Process for preparing entecavir and intermediate for said process

Claim 1: A method for preparing entecavir of formula (1), comprising the steps of: (a) performing olefination of an alpha-hydroxy ketone compound of formula (2) to obtain an exomethylene compound of formula (3 ); (B) conducting a Mitsunobu reaction of the exomethylene compound of the formula (3) with a purine derivative to obtain a nucleoside compound of the formula (4); (C) removing the protecting groups from 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, alky, and alilsililo; and X is selected from the group consisting of chlorine, iodine, and benzyloxy. Claim 12: A ketone compound characterized in that it is of formula (6) 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.