Instead of developing user interfaces (UIs) directly, we argue for specifying an interaction design from which UIs can be automatically synthesized. We present an approach to using communicative acts in high-level specification of interaction design, which is implemented and allows automated synthesis of interfaces for multiple devices. Communicative acts derive from speech act theory and carry desired intentions in interactions. Models of communicative acts, UI domain objects and interaction sequences comprise interaction design specifications in our approach and are based on a metamodel that we have defined. As a result, the usability of a synthesized user interface of a real-world application turned out to be good.