Acyloxonium intermediate

Recently, Wiesner et al. [108] succeeded in a stereoselective P-glycosylation in the D-ribo series by use of an anchimeric assistance from the 3 position. The 3-O-p-methoxy-benzoyl group is supposed to form the 1,3-acyloxonium ion intermediate 215 either starting from an at- or P-configurated ethyl thioglycoside precursor (214). 

Acyloxylation of aryl olefins probably involves radical cations as intermediates. Acetoxylation of frans-stilbene in anhydrous acetic acid/sodium acetate yields mainly meso-diacetate, while in moist acetic acid mainly threo-2-acetoxy-l,2-di-phenylethanol is formed 100 Anodic oxidation of trans- and ds-stilbene in ace-tonitrile/benzoic acid produces with both olefins the same mixtures of meso-hydrobenzoin diacetate (62) and f/ireo-2-benzoyloxy-l,2-dip]ienylethanol (63) l01 Product formation is best rationalized by a ECiqE-sequence leading to theienerge-tically most favorable acyloxonium ion (64) (Eq. (125) ) ... 

Exposure of 3-(tetrahydro-2-furyl)-3-trimethylsilylpropanoic acids 178 to trifluoroacetic anhydride allows intramolecular acylative ring-opening reaction to give the corresponding eight-membered lactones 180 in moderate to good yields (Scheme 46) <1999SL1757>. However, when R1 = R2 = Me, lactones 180 are not formed. The acyloxonium ion 179 is a probable intermediate in these reactions. Similarly, acids 182 afford lactones 183 (Scheme 47). 

Reactive glycosyl halides which cannot form an intermediate acyloxonium ion, such as 2-de-oxysugar halides, usually give mixtures of a- and j8-nucleosides. 

The Lewis acid catalyst, such as trimcthylsilyl triflate, reacts simultaneously with the peracylated sugar bearing a 2-acyloxy group to generate in situ the electrophilic 1,2-acyloxoni-um ion D and with the silylated purine A to form in situ the trimcthylsilyl tr-complex at Nl, the center of highest electron density, B. The a-complex B can also exist in equilibrium with the Nl trimethylsilyl derivative C. Either of the silylated intermediates reacts with the acyloxonium cation to result in a 1-, 3-, 7-, and 9-ribosylation. Under optimum reaction conditions, the 1-, 3-, and 7-isomers rearrange to the thermodynamically most favored N9 isomer... 

It is highly probable that the initial step in the overmercaptalation of 27 involves formation of a 2,3-acyloxonium ion however, formation of the 1,2-episulfonium ion 28 as an intermediate in the replacement of the 2-hydroxyl group has been established130 through the conversion of 27 into 3,4,5,6-tetra-0-benzoyl-2-S-ethyl-2-thio-D-man-nose ethyl phenyl dithioacetal (34) by the action of benzenethiol and an acid catalyst. The presence of an ethylthio group on C-2 in 34 was further established by conversion of 34 into 3,4,5,6-tetra-O-benzoyl-2-S-ethyl-2-thio-D-mannose dimethyl acetal. 

Acyloxonium ions (Section 7.3) are stabilized by heavy anions. The formation of acyloxonium ions is also assumed, this time as non-isolated intermediates, in the solvolysis of some tosylates. In reaction (7,12) with sodium acetate, the introduction of an acetyl group at position 6 and migration of benzoate to position 5 testify to the presence of a benzoxonium intermediate. Likewise, solvolysis of sulfonates presenting a frani -vicinal acetamido group involves an oxazolinium cation 7.28, the analogue of an acyloxonium. 

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