Seven-membered heterocycles with three heteroatom

Seven-Membered Heterocycles with Three Heteroatoms... 

Three possible isomeric forms exist for benzheterocycles containing seven- or eight-membered rings, as depicted by structures (VII), (VIII), and (IX). Most of these heterocycles contain an Si heteroatom, but derivatives of (IX) with several different heteroatoms are known. Only one benzheterocycle with an eight-membered ring has been reported [(IX), M = Si]. 

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