Typescript 1.4 has been released recently, providing new possibilities like unions.
I installed the new version for Visual Studio, however the syntax highlighting do
I've had the same issue, and for me it was Resharper that was causing the problem.
If you have Resharper installed then simply disable it while working on TypeScript 1.4, at least until JetBrains releases an update with 1.4 support.
To disable Resharper: Tools -> Resharper -> General -> Suspend
Tools -> Resharper -> General -> Suspend