SEARCH Articles Figures Tables 6- membered systems Four-membered Four-membered heterocycles Four-membered ring heterocycles Four-membered ring systems Heterocyclic 5- membered ring Heterocyclic ring systems Heterocyclic systems Heterocyclic systems four-membered Phosphorus 4-membered heterocycles Phosphorus and silicon Phosphorus heterocycles four-membered rings Phosphorus heterocycles, four-membered Phosphorus heterocyclics Phosphorus rings Phosphorus systems Silicon 4-membered heterocycles Silicon heterocycles Silicon heterocycles, four-membered Silicon-phosphorus rings