The present invention provides processes for preparing a compound of formula (I) and pharmaceutically acceptable salts thereof, comprising deprotecting a compound of formula (II), wherein each R3 independently represents a tertiary alkyl group, preferably wherein each R3 is tertiary butyl. The invention also provides intermediates useful for preparing compounds of formula (I) and processes for preparing these intermediates. Additionally the invention provides polymorphic forms of the dichloride salt of the compound of formula (I) and their use in the treatment of proliferative disorders.