first i converted your common name to a latin name at this link

https://www.google.com/search?q=purple+milkweed&ie=utf-8&oe=utf-8

then i added SHADE to such.. and the second blurb seems to answer your question

https://www.google.com/search?q=Asclepias+purpurascens&ie=utf-8&oe=utf-8#q=Asclepias+purpurascens+shade

the problem when talking about shade.. is that its specific to your micro climate .... shade can be ridiculously bright.. but still shade.. or it can be a grotto ...

no plant dies in shade.. it just might not perform to expectation ... which leaves you with just giving it a go.. in your garden.. in your micro climate ...

ken

There are a lot of milkweed plants, and many are unique or at least thrive in different areas. So, I think you may need to narrow down the specific "purple milkweed" you are referring to, and see if indeed it can thrive in your particular micro climate, as ken points out. You might want to check out some of the other sub-forums here, particularly milkweed and butterfly gardening (which is more active).

Uhh, thanks that was exactly the type of advice I was looking for when I asked to hear personal experience growing one vs the other... :)