Most existing high-level synthesis systems attempt to generate a circuit from a behavioral description ``out of the void'', using the entire design space as the search domain. Because of the vastness of the search space, it is impossible to do more than a coarse grain search, often resulting in inefficient designs. This approach, ignores the designer's knowledge of the general structure of the circuit to be synthesized. In this paper, we describe the High-level synthesis system Sider (Synthesis by Initial Design Extension and Refinement). Sider utilizes designer knowledge about the design space in the form of an initial circuit. By limiting search to the neighborhood of this initial circuit, much finer grain search can be performed yielding a higher quality design. The effectiveness of the Sider approach is shown by high-level synthesis of a 300 line C description of 27 instructions from a MC6502 CPU.