list1 = [4356, 'abc']
list2 = {4356: ('5986', '5987'), 6598: ('6593', '2659')}
output = {x: True for x in list2[list1[0]]}
print(output) # {'5986': True, '5987': True}
Или, используя list2
из вашего комментария:
list1 = [4356, 'abc']
list2 = [(4356, '5986'), (4356, '5987'), (6598, '6593'), (6598, '2659')]
output = {val: True for key, val in list2 if key == list1[0]}
print(output) # {'5986': True, '5987': True}