What should the chemist who synthesized a drug expect regarding coauthorship of papers on clinical trials?
The ICMJE guidelines says that authors must meet four requirements:

Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
Drafting the work or revising it critically for important intellectual content; AND
Final approval of the version to be published; AND
Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Without knowing your field, it sounds like you think you satisfy the first requirement. It doesn't sound like you meet the final three. While I disagree with the guidelines and think hat everyone who meets the first requirement should be given the option to meet the other three, the guidelines do not require that.
This sounds like a situation where you should talk to the lead investigator. Ideally the conversation should have happened early on, but now is better than never.