A method for preparing a compound represented by the following general formula (5) ##STR00001## where, n is an integer of 0 to 3; and Y represents a protecting group for an amino group, the method including the steps of reacting a compound represented by the following general formula (3) ##STR00002## where n and Y are as described above, with a silylating agent, and subsequently reacting it with P (.dbd.O) T.sub.3, where T represents a halogen atom, and further with ammonia. A method for preparing an optically active intermediate of sulphostin or an analogue thereof, which is an optically active amine salt of an optically active compound represented by the following general formula (8) ##STR00003## where n is integer of 0 to 3; Y represents protecting group for the amino group; and each configuration at C* and P* may be the same or different and indicates S or R, the method including reacting a compound represented by the following general formula (7) ##STR00004## where n and Y are as described above; and the configuration of C* indicates either of S or R, with an optically active amine, and resolving the formed diastereomeric salt by fractional crystallization.