✍ Scribed by Lichtenthaler, Frieder W. ;Hahn, Susanne ;Flath, Franz-Josef
No coin nor oath required. For personal study only.
The reductive elimination of the benzoylated β‐D‐fructopyranosyl and α‐L‐sorbopyranosyl bromides 10 and 23 has been examined in order to determine suitable conditions for effecting the elimination from the exo‐ as well as the endo‐positions. Exposure of 10 and 23 to Zn/HOAc or Zn/N‐methylimidazole gave the tribenzoates of exo‐D‐fructal (1) and exo‐L‐sorbal (25), in yields of 79 and 80%, respectively; minor products (ca, 10% each) were the corresponding endo‐D‐fructal (6) and endo‐L‐sorbal esters (26). For the reductive elimination to occur primarily from the endo‐position, enhancement of the leaving group capacity of the respective ring acyloxy group was required, and materialized via the 3‐O‐mesyl analogues of fructosyl bromide 10, which smoothly reacted with Zn/HOAc to give the endo‐D‐fructal esters 20 and 21; their yields, over the five steps from diacetone‐fructose, were in the 50% range. Accordingly, the methodology for generating ketose‐derived glycal esters in exo‐ or endo‐version, is now available. Some ensuing reactions of the exo‐glycals have been exploited, that is, their ozonolysis to the respective pyranoid pentonolactones, and their hydrogenation to 1,5‐anhydro‐6‐deoxy‐hexitols.
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