We are always taught that molecules which can exist in a number of existing tautomeric forms may be prone to polymorph formation as a result, but I came across a real example at our recent Winter Process Conference held at the University of Strathclyde in Glasgow in a presentation from Ivan Marziano of Pfizer.
The compound is shown below and in principle can exist in 8 different tautomeric forms, although only 3 tautomers were observed in the solid state, but each of these 3 tautomers corresponded to a different polymorphic form (see below). Form A which is the tautomer most of us would draw first is a metastable form, while form B and C are enantiotropic polymorphs, which is to say that form C is the thermodynamically stable form £ 40°C and form B is the thermodynamically stable form ³ 40°C.
I. Marziano (Pfizer) presentation at The 4th Winter Process Conference, Glasgow, December, 2016.