[Amino acids and peptides. VII. Studies on the synthesis of 3,6-epidithio-2,5-dioxopiperazine antibiotics: gliotoxin, sporidesmin, aranotin, chaetocin, and verticillin. VII. The introduction of oxygen into prolyl-prolinanhydride via lead acetate: a new approach to the epidisulfide of prolyl-prolinanhydride].