An Efficient Approach to the 2-Nonulosonic Acid Skeleton Through a Hetero-Diels-Alder Reaction

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Abstract
The hetero-Diels-Alder reaction of 1,3-diene-polyols with glyoxylates is an expeditious way to build up the skeleton of ulosonic acids. This paper reports an efficient means to improve the yields and the stereoselectivity associated with the hetero-cycloaddition of the activated 1,3-diene, 2S,3S-4E-1-O-Benzyl-2,3-O-isopropylidene-6-ter-butyldimethylsilyloxyhepta-4,6-diene-1,2,3-triol (3) with the dienophile ethyl glyoxylate. The effects of temperature, catalyst, and solvent on the outcome of the Diels-Alder reaction were investigated. The cycloaddition can proceed smoothly even at −78°C and the complex bis [3-(heptafluorobutyryl)camphorato]oxovanadium (8) was found to be a very efficient catalyst for the cycloaddition reaction. The satisfactory selectivity (endo/exo: 97:3 and re/si: 82:18) observed in this catalytic reaction shows that there is a match effect between the substrate and catalyst.