Rapid Preparation of Variously Protected Glycals Using Titanium(III)
摘要:
Glycosyl chlorides and bromides can be rapidly converted to glycals in high yield by reaction with (Cp2Ti[III]Cl)(2). This reagent tolerates a wide range of common carbohydrate protecting groups, including silyl ethers, acetals, and esters; the methodology provides a general route for the preparation of glycals substituted with both acid- and base-labile functionality. A reaction mechanism is proposed that is based on heteroatom abstraction to give an intermediate glycosyl radical. This radical reacts with a second equivalent of Ti(III) to yield a glycosyltitanium(IV) species. P-Heteroatom elimination from the glycosyltitanium(IV) complex gives the glycal.
The repeatingunits 2-O-beta-D-glucopyranosyl-L-ribitol 4'- and 1-phosphate of Haemophilusinfluenzaetype a capsularantigen have been synthesised by condensation of an alpha-D-glucopyranosyl bromide derivative with 5-O-allyl-1,2,3-tri-O-benzyl-D-ribitol followed by selective deprotection of HO-4' or HO-1, phosphorylation, and removal of the blocking groups.