Intermediates useful in processes for the diastereoselective synthesis of nucleosides

The present invention relates to intermediates useful in highly diastereoselective processes for production of cis-nucleosides and nucleoside analogues and derivatives in high optical purity. A diastereoselective process for producing optically active cis-nucleosides and nucleoside analogues and derivatives of formula (I), wherein R1 is hydrogen or an acyl group of the formula C1-6 alkyl-C(O)- or halo-C1-6 alkyl-C(O)-; R2 is a purine or pyrimidine base or a halo, hydroxyl, C1-6 alkyl or amino derivative thereof; W is 0; X is CH2, or CHF; and Y is CH2, or CHF; or both X and Y are CH linked together by a double bond; the process comprising the step of glycosylating the desired purine or pyrimidine base or derivative thereof with a single enantiomer of the compound of formula (II) wherein R3 is carboxyl, C1-6 alkoxycarbonyl, C1-6 alkylcarbonyloxycarbonyl, or halo-C1-6 alkylcarbonyloxycarbonyl; and L is a leaving group, using a Lewis acid of the formula (III) wherein R5, R6 and R7 are independently selected from the group consisting of hydrogen; C1-20 alkyl optionally substituted by fluoro, bromo, chloro iodo, C1-6 alkoxy or C6-20 aryloxy; C6-20 aralkyl optionally substituted by halogen, C1-20 alkyl or C1-20 alkoxy; C6-20 aryl optionally substituted by fluoro, bromo, chloro, iodine, C1-20 alkyl or C1-20 alkoyl; tri-C1-6 alkylsilyl; fluoro, bromo; chloro and iodine; and R8 is selected from the group consisting of fluoro; bromo; chloro; iodine; C1-20 sulphonate esters, optionally substituted by fluoro, bromo, chloro or iodo; C1-20 alkyl esters optionally substituted by fluoro, bromo, chloro or iodine; triiodate; saturated or unsaturated C6-20 aryl selenenyl; C6-20 arylsulfenyl; C6-20 alkoxyalkyl; and tri-C1-6 alkylsiloxy; and reducing R3 of the glycosylated purine or pyrimidine base or derivative thereof to produce optically active cis-nucleoside or nucleoside analogue or derivative of formula (I).