Intermediate and process for preparing entecavir using same

A method of preparing entecavir of formula (1), comprising the steps of

• (a) conducting olefination of an α-hydroxy ketone compound of formula (2) to obtain an exomethylene compound of formula (3);
• (b) carrying out a Mitsunobu reaction of the exomethylene compound of formula (3) with a purine derivative to obtain a nucleoside compound of formula (4);
• (c) removing the protecting groups of the nucleoside compound of formula (4) to obtain a compound of formula (5);
• and
• (d) hydrolyzing the compound of formula (5):
• wherein
• R1 and R2 are each independently H or a hydroxy-protecting group, or R1 and R2 are fused together to form a cyclic hydroxy-protecting group;
• R3 is selected from the group consisting of H, alkylsilyl, and allylsilyl;
• and
• X is selected from the group consisting of chloro, iodo, and benzyloxy.