Method for preparing entecavir and intermediate used therein

A method for preparing entecavir of formula (I), comprising the steps of:

• (a) hydrolyzing an α-exomethylene derivative of formula (II) to obtain a compound of formula (III);
• (b) carrying out a Mitsunobu reaction of the compound of formula (III) with a purine derivative of formula (IV) to obtain a nucleoside compound of formula (V);
• (c) subjecting the nucleoside compound of formula (V) to a reaction with tetrabutyl ammonium fluoride to obtain a compound of formula (VI);
• and
• (d) hydrolyzing the compound of formula (VI):
• wherein
• R1 and R2 are each independently a hydroxy-protecting group, or R1 and R2 are fused together to form a cyclic hydroxy-protecting group;
• R3 is benzoyl or arylbenzoyl;
• and
• X is halogen or benzyloxy.