A Simplified,One-Pot Preparation of Acetobromosugars from Reducing Sugars

ABSTRACT

Acetobromoglycoses continue to be important as glycosyl donors in the synthesis of simple glycosides as well as complex oligosaccharides. From reducing sugars they are usually prepared via their peracetates in two steps. In the first step, sugars are converted to their peracetates using pyridine and acetic anhydride^1,2 and the acetates are then converted in a second step to acetobromosugars using a solution of hydrogen bromide in glacial acetic acid(HBr/HOAc).² Although not in use very often Redemann and Niemann³ as well as Lemieux⁴ have described one-pot methods for the preparation of acetobromoglucose wherein the reducing sugar is first treated with acetic anhydride in the presence of sulfuric acid³ or perchloric acid⁴ respectively to afford the peracetate. Direct conversion of the peracetate to its 1-bromo-derivative, in yields ranging from 80-87%, was then achieved by either treating the solution of the peracetate with gaseous HBr³ or with bromine in the presence of red phosphorus.⁴ In another approach to a one-pot method Humoller⁵ prepared tri-O-acetyl-β-L-arabinopyranosyl bromide in 40% yield by passing anhydrous HBr gas through a suspension of L-arabinose in acetic anhydride. By an essentially similar method Dale⁶ obtained acetobromosugars in yields ranging from 26-77% depending upon the sugar. Because acetobromosugars are the compounds most frequently used for synthesizing numerous other sugar derivatives, we thought it would be worthwhile to report our recent observation that acetic anhydride and HBr/HOAc can be used with advantage for the preparation of acetobromosugars.