Rearrangement of acyloxonium ions

In continuing their studies on the rearrangements of acyloxonium ions in carbohydrate chemistry, Paulsen s group has examined the ions formed from 2,3,4-tri-O-acetyl-l,5-anhydro-D-arabinitol. The acetoxonium ion (110) can undergo a 1,2-rearrangement to give the chiral ion (111) or a competitive 1,3-rearrangement... 

From the results given in Table I, it may be seen that the acyloxonium ion rearrangement proceeds the most readily in the (1,2/3)-cyclopentanetriol system. This type of study has been extended to polyhydroxycyclopentane derivatives that are capable of such rearrangements. Both tetra-O-acetyl- and tetra-0-pivaloyl-(l,3/2,4)-cyclopentanetetrol (38a or 38b) react with antimony pentachloride... 

In contrast, if 56,57, or 55 in dichloromethane is treated at 20° with antimony pentachloride, a rapid acyloxonium rearrangement of the D-gluco ion 59 ensues, first by neighboring-group participation to give the D-manno ion (60), and, subsequently, to the D-altro ion (61), and then to the D-ido ion (62). In the solution, an equilibrium 59 60 61 62 exists between the ions. The equilibrium com-... 

To explore the dependence of the equilibrium on the nature of the anion, the fluoroborate salt of 59 was prepared. It was obtained at low temperature (0 ) either by the reaction of tetra-0-acetyl-/3-D-glu-copyranosyl fluoride (53) with boron trifluoride in carbon tetrachloride, or by treatment of tetra-O-acetyl-a-D-glucopyranosyl bromide (54) with silver fluoroborate in nitromethane. In edier, compound 54 and silver fluoroborate give a mixture of tetra-O-acetyl-a- and /3-D-glucopyranosyl fluoride, because the ether complexes with the boron trifluoride, and so a fluoroborate anion is no longer available. On dissolution of the fluoroborate salt of the D-gluco acetoxonium ion (59) in nitromethane or acetonitrile, an acyloxonium-ion rearrangement, is observed with consequent equilibration, between 59 60 61 62, that is not significantly different from... 

The reactions of esters of monosaccharides in hydrogen fluoride have been studied intensively by Pedersen. Besides making analyses of the rearranged products, it has been found of great value to monitor the progress of the reactions by n.m.r. spectroscopy. Measurements can be made directly on the reaction mixtures in hydrogen fluoride by use of a Teflon tube. The acyloxonium ions formed as intermediates can be detected, and their n.m.r. spectra analyzed. 

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... 

A hexachloroantimonate salt of the D-gluco acetoxonium ion is isolable only if the reaction is performed at low temperature (—10°) in a nonpolar solvent (such as carbon tetrachloride) from which the salt is precipitated, by virtue of its insolubility, as fast as it is formed. Under these conditions, the ion 59 has no time to undergo successive acyloxonium rearrangements to 60, 61, and 62, because the rate of precipitation is greater than the rate of rearrangement. The salt thereby obtained contains over 90% of the D-gluco derivative. ... 

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