Ribopyranosyl bromide

An amorphous 2,3,4-tribenzoyl-D-ribose has been reported as a product of the hydrolysis of aniline D-ribopyranoside tribenzoate29 as well as of the hydrolysis of tribenzoyl-/3-D-ribopyranosyl bromide with moist acetone in the presence of silver carbonate.110 In the latter case the structure of the product was demonstrated through methylation to the known methyl /9-D-ribopyranoside tribenzoate. Recently Ness, Fletcher and Hudson111 have succeeded in obtaining a 2,3,4-tribenzoyl-D-ribose in crystalline form. 

The benzoylated D-ribopyranosyl halides appear to be considerably more stable than their acetyl analogs and tribenzoyl-/3-D-ribopyranosyl bromide (XXXIX), obtained in crystalline form through the action of hydrogen bromide on /3-D-ribopyranose tetrabenzoate (XLI) in glacial... 

The reaction of /3-D-ribopyranose tetrabenzoate with hydrogen bromide in glacial acetic acid solution has recently been reinvestigated by Ness, Fletcher and Hudson111 and a new, crystalline tribenzoyl-D-ribopyranosyl bromide, isomeric with that previously known, obtained in 5% yield. In contrast to the major product of the reaction, the new halide proved to have a dextrorotation of [a]20D + 78° in chloroform and, since it reacted readily with methanol to give methyl 0-D-ribo-pyranoside tribenzoate (XLII), it was concluded that it is tribenzoyl-... 

As mentioned above, the triacyl-/3-D-ribopyranosyl halides may react with alcohols in the presence of an acid acceptor to give orthoester derivatives. Thus Levene and Tipson56 showed that triacetyl-/3-D-ribopyranosyl bromide reacts with methanol in the presence of silver carbonate to give 3,4-diacetyl-D-ribopyranose methyl 1,2-orthoacetate (LXIV) while Klingensmith and Evans135 obtained an analogous compound (LXV) upon condensing dihydroxyacetone monoacetate with the same halide. The structures of these two orthoesters, LXIV and LXV, have recently been discussed by Pacsu.186... 

Because of the great interest in the naturally occurring nucleosides, studies of the synthesis of D-ribosides of purines and pyrimidines have been numerous. The first attempt in this direction was made by Levene and Sobotka146 who condensed the silver salt of theophylline with tri-acetyl- 8-D-ribopyranosyl bromide to obtain what was probably triacetyl-7- 8-D-ribopyranosyl theophylline (LXXVI). Subsequently Hilbert and Rist94 condensed 2,4-diethoxypyrimidine (LXXVII) with triacetyl-0-D-ribopyranosyl bromide (LXXVIII) to form ethyl bromide and 1-triacetyl-D-ribopyranosyl-4-ethoxyuracil (LXXX), which, on deacetylation with hydrogen chloride, gave 1-D-ribopyranosyluracil (LXXIX), an isomer of... 

Similarly, with 2,3,4-tri-0-benzoyl-/ -D-ribopyranosyl bromide, a mixture of 1-a-D- and l-/3-D-ribopyranosylthymines was obtained.12... 

The neighboring-group concept of Winstein has been used to explain the difference in reactivity exhibited by anomeric pairs of halides. This suggestion is by no means improbable, and would explain the facts that tri-0-benzoyl-/3-D-ribopyranosyl bromide is 19 times more reactive at 20° in 1 9 dioxane-methanol than is the a anomer, and that the rate for the trans-l3 chloride shows an 85-fold increase over that of the cis-a chloride. However, in view of the properties of the series XXVII to XXX, steric effects from the whole molecule may be equally important, although the disadvantage of this concept is that hindrance as assessed from models is difficult to define precisely. It is of interest to compare the rates of methanolysis of tri-O-benzoyl-a-D-xylopyranosyl bromide and of tri-O-acetyl-a-D-xylopyranosyl bromide [10 fc ° = 51(sec ) in 1 9 dioxane-methanol and = 139(sec 0 in 100% methanol, respectively]. Although the... 

Tribenzoyl- -D-ribopyranosyl bromide (prepn. s. 405) dissolved in abs. methanol by refluxing with shaking for 1 min., then allowed to... 

A study of the stability of compounds derived from D-glucose and D-fructose with boric acid or phenylboronic acid has found that the stability constants for the fructose compounds are much greater than those for the glucose compounds. The synthesis of crystalline 3-0-acetyl-2,4-0-phenylboranediyl- -D-ribopyranosyl bromide in a two pot process from D-ribose has been described along with some of its selective glycosylation reactions. Ethyl (dimethoxy)borane has been used in the synthesis of anomerically pure furanosides from the free sugars. Thus methyl 2,3-0-ethylboranediyl-tt-D-lyxofuranoside was prepared and then de-esterified under essentially neutral conditions to the glycoside in 80%... 

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