Prevent mob despawning

i am making an rpg world/map i placed a tructure that contains a lot of mobs and a boss and i want to go build somewhere else and i am scared that the mobs will despawn, i cannot name or give items to every one of them also the player in the map will spawn away from this structure how to make it that these mobs will not despawn or disppear? like i want to make them exsit but unloaded until a player enter the place