Unsaturated Sugars: A Rich Platform for Methodological and Synthetic Studies

Bert      Fraser-Reid; J.   Cristobal   Lopez

Abstract

The aim of this review is to emphasize the value of unsaturated carbohydrate derivatives as a ground floor for useful synthetic transformations. In this context, we highlight the rich chemistry of Δ2,3 unsaturated pyranosides by way of the different transformations carried out in the Fraser-Reid laboratories during the past four decades. These transformations have all led to the synthesis of natural products. Cyclopropanated derivatives were used in an enantiodivergent approach to chrysantemic acids. The former, also paved the way to a stereoselective synthesis of sucrose. An investigation on the aldehydic nature of tri-O-acetyl glucal triggered an approach to trichothecenes. The allylic hydroxyl of unsaturated sugars was used fulcrum to direct the entry of the cis nitrogen in synthetic approaches to ristosamine and daunosamine. The discovery of the photochemically induced addition of alcohols and aldehydes to unsaturated enones was employed in an unequivocal synthesis of pillarose, the sugar component of antibiotic pillaromycin A. Unsaturated enones also proved to be precious synthetic intermediates in Diels-Alder reactions: an intermolecular Diels-Alder transformation was used in the synthesis of actinobolin, whereas its intramolecular version was employed in the synthesis of an advanced intermediate of forskolin. The concatenation of unsaturated sugars was key in the development of the “pyranosidic homologation” concept that led to the synthesis of the ansa chain of streptovaricin A. Finally, radical cyclization and serial radical reactions on unsaturated sugars were used in the synthesis of cyclophellitol and reserpine.

Keywords: Unsaturated Sugars, pyranosides, synthesis, natural products, Diels-Alder reactions, pyranosidic homologation, cyclophellitol, reserpine