Complex sentence: He won the San Vicente Stakes and the grade II San Rafael Stakes then after winning the 1990 Santa Anita Derby he was considered the early favorite for one of the World's most prestigious races: the Kentucky Derby.
Split into simpler sentences: He won the San Vicente Stakes and the grade II San Rafael Stakes. After winning the 1990 Santa Anita Derby he was considered the early favorite for one of the World's most prestigious races: the Kentucky Derby.