The 3,6,9-Trioxanthracene and 3,6-Dioxa-9-thianthracene Ring Systems by Cyclization of 1,1-Bis[4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl]alkanes - Moreno-Mañas, Marcial; Ribas, Jordi; Virgili, Albert (Synthesis, 1985)