Selective 1,2-syn alkynylation of an open chain sugar: the prominent role of zinc chloride addition

Depending on the metal, solvent and protective groups, functionalized aldehydes may react through either Felkin or chelated transition states leading to the Felkin–Anh product or the 1,2-anti Felkin–Anh product, respectively. Zinc chloride addition proved to be a key aspect in order to favor 1,2-syn nucleophilic attack.