Animating the rotation

I've recorded an animation that rotates a plane -90 along the Z axis, while moving it forth and back to the same position again.

During runtime, I parent this child to an empty gameObject. That way, I solve the problem of playing the animation in world space. It now plays normally on local space.

This isn't the same with the rotation, though. Whatever the current rotation of my object, it resets its Z to 0 and plays the animation until it reaches -90. I'd like, if e.g. the rotation of my object along the Z axis is -90 to be -180 after the animation plays.

Video demonstrating the problem:

http://www.youtube.com/watch?v=nBXT8J2xxjw

Any ideas?