44 W. G. OVEREND AND M. STAGEY 



a useful intermediate in the synthesis of naturally occurring ribonucleosides 

 and this offers some confirmation for the structure assigned. The method was 

 improved, so that the product could be isolated directly by accomplishing 

 simultaneous detritylation and acetylation of XLI with acetyl bromide in 

 acetic anhydride.^^* 5-0-Benzyl-D-ribofuranose can also be used as an 

 intermediate for the preparation of this acetate. ^'^ 



Some confusion has arisen in the literature concerning D-ribofuranose tetra-0- 

 acetate. Thus, for this compound Howard et a/.'^^ report m.p. 58° and [a]^^ +20° 

 (in chloroform), Bredereck and Hoepfner"« state that the physical constants are 

 m.p. 56° and [afo —3.6° (in methanol), whereas Zinner'" describes a product of m.p. 

 82° and [a\o -12.6° (in chloroform) and —15.4° (in methanol). Zinner'". "6 referred 

 to his product as /3-tetra-O-acetylribofuranose, and it obviously differed from the 

 acetate prepared by Howard et al.^** and Bredereck and Hoepfner.^e Davoll et al.'^^'' 

 have described some experiences with the preparation of this acetate by the Bredereck 

 method. The first three portions prepared had m.p. 56-58°, but the fourth and all 

 subsequent batches had m.p. 85° and [a]" —12° (in chloroform) and —13.5° (in meth- 

 anol), and were obviously identical with the product prepared by the method of 

 Zinner. After the isolation of the latter product the melting points of earlier samples 

 spontaneously changed to 85° and it became impossible to prepare samples of the 

 acetate of lower melting point. Similarly, samples of products of lower m.p. forwarded 

 to Davoll from other laboratories changed into the higher melting form. According 

 to Davoll et al.'^^'' the change was accompanied by a change in optical rotation. The 

 furanose structure of the material of higher melting point was proved by its conver- 

 sion through the aceto-chloro-compound to adenosine in fields comparable to those 

 obtained using tetra-O-acetyl-D-ribofuranose of m.p. 56-58° as the initial material. 

 Davoll and his colleagues were unable to determine the difference between the iso- 

 mers, but did not think that it was due to simple a,/3-isomerism, since according to 

 them measurements of optical rotation changes indicated that a complex process was 

 operating in which at least three molecular species were involved. They considered it 

 possible that one of the isomers had an orthoacetate anhj'dride structure, but this is 

 difficult to reconcile with the stability of both forms to water and ethanol. Farrar^"* 

 considers that speculations regarding possible structures of the two forms are un- 

 necessary, and all that is involved is a simple, but interesting, case of dimorphism, 

 since she found that isomerization proceeded without any significant change in 

 optical rotation. Obviously a contradiction exists between this result and the rotation 

 changes reported by Davoll et a/.,^'^ and the problem needs to be investigated further. 



2 , 3 , 4 , 5-Tetra-0-acetyl-a/c/e/i?/£/o-D-ribose has been obtained crystalline 

 and can be prepared by several routes. Demercaptalation of 2,3,4,5-tetra- 

 0-acetyl-D-ribose diethyl thioacetal,-^* hydrogenolysis with Raney nickel 

 of ethylthio-D-ribonate tetra-O-acetate^^^ or Rosenmund reduction of 

 tetra-0-acetyl-D-ribonyl chloride, all yield this compound. ^^^ Attempts to 



236 H. Bredereck and Eva Hoepfner, Chem. Ber. 81, 51 (1948). 



2" J. Davoll, G. B. Brown, and D. W. Visser, Nature 170, 64 (1952). 



""* Kathleen R. Farrar, Nature 170, 896 (1952). 



238 H. Zinner, Che7n. Ber. 83, 418 (1950). 



"9 M. L. Wolfrom and J. V. Karabinos, /. Am. Chem. Soc. 68, 724, 1455 (1946). 



