A 58-year-old man from Delhi presents to the emergency department with sudden-onset severe chest pain radiating to the back, associated with diaphoresis and hypertension (BP 180/110 mmHg). On examination, he has a blood pressure differential of 20 mmHg between the right and left arms. Chest X-ray shows widening of the mediastinum. CT angiography confirms a Type A aortic dissection involving the ascending aorta. During surgical planning, the cardiothoracic surgeon needs to identify the mediastinal compartment containing the ascending aorta and the structures at immediate risk. Which mediastinal division is primarily involved in this pathology?