Previously it has been reported from our group that thiouracil catalyses stereoselective glycosylations with galactals in loadings as low as 0.1 mol%. However, glycosylations with glucal and rhamnal gave an inseparable mixture of Ferrier products and desired disaccharide (Scheme 1).
This project focuses on the Ferrier rearrangement which occurs in the reaction of the glycals with the thiouracil catalyst. The reaction has been optimised and benzyl protected glycals, such as glucal, gulal, allal and rhamnal gave the Ferrier product with satisfactory αselectivity. Furthermore, making use of the bulkier tert-butyldimethylsilyl protecting group, the reaction gave two easily separable glycosides, the Ferrier rearrangement product as minor and the desired disaccharide as major. To our delight, introducing the tert-butyloxycarbonyl protecting group, the glycosylation reaction with sugar, non-sugar and amino-acid acceptors led to the desired products after an intermolecular Ferrier rearrangement (Scheme 2).
- G. A. Bradshaw, A. C. Colgan, N. P. Allen, I. Pongener, M. B. Boland, Y. Ortin, E. M. McGarrigle, Chem. Sci., 2019,10, 508-514