An oxaphosphetane is a molecule containing a four-membered ring with one phosphorus, one oxygen and two carbon atoms.

[2] Edwin Vedejs's NMR studies first revealed the importance of oxaphosphetanes in the mechanism of the Wittig reaction in the 1970s.

[3][4] In 2005 the first isolation of 1,2-Oxaphosphetanes (typical Wittig intermediates) was reported.

[5] One of the compounds was characterized by X-ray crystallography and NMR.

Although relatively stable, thermal decomposition of these oxaphosphetanes gave a phosphonium salt, which slowly dissociated to the Wittig reaction starting materials, the carbonyl and olefin compounds.