Dear candidate for admission into B.Pharmacy courses you need to give entrance examination like state Joint Examinations or JEE Mains. Now for specialization like Pharmaceutical Chemistry you need to crack GPAT Examination and then only you can pursue Masters of Pharmacy course with specialization in Pharmaceutical Chemistry. So after M.Sc Chemistry, shifting to Pharmaceutical Chemistry would be difficult to impossible but one suggestion is that if you go through the courses such as Pharmaceutical Chemistry, you would get much similarity with your M.Sc Chemistry curriculum. Only few QSAR Medicinal Chemistry portions might differ.

Good luck!!