Glycosidic radical

In addition to the use of substrates similar to those shown in Figure 5.1.1, glycosyl halides have found much utility in the chemistry of glycosidic radicals. 

In 1992, the reported preparations of C-disaccharides included the utilization of free radical reactions. Additionally, the use of various polyol cyclizations were discussed. In the application of radical chemistry to the synthesis of C-disaccharides, Bimwala, et al.,35 expanded upon his earlier work illustrated in Scheme 8.9.5. As shown in Scheme 8.10.1, glycosidic radicals were added a,p-unsaturated carbonyl compounds producing yields ranging from 47% to 73%. Furthermore, the illustrated C-disaccharide precursors exhibited diastereomeric ratios of approximately 5.5 1. 

The carbohydrate-based ether auxiliaries have been used to develop a radical-based asymmetric aldol reaction. Glycoside radicals such as 37 are generated from Barton ester precursors 39 [40], and the addition of these radicals to 2-nitropropene gives a thiopyridyl adduct 40 that can be converted to an aldol product [41]. The yield for the conversion is moderate to good and the diastereoselectivity ranges from 5 1 to 8 1, depending on R and temperature. 

An unexpectedly high proportion of 1,5-trans products was obtained in the cyclization of the a-allyl (axial) glycoside radicals 47a and 47b [25]. As the previously discussed model would have predicted, a significant portion of the reaction must proceed through the chair-chair conformation leading to this product. Since the triacetoxy sugar is conformationally flexible (hydroxy and acetoxy groups are small. 

---