It's a nice idea, but I suspect that if you were to tweak copyright law to apply to chemical compounds used as drugs (e.g. field of use restrictions) you'd end up with patents.
patents protect the mode of action rather than the compound, and the term can be extended in court with simple little changes
it isn't as cut-and-dry in the pharma business, which is why most patent lawsuits and active trial involve pharma. the patent system is supposed to be simple, but the companies and the generic manufacturers are suing each other all the time.
copyright would just apply per-compound at a fixed term. the other way to do it is for the FDA (who are already approving drugs anyway) grant exclusive periods to new drugs before generics are allowed
What I was getting at is that most(?) compounds probably exist somewhere in nature already, so copyright wouldn't apply without modification. I think that means you'd have to copyright the compound in a novel field of use e.g. "for use in treating cancer".
Further, you'd want your protection to cover many modifications, so that a competitor can't just make a change to a non-functional aspect and piggy-back off your research. Otherwise you'd have the problem that new R&D wouldn't pay off, because competitors could just devise drugs that would use whatever mechanism you discovered, even if they didn't have the same physical embodiment.
What you end up with is a "copyright" that is - in practice - a patent.
Of course, the drug companies would probably love for their protection to last 75 years!
On the "found in nature" aspect, I believe some of the patented gene sequences are indeed derived directly from existing organisms, but are considered novel 'inventions' due to the labour and skill involved in identifying, isolating and applying them. Copyright doesn't really fit for that sort of use-case.
