Arabinopyranose 1.2.3.4- tetraacetate

The formation of 2,3,4-tri-0-acetyl-5-5-ethyl-5-thio-L-arabinose diethyl dithioacetal (67) when a-L-arabinopyranose tetraacetate (57) was treated o) th ethanethiol in the presence of either zinc chloride or boron trichloride was rationalised by the sequence of reactions shown subsequent work (see below) suggests that an orthoester may be inter-... 

D-Arabinopyranose tetraacetate Methyl D-arabinopyranoside triacetate D-Galactopyranose pentaacetate Methyl 2,3-di-0-acetyl-4,6-0-benzylidene-a-D-galactopyranoside... 

D-Glucitol (sorbitol) hexaacetate, n-inannitol hexaacetate 0-D-Glucopyranose pentaacetate a-D-Arabinopyranose tetraacetate... 

From a consideration of detailed results on the conformational equilibria of aldopentopyranose derivatives, it has been pointed out92- 9 that a more sophisticated model is required before conformational populations can be reliably predicted, at least with acylated derivatives. Even with adjustment of the original parameters in order to take revised values for the anomeric equilibrium of D-lyxopyranose tetraacetate and the conformational equilibrium of /3-D-arabinopyranose tetraacetate into account, the observed data cannot be accommodated within the framework of this model, except on a very broad, qualitative basis. Other possible factors that should be considered " include polar contributions from substituents other than that on C-1, attractive interactions between syn-diaxial acyloxy groups, non-bonded interactions between atoms that have unshared pairs of electrons,repulsive interactions between gauche-vicinal groups, the effect of solvent pressure, and differences between the molar volume of conformers. 

Durette and Horton295(c) studied, by nuclear magnetic resonance spectroscopy, the anomeric equilibria at 27° of the aldopentopyranose tetraacetates in 1 1 acetic anhydride-acetic acid that was 0.1 M in perchloric acid. They found the following values for the ratios of the anomeric tetraacetates (/3la) at equilibrium, with the free energy A G° values for the J3 equilibrium (in parentheses) D-ribopyra-nose, 3.4 (—0.73 0.03) D-arabinopyranose, 5.4 (—1.01 0.03) D-xylopyranose, 0.23 (+0.89 0.03) and D-lyxopyranose, 0.20 (+ 0.98 0.05). 

Surprisingly, the equilibration studies consistently show a substantial proportion of the -D-arabino ion (75), as indicated by a corresponding proportion of tetra-0-acetyl-/3-D-arabinopyranose obtained after hydrolysis and subsequent analysis of the product mixture. The /3-D ion (75) cannot be formed through acetoxonium rearrangement of 72, but must arise through subsequent anomerization of the a-D ion (74) formed initially, because the acetoxonium salt functions as an effective catalyst for anomerization If solutions of anomerically pure aldopentopyranose tetraacetates in nitromethane are treated with 2-metiiyl-l,3-dioxolan-2-ylium hexachloroantimonate (2,R=Me), a rapid onset of anomeric equilibration is observed. Under these conditions, tetra-O-acetyl-D-arabinopyranose gives an a /3 ratio of 9 91, and tetra-O-acetyl-D-xylopyranose, an a /3 ratio of 96 4. 

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