Complex sentence: In 1972, when Mission Viejo Co. and its remaining undeveloped area in Mission Viejo were sold, Moiso began managing the remaining of Rancho Mission Viejo.
Split into simpler sentences: In 1972, Mission Viejo Co. and its remaining undeveloped area in Mission Viejo were sold. After the initial sale, Moiso began managing the remaining of Rancho Mission Viejo.