A process for producing predominantly cis nucleosides or nucleoside analogues and derivatives of formula (A): wherein Rl is a pyrimidine base or pharmaceutically acceptable derivative thereof and Q is oxygen, carbon or sulphur, comprising a coupling step of the pyrimidine base with a molecule of formula (B) described herein in a suitable coupling solvent, in the presence of catalytic amounts of an element or combination of elements of groups IB or IIB of the periodic table, a tertiary amine and a Lewis acid to obtained an intermediate of formula (D) which is deprotected in the subsequent step to generate the product of formula (A).
