Complex sentence: In his letter to Autran of August 7,1852, while orchestrating the four chorus pieces, Liszt wrote that the idea had come to him to join a rather long Overture to them.
Split into simpler sentences: In his letter to Autran of August 7,1852, Liszt wrote, he had orchestrated the four chorus pieces. While doing it, the idea had come to him to join a rather long Overture to them.