why is it neccasary to proceed via the nitro-alcohol?
it would seem to me that a simpler procedure would be to condense the nitronate ion onto a benzylhalide.
while there could be some reaction between the benzylhalide and the base, it would seem that this condition could be managed by combing the 1equiv of nitropropane, 1 equiv of KOH in the ethanol, allow the mixture to sit long enough for deprotonation to occur, and then slowly add in 1 equiv of benzylhalide.
to insure that all the presumably more expensive, 2-nitroprpane is consumed, one could then add for example 0.1equivs of KOH to the ethanol, and slowly drip in another 0.1 equivs of benzylhalide.
since the nitro group is located on a tert carbon, it would seem there would be little danger of the nitro-phentermine undergoing any side reactions.