Illustrated Glossary of Organic Chemistry

Oxaphosphetane: A four-membered ring consisting of two carbon atoms, one oxygen atom, and one phosphorous atom. Most frequently encountered as an intermediate in the Wittig reaction or related processes.

General oxaphosphetane structure.

In the final mechanism step of this Wittig reaction, an oxaphosphetane intermediate decomposes to form propene (an alkene) and triphenylphosphine oxide.