Consider a requirements model with a set of interfaces representing use cases, and a set of system sequence diagrams specifying the scenarios of the interfaces, this paper discusses how to automatically transform such a model into an inial design model by generating a protocol state machine for each interface from its sequence diagrams, and deriving a controller class as an implementation of the interface. The generated model can be severed as the starting point for the detailed object-oriented design. The contracts of the controller class ensure that the behaviour defined by the sequence diagrams is fully complied with through pre/post-conditions. The transformation takes into account the concurrency and critical area of the respective diagrams. We present the design and implementation of the transformation, which is specified using the graphical notation of QVT Relations and integrated into a CASE tool.