Neighboring‐Group Participation by C‐2 Ether Functions in Glycosylations Directed by Nitrile Solvents

Abstract

Ether-protecting functions at C-2 hydroxy groups have been found to play participating roles in glycosylations when the reactions are conducted in nitrile solvent mixtures. The participation mechanism is based on intramolecular interaction between the lone electron pair of the oxygen atom of the C-2 ether function and the nitrile molecule when they are positioned in a cis configuration. A 1,2-cis glycosyl oxazolinium intermediate is formed. This participation, in conjunction with the anomeric effect of the glycosyl donor, confers high 1,2-trans selectivities on glycosylations. Further application of this concept has led to efficient preparations of α-(1→5)-arabinan oligomers.

AuIII‐Halide/Phenylacetylene‐Cata­lysed Glycosylations Using 1‐O‐Acetyl­furanoses and Pyranose 1,2‐Ortho­esters as Glycosyl Donors

“…But this gave a complex mixture (Table 3, entry 1). We therefore tested benzyl-protected methyl 1,2-orthoester 9 [23] in the AuCl 3 /phenylacetylene-catalysed glycosylation, and this gave the desired glycoside (i.e., 12a) in 61 % yield (Table 3, entry 2) within 10 min. We then carried out the same reaction using AuBr 3 , which appeared to be a better catalyst than AuCl 3 as we obtained a higher yield (76 %) of 12a.…”

“…Benzyl-protected D-galactose 1,2-orthoester 10 [23] and Dmannose 1,2-orthoester 11 [23] were also tested for glycosylations using the AuBr 3 /phenylacetylene reagent system (Table 4). All the reactions of benzyl-protected pyranose 1,2-orthoesters were found to be 1,2-trans selective.…”

Synthesis of lupane saponins from acetylated glycosyl donors by acetonitrile directed glycosylation

“…It is well known that, in the presence of acetonitrile which forms a nitrilium ion during the donor activation step, the neighbouring acetyl group is unable to form an acyloxonium ion during glycosylation and, as a result, orthoesters are not formed. 35,36 Therefore, we examined the influence of acetonitrile as a solvent in the above reaction. Due to very poor solubility of lupeol and betulin derivatives in acetonitrile at low temperature, addition of dichloromethane as co-solvent was necessary.…”

Synthesis of a Tristearoyl Lipomannan via Preactivation-Based Iterative One-Pot Glycosylation

“…The following NMR data agreed well with that of the reported. 30 1 H NMR (600 MHz, CDCl 3 ) δ 7.38-7.25 (m, 15H, Ph), 7.20 (d, J = 7.2 Hz, 2H, Ph), 7.05 (d, J = 7.2 Hz, 2H, Ph), 5.60 (d, J = 1.8 Hz, 1H, H-2), 5.46 (s, 1H, H-1), 4.89 (d, J = 10.8 Hz, 1H, Bn), 4.73 (d, J = 10.8 Hz, 1H, Bn), 4.66 (d, J = 12.0 Hz, 1H, Bn), 4.57 (d, J = 11.4 Hz, 1H, Bn), 4.51 (d, J = 10.8 Hz, 1H, Bn), 4.46 (d, J = 12.0 Hz, 1H, Bn), 4.34 (m, 1H, H-5), 3.96-3.93 (m, 2H, H-3,4), 3.85 (ddd, J = 10.8, 4.8, 1.8 Hz, 1H, H-6), 3.73 (d, J = 10.8 Hz, 1H, H-6′), 2.30 (s, 3H), 2.13 (s, 3H). 13 C NMR (150 MHz, CDCl 3 ) δ 170.3, 138.3, 138.2, 137.8, 137.6, 132.3, 129.9, 129.8, 128.4, 128.3, 128.2, 128.1, 127.8, 127.7, 127.6, 127.5, 86.5 (C-1), 78.5, 75.2, 74.6, 73.3, 72.4, 71.9, 70.3, 68.9, 21.07, 21.06.…”