Synthesis of New Hetero-Annulated Coumarins: 2,3-Dihydro-4-oxothiapyrano [2,3-c]-benzopyran-2-ones, 4-Oxothiapyrano[2,3-c]-1-benzopyran-2-ones and 2-Oxo-3,4-dihydrobenzopyrano[4,3-b]-4-piperidones - van den Goorbergh, J A M; van der Steeg, M; van der Gen, A (Synthesis, 1987)