Processes for the diastereoselective synthesis of nucleoside analogues

A diastereoselective process for producing optically active cis-nucleosides and nucleoside analogues and derivatives of formula (I) ##STR26## wherein R.sub.1 is hydrogen or acyl;

• R.sub.2 is a desired purine or pyrimidine base or an analogue or derivative thereof;
• W is S, S=O, or SO.sub.2 O, NZ, or CH.sub.2 ;
• X is O, S, S=O, or So.sub.2 O, NZ, CH.sub.2, CHF, CH, CHN.sub.3, or CHOH;
• Y is O, S, CH.sub.2, CH, CHF, or CHOH;
• and
• Z is hydrogen, hydroxyl, alkyl or acyl;
• provided that W is not O, S, S=O or SO.sub.2 when Y is CH.sub.2 and X is O, S, S=O or S.sub.2 ;
• the process comprising the step of glycosylating the desired purine or pyrimidine base or analogue or derivative thereof with a single enantiomer of the compound of formula (II) ##STR27## wherein R3 is a substituted carbonyl or carbonyl derivative;
• and
• L is a leaving group, using a Lewis:
• acid of the formula (III) ##STR28## wherein R.sub.5, R.sub.6 and R.sub.7 are independently selected from the group consisting of hydrogen;
• C.sub.1-20 alkyl optionally substituted by fluoro, bromo, chloro, iodo, C.sub.1-6 alkoxy or C.sub.6-20 aryloxy;
• C.sub.7-20 aralkyl optionally substituted by halogen, C.sub.1-20 alkyl or C.sub.1-20 alkoxy;
• C.sub.6-20 aryl optionally substituted by fluoro, bromo, chloro, iodo, C.sub.1-20 alkyl or C.sub.1-20 alkoxy;
• trialkylsilyl;
• fluoro;
• bromo;
• chloro and iodo;
• and R.sub.8 is selected from the group consisting of fluoro;
• bromo;
• chloro;
• iodo;
• C.sub.1-20 sulphonate esters, optionally substituted by fluoro, bromo, chloro or iodo;
• C.sub.1-20 alkyl esters optionally substituted by fluoro, bromo, chloro or iodo;
• polyvalent halides;
• trisubstituted silyl groups of the general formula (R.sub.5) (R.sub.6) (R.sub.7)Si (wherein R.sub.5, R.sub.6, and R.sub.7 are as defined above);
• saturated or unsaturated selenenyl C.sub.6-20 aryl;
• substituted or unsubstituted C.sub.6-20 arylsulfenyl;
• substituted or unsubstituted C.sub.6-20 alkoxyalkyl;
• and trialkylsiloxy.