glycooxazolines are exploited for the generation of N-glycan structures [35]. In O-glycosylation reactions, an oxacarbenium-ion intermediate interacts

• , the complete consumption of the starting material and the formation of spiro 2-substituted-2-oxazoline 6a in 69% yield was observed. This Ritter-like reaction is known to proceed via TMSOTf-mediated in situ cleavage of the 1,2-O-isopropylidene group, generating an oxacarbenium ion and its nucleophilic

Catalysis: transition-state molecular recognition?

• Ian H. Williams

Beilstein J. Org. Chem. 2010, 6, 1026–1034, doi:10.3762/bjoc.6.117

• displacement mechanism involving a covalent glycosyl-enzyme intermediate. Formation and hydrolysis of this covalent intermediate occur via oxacarbenium ion-like TSs, with the assistance of two key active site glutamic acid residues [20]. Glu78 is deprotonated in the noncovalent enzyme-substrate reactant