Im not 100% sure on this, so please correct me if im wrong but i dont think its possible. Even if its possible i dont think the effort would be worth it. there are so many structural changes you would have to do to get from methylphenidate to methamphetamin. Its way easyer to start from phenylaceton or pseudoephedrin/ ephedrin. there are good synthesis descriptions here on the forum but i think you would go better to just sell the ritalin as it is and not to try to turn it into methamphetamin
