May 2 (Reuters) - Changchun Sinoenergy Corp 600856.SS :
* SAYS UNIT PLANS TO BUY 49.74 PERCENT STAKE IN PETROLEUM INVESTMENT FIRM FOR 1.8 BILLION YUAN ($283.15 million)
* SAYS IT WILL INDIRECTLY OWN THE PETROLEUM INVESTMENT FIRM AND CANADA'S LONG RUN EXPLORATION LTD AFTER TRANSACTION