AbstractA new approach to ketose‐derived nucteosides is described. It is based upon a chain elongation of 1‐deoxy‐1‐nitroaldoses, followed by activation of the nitro group as a leaving group, and introduction of a pyrimidine or purine base. Thus, the nitroaldose 7 was prepared from 3 by pivaloylation (→4), synthesis of the anomeric nitrones 5/6, and ozonolysis of 6 (Scheme 1). Partial hydrolysis of 4 yielded 8/9, which were characterized as the acetates 10/11 and transformed into the nitrones 12/13. Ozonolysis of 12/13 gave 14/15, which were acetylated to 16/17. Henry reaction of 7 lead to 19 and 20, which were acetylated to 21 and 22 (Scheme 2). Michael addition of 7 to acrylonitrile and to methyl propynoate yielded the anomers 23/24 and 25/26, respectively. Similar reactions of 16/17 were prevented by a facile β‐elimination. Therefore, the nitrodiol 15 was transformed into the orthoesters 27 and then, by Henry reaction, partial hydrolysis, and acetylation, into 28 and 29 (Scheme 2). The structure of 19 was established by X‐ray analysis. It was the major product of the kinetically controlled Henry reaction of 7. Similarly, the β‐D‐configurated nitroaldoses 23 and 25 were the major products of the Michael addition. This indicates a preferred ‘endo’‐attack on the nitronate anion derived from 7. AMI calculations for this anion indicate a strong pyramidalization at C(1), in agreement with an ‘endo’‐attack. Nucleosidation of 21 by 31 afforded 32 and 33. Yields depended strongly upon the nature and the amount of the promoter and reached 77% for 33, which was transformed into 34, 35, and the known ‘psicouridine’ (36; Scheme 3). To probe the mechanism, the trityl‐protected 30 was nucleosidated yielding 37, or 37 and 38, depending upon the amount of FeCl3. Nucleosidation of the nitroacetate 28 was more difficult, required SnCl2 as a promoter, and yielded 39 and 40. The β‐D‐anomer 40 was transformed into 36. Nucleosidation of 23 (SnCl4) yielded the anomers 41 and 42, which were transformed into 43 and 44, and hence into 45 and 46 (Scheme 4). Similarly, nucleosidation of 25 yielded 47 and 48, which were deprotected to 49 and 50, respectively. The nucleoside 49 was saponified to 51. Nucleosidation of 21 by 52 (SnCl2) afforded the adenine nucleosides 53 and 54 (Scheme 5). The adenine nucleoside 53 was deprotected (→55→56) to ‘psicofuranine’ (1), which was also obtained from 58, formed along with 57 by nucleosidation of 28. The structure and particularly the conformation of the nitroaldoses, nitroketoses, and nucleosides are examined.
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Apr 9, 1996
N Arai + 1
Open Access