Highly stereoselective addition of organozinc reagents to pentopyranose derived glycals: effect of protecting group and assignment of C-glycoside stereochemistry

The nucleophilic addition of ethyl 3-propionylzinc iodide to a variety of differently protected pentopyranose derived d-glycals proceeds with good to high levels of diastereoselectivity to provide the corresponding β-C-glycosides . The stereochemistry of the para-nitrobenzoate derivative has been confirmed by X-ray crystallography, and the stereochemistry of the other β-C-glycoside products has been correlated to . The stereochemical outcome observed supports the earlier suggestion by Isobe that through-space effects are important in stabilising and controlling the reactivity of the intermediate oxonium species represented by .