In 1452, Cardinal Isidore of Kiev stopped at Lesbos on his way to Constantinople to conduct negotiations for a union between the Catholic and Greek Orthodox churches. Stopping on Lesbos, Isidore invited Leonard to join him. The latter accepted, and arrived at Constantinople with the papal delegation on 26 October 1452. As a result, he was an eyewitness of the subsequent siege and capture of Constantinople by the Ottoman Sultan Mehmed II in May 1453. It is not known what role he played in the defence; he evidently accompanied Cardinal Isidore, and thus participated both in the councils of the defenders, and in the defence of the sector of Saint Demetrios, that Isidore commanded. During the sack of the city on 29 May, both Isidore and Leonard were captured, but soon were released or ransomed. Leonard was even able to buy some books from the looting Turks on the same day of the sack.