Joao (1451-1451) was the oldest son of King Afonso V of Portugal and his wife Isabella. He died young. His younger brother (1455-1495) was also named Joao and succeeded his father as King Joao II of Portugal.
Joao (1688-1688) was the oldest son of King Pedro II of Portugal and his second wife Marie Sophie. He died young. His younger brother (1689-1750) was the also called Joao and later became King Joao V of Portugal.
Joao was the name of three sons of Queen Maria I of Portugal and her husband Pedro. 1. Joao (1762-1762) was stillborn 2. Joao (1763-1763) died as a baby 3. Joao (1767-1825) was later King Joao IV of Portugal.