the present invention describes an industrially advantageous process to prepare highly pure 1 - methyl - 3 - phenylpiperazine of formula i that makes use of a novel piperazine derivative , 4 - benzyl - 1 - methyl - 2 - oxo - 3 - phenylpiperazine , represented by formula ii 1 - methyl - 3 - phenylpiperazine is a useful intermediate in the preparation of antidepressant mirtazapine .