Provided herein are impurities of paliperidone, 3-[2-[4-[1-(4-fluoro-2-hydroxyphenyl)methanoyl]piperidinyl-1-yl]ethyl]-2-methyl-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-one (methanoyl impurity), 3-[2-[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1-piperidinyl]ethyl]-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one (dehydroxy impurity) and 3-[2-[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1-piperidinyl]ethyl]-2-methyl-7,8-dihydro-6H-pyrido[1,2-a]pyrimidin-4,9-dione (9-keto impurity), and processes for preparing and isolating thereof. Provided further herein is a highly pure paliperidone or a pharmaceutically acceptable salt thereof substantially free of methanoyl, dehydroxy and 9-keto impurities, process for the preparation thereof, and pharmaceutical compositions comprising highly pure paliperidone or a pharmaceutically acceptable salt thereof substantially free of methanoyl, dehydroxy and 9-keto impurities. Provided also herein are improved and efficient processes for preparing paliperidone intermediates.
