What's wrong with using tin in medicinal chemistry? I just read this In the Pipeline post and I was slightly confused by a statement on the use of tin. Lowe reports on this paper, which describes a synthetic route to spiro heterocycles using tin compounds, and caveats the finding with the comment that


  No one's crazy about using tin, but the transformation is too useful to pass up.


Why is the use of tin undesirable in synthetic chemistry?
Organotin compounds are rather toxic.  They are also persistent in the environment and have a long biological half-life.

The problem is that trialkyltin byproducts from your reaction are difficult to separate from the product.  In the lab, this is painful and (usually) involves multiple columns, but when you want to get a drug past the FDA into the clinic you have to meet purity standards for heavy metal contamination.  To a medicinal chemist, a transformation that won't make it into pilot plant and production is useless.