Applications of Free Radical Reactions in Synthetic Carbohydrate Chemistry

Abstract

The copper-catalysed reactions of some unsaturated per-O-acylated hexopyranose derivatives with t-butyl perbenzoate were investigated. Products of substitution (of benzoyloxy groups) at allylic methylene positions were obtained and when the double bonds were within vinyl ether groups there was competitive addition across the double bonds. The remainder of the thesis is a report of an investigation of the use of free radical processes for the stereospecific introduction of one, two and three carbon branches into carbohydrate derivatives by the use of both inter- and intra-molecular radical reactions: Treatment of ethyl 2,3-dideox-α-D-glycero-hex-2-enopyranosid-4-ulose with functionalised alkyl halides in the presence of tributyltin hydride afforded branched-chain sugars by way of the former intermolecular radical addition processes. The majority of the described work involved the use of intramolecular radical cyclisation processes. Reaction of some 2-bromoethyl hex-2-enopyranoside and 4-O-haloethyl-hex-2-enopyranoside derivatives with tri-n-butyltin hydride with radical initiation afforded carbohydrates with a single branch point. Repetition of these reactions in the presence of methyl acrylate or by the use of allyltributyltin instead of tri-n-butyltin hydride gave access to a number of di-branched sugars. An extension in the case of one of the di-branched derivatives led to the formation of triply-branched carbohydrates. Treatment of 2,3,4,6-tetra-O-acetyl-2-hydroxy_D_glucal with allyltrimethylsilane in the presence of boron trifluoride etherate afforded a novel unsaturated allyl C-glycoside. Chemical manipulation of this compound resulted in the production of mono- and di-branched C-glycoside derivatives. In particular this C-glycoside was used in a synthetic approach to the formation of the 2-oxabicyclo[3.2.1]octane ring system present in the trichothecene family of sesquiterpene compounds.