Integrated Modular Avionics (IMA) architecture has been widely adopted by the avionics industry due to its strong temporal and spatial isolation capability for safety-critical realtime systems. The fundamental challenge to integrating an existing set of single-core IMA partitions into a multi-core system is to ensure that the isolation of the partitions will be maintained without incurring huge redevelopment and recertification costs. To address this challenge, we developed an optimized partition scheduling algorithm which considers exclusive regions to achieve the synchronization between partitions across cores. We show that the problem of finding the optimal partition schedule is NP-complete and present a Constraint Programming formulation. In addition, we relax this problem to find the minimum number of cores needed to schedule a given set of partitions and propose an approximation algorithm which is guaranteed to find a feasible schedule of partitions if there exists a feasible schedule of exclusive regions.