Ribopyranose tetraacetate

The reaction of p-D-ribopyranose tetraacetate (63) with methanethiol and zinc chloride gave 32), after deacetylation, methyl 1-thio-p-D-ribo-pyranoside (64), methyl 1,5-dithio-p-D-ribopyranoside (72, R=H), 4-S-methyl-4-thio-L-lyxose dimethyl dithioacetal (77, R=H), and a small... 

Figure 9. The low-field portion of the 220 MHz NMR spectrum of fi-D-ribopyranose tetraacetate in acetone-d6 at 20, —60, —70, —84°C...

The acetylation of D-ribose in pyridine solution at low or ordinary temperatures leads to the formation of a crystalline tetraacetate melting at 110° and showing [ ]26D — 54.3° in chloroform.56-14 The ring structure of this substance is retained on conversion to triacetyl-D-ribosyl bromide since the latter compound may be reconverted to it by treatment with silver acetate.66 While indirect evidence early indicated94 that the bromide as well as the tetraacetate possessed a pyranose structure the question was not settled unequivocally until the bromide was used in the synthesis of nucleosides which were known to be pyranosides through quantitative periodate oxidation.95 While the configuration of the crystalline tetraacetate at carbon one will not be known with certainty until its anomer is obtained, the compound may provisionally be considered jS-D-ribopyranose tetraacetate because of its strong levorotation. 

C. S. Hudson, J. Am. Chem. Soc., 46, 462 (1924). Incidentally, the assignment of these two strongly levorotatory halides to the 0-n-series supports the previous allocation (p. 148) of the crystalline D-ribopyranose tetraacetate to the /J-D-series. 

Other workers have studied conformational and configurational equilibria by nuclear magnetic resonance spectroscopy. Horton and coworkers295room temperature in the CE and CA conformations, in rapid equilibrium, in the ratio of 9 11. At —84°, the ratio of the CE and CA conformers was 2 1 at —60°, the rate of interconversion of the two conformers was found to be 130 times per second. The equilibrium... 

By this method, it was found that a solution of j3-D-ribopyranose tetraacetate in acetone-de at room temperature contains 45% of the 1C(d) conformer and 55% of the C1(d) conformer, thus indicating that the equilibrium constant for the 1C(d) C1(d) process... 

The nature of the solvent and its polarity do not appear to affect in any regular way the position of the conformational equilibria for example, /3-D-xylopyranose tetrabenzoate, which exists in ace-tone-de (dielectric constant, e, = 20.7) as a 1 1 mixture of the CJ(d) and 1C(d) conformations, shows practically the same conformational population in a range of solvents, including benzene-rie = 2.3), toluene-dg ( = 2.4), chloroform-d (e = 4.8), pyridine-dg (e = 12.3), hexachloroacetone, and methyl sulfoxide-dg ( = 48.9). Similar results, showing negligible dependence of conformational populations on the polarity of the solvent, were obtained with jS-D-ribopyranose tetraacetate, tri-0-acetyl-/3-D-xylopyranosyl chloride, and some simpler derivatives of tetrahydropyran. In contrast, as the polarity of the solvent was increased, there was observed with methyl 2,3,4-tri-0-benzoyl-/3-D-xylopyranoside a broad trend in favor of that conformation having the C-1 substituent equatorially attached. 

This is clearly the case with neodisyherbaine 224 and it is not surprising that ribopyranose-tetraacetate 225 was chosen as the starting material for the [73] left part of the natural product. 

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