Benzoxonium ions

Another internal acetal, methyl 2-0-benzoyl-3,4-0-benzylidene-/ -D-arabinopyranoside 103 (84), behaves in an analogous manner, except that the isomeric 4-bromo (104) and 3-bromo (105) benzoate derivatives are formed in a ratio of 1 1. The identity of 104 (l-xylo) can be deduced from its mode of formation from a presumed benzoxonium ion (106)... 

C-6, the benzoxonium ion from 113 is attacked by bromide ion preferentially and selectively at the less hindered acyclic C-5 position, rather than the C-3 ring position. 

Extension of the C-glycosylation reaction to the D-ribo series was at first disappointing. The sole product, obtained in high yield, from the reaction of 2,3,5-tri-0-benzoyl-/3-D-ribofuranosyl chloride (154), or the corresponding bromide (81), with diethyl sodiomalonate in 1,2-dimethoxyethane was the acetal 155, resulting102 from an attack on the dioxolenium carbon atom in the intermediate benzoxonium ion (156). 

Reactions that have led to other deoxyhalogeno sugars do not necessarily lead to deoxyfluoro sugars, as, for example, in the attempted decomposition of fluoroformates, the treatment of diazoketones and of 2-deoxy-2-diazohexonates with hydrogen fluoride, and the reaction of benzoxonium ions with halide ions. The reaction2281229 by which fluoroformates are thermally or catalytically decarbonylated to give alkyl fluorides has been applied to carbohydrates. Both thermal and catalytic treatment of 6-0-(fluoroformyl)-l,2 3,4-di-0-isopropylidene-... 

I, 3-Benzoxonium-Ions 52) lieB sich nicht bestatigen 5). 209b wird offen-... 

C-3 (or C-4) from a 1,3-dioxolane actually, this reaction (see Section HI, 14) was observed for 2,2-disubstituted 1,3-dioxolanes, and it may be assumed that, if there is any competition between removal of a proton from C-2 and from C-3 (or C-4), the rate of the first reaction is far greater than that of the second. The possibility of dioxolanylium ions reacting with nucleophilic reagents was first demonstrated by Hanes-sian and Staub,152 and the scope of these reactions was independently investigated by Pedersen and coworkers.153-157 Hanessian and Staub152 treated methyl 5-0-benzoyl-2,3-0-benzylidene-j3-D-ribofuranoside (125) with triphenylmethyl fluoroborate in acetonitrile, and obtained, after hydrolysis, a 1 1 mixture of the two benzoates 127 and 128 (yield 83%), probably through the benzoxonium ion 126. The same reaction... 

Jacobsen and Pedersen153 confirmed the generation of benzoxonium ion 126 by treatment of compound 125 with triphenylmethyl fluoroborate in acetonitrile, but they treated it with tetraethylammonium bromide, obtaining a 3 2 mixture of the a and /3 anomers of methyl 2,5-di-0-benzoyl-3-bromo-3-deoxy-D-xylofuranoside (134) in a yield of 74%. This mixture could be anomerized to give the pure )8 anomer (overall yield 60%) by treatment with hydrogen bromide followed by the action of methanol. 

The first example in the pyranoid series was afforded153 by methyl 2-0-benzoyl-3,4-0-benzylidene-j8-D-arabinopyranoside (92), which gave the corresponding benzoxonium ion (93) this ion was shown to be in equilibrium158 with its regio- and stereo-isomer 94, the latter... 

J) For instance, hydrolysis of benzoxonium ions 93 and 93a gave, respectively, the 4-O-benzoyl derivatives 135 and 135a (respective yield, 67 and 46%). The preservation of the configuration of the start-... 

Previous studies had indicated that, in general, although some differences were sometimes observable, the products formed by photobromination of carbohydrate derivatives using bromine or yV-bromosuccinimide were similar. When compound 31 or 32, however, was treated with the latter reagent, a major difference was found, and the main product (74% isolated) was the orthoamide 35, formed, it was concluded, by way of the bromides 33 and 34 and, thence, a cyclic 3,4-benzoxonium ion.35 Support for this route was obtained by observing that treatment of a mixture of the bromides with yV-bromosuccinimide in refluxing carbon tetrachloride without irradiation resulted in their complete conversion into the orthoamide 35. yV-Bromoacet-... 

From this point of view, a brief comparison of the acyloxylation of cis or trans stil-benes under electrochemical and chemical conditions is also relevant. Anodic oxidation (Pt) of cis or trans stilbenes in the presence of acetic or benzoic acid gives predominantly mesa diacylates of hydroxybenzoin or, if some water is present, threo monoacylate. None of stereoisomeric erythro monoacylate and rac diacylate was obtained in either case. There was no evidence of isomerization of cis-to-trans stilbene under the electrolytic conditions employed (Mango Bonner 1964 Koyama et al. 1969). The sequence of reaction steps in Scheme 2-23 was proposed. Adsorption-controlled one-electron oxidation of the substrate takes place. Then an adsorption-controlled rotation proceeds of cis stilbene monoadduct into thermodynamically more stable trans benzoxonium ion. The trans benzoxonium ion is the common intermediate for conversions of both cis and trans stilbenes and, of course, for all the final products (Scheme 2-24). Hence, oxidized molecules of stilbene are, at the electrode, involved in a reaction with acylate ions. There is no passing into the solution volume, with the following electron being exchanged there with unoxidized molecules of stilbene. 

A neighbouring-group participation mechanism prevents the immediate nucleophilic substitution of iodine by a second equivalent of benzoate that would lead to a syn-substituted product Instead, a cyclic benzoxonium ion intermediate is formed ... 

Equilibrium Compositions of the Acetoxonium Ions 59 60 61 62 and of the Corresponding Benzoxonium Ions... 

The mixture of benzoxonium salts is subject, in various solvent media, to an equilibration reaction similar to that involving the acetoxonium salts 59 and 62. The equilibrium proportions of the four benzoxonium ions (see Table II) correspond approximately to those found for the four acetoxonium ions 59, 60, 61, and 62. The content of the n-gluco ion is somewhat greater, and that of the D-ido ion is proportionately lower, but, evidently, a change in the nature of the acyl group does not lead to substantial alteration in the equilibrium composition. 

The rearrangement of a benzoxonium intermediate can be convincingly demonstrated in the reaction of 3,5-0-benzylidene-1,2-0-isopropylidene-a-D-glucofuranose (120) with N-bromosuccini-mide, whereby the 6-bromo derivative 124 and the 3,6-anhydride 123 are obtained. It may be presumed that the benzoxonium ion 121 rearranges into the ion 122 through intramolecular attack by... 

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