Hi Zeeshan, first thing is you want to do research on Phytochemicals and their specific activity (based on available technique in your chosen lab), broadly called Natural product Chemistry. Second thing is you have interest on Heterocycles, but why not other cycles or alkaloids or terpenoids? You know all heterocycles are under natural product if you find that from natural source (say from plant), so dont specify unless it is required because more you specify isolation, less probability to achieve goal within specific time. Why not you choose just isolation, characterization and activity tests from natural sources (like plant, algae, microbes etc.)

Agreed with Rahman.

You select any specify only because more work alread reported on heterocyclic compounds. you concentrate on activity and select compound that one more usefull for society.

Thanks sir