1,3-dipolar cycloadditions of diazomethane to chiral electron-deficient olefins: The origin of the pi-facial diastereoselection

The stereochemical outcome of diazomethane cycloadditions to several chiral electron-deficient olefins has been investigated in order to establish the origin of the Jr-facial diastereoselection. Nitro olefins, vinyl sulfones, enoates, and 2-amino enoates have been used for such a purpose. These subs trates have been prepared from D-glyceraldehyde acetonide through Wittig-ty pe condensations and present an alkoxy substituent, provided by the bulky d ioxolane ring, attached to the stereogenic allylic carbon. Syn-adducts have been obtained in all cases as the major isomers, independently of the Z/E stereochemistry of the double bond and the number and the nature of the sub stituents, the chirality of the asymmetric allylic carbon being the only th ing responsible for the diastereoselection. Theoretical calculations show t hat steric hindrance due to the bulky dioxolane group is the main factor go verning the preference for the syn-attack of diazomethane to the olefinic d ouble bond.