DE 2851440, Cairns, Hugh; Lee, Thomas Brian & Ingall, Anthony Howard et al., "Neue Salze von 2,8-Dicarboxy-4,6-dioxo-10-propyl-4H,6H-benzo[1,2-b zu 5,4-b]dipyran und Verfahren zu deren Herstellung [New salt of 2,8-dicarboxy-4,6-dioxo-10-propyl-4H,6H-benzo[1,2-b to 5,4-b]dipyrane and method for manufacturing thereof]", published 1979-05-31, assigned to Fisons Ltd.