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 6a-g proceeds with good to high levels of diastereoselectivity to provide the corresponding P-C-glycosides 7. The stereochemistry of the para-nitrobenzoate derivative 7d has been confirmed by X-ray crystallography, and the stereochemistry of the other P-C-glycoside products has been correlated to 7d. 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 11. (C) 2004 Elsevier Ltd. All rights reserved.