The idea of synthesizing statecharts out of a collection of scenarios has received a lot of attention in recent years. However due to the poor expressive power of first generation scenario languages, including UML1.x sequence diagrams, the proposed solutions often use ad hoc tricks and suffer from many shortcomings. The recent adoption in UML2.0 of a richer scenario language, including interesting composition operators, now makes it possible to revisit the problem of statechart synthesis with a radically new approach. Inspired by the way UML2.0 sequence diagrams can be algebraically composed, we first define an algebraic framework for composing state charts. Then we show how to leverage the algebraic structure of UML2.0 sequence diagrams to get a direct algorithm for synthesizing a composition of state charts out of them. The synthesized statecharts exhibit interesting properties that make them particularly useful as a basis for the detailed design process. Beyond offering a systematic and semantically well founded method, another interest of our approach lies in its flexibility: the modification or replacement of a given scenario has a limited impact on the synthesis process, thus fostering a better traceability between the requirements and the detailed design.