C07F 9/65719

Heterocyclic compounds, e.g. containing phosphorus as a ring hetero atom having phosphorus atoms, with or without nitrogen, oxygen, sulfur, selenium or tellurium atoms, as ring hetero atoms having phosphorus and oxygen atoms as the only ring hetero atoms {the ring phosphorus atom being bound to at least one carbon atom the ring phosphorus atom and, at least, one ring oxygen atom being part of a (thio)phosphonous acid derivative}