flashback table

I am trying to flashback a table using below command
SYS@rmsdev1>INSERT INTO CUSTOM.ITEM_HTS_ASSESS_TEMP SELECT * FROM CUSTOM.ITEM_HTS_ASSESS AS OF TIMESTAMP ('07-AUG-13 06:00:00','DD-MON-YY HH24: MI: SS');
INSERT INTO CUSTOM.ITEM_HTS_ASSESS_TEMP SELECT * FROM CUSTOM.ITEM_HTS_ASSESS AS OF TIMESTAMP ('07-AUG-13 06:00:00','DD-MON-YY HH24: MI: SS')
                                                                                                                  *
ERROR at line 1:
ORA-00907: missing right parenthesis

What is wrong with the syntax?

your help is appreciated..