word of the day: oxa

The word of the day is oxa:

Chem.

Forming names of compounds in which an oxygen atom is regarded as having replaced a methylene (—CH₂—) group.  (OED)

"Furthermore, addition of a benzene ring at C5 adjacent to an oxa group in the carbon chain, as in FTY20, does not prevent binding and phosphorylation by SK2 or SK1, but does affect the reaction efficiency."

 - M. R. Pitman and S. M. Pitson, "Inhibitors of the Sphingosine Kinase Pathway as Potential Therapeutics", Current Cancer Drug Targets 10:354 (2010)


Here's the structure of FTY20:

And of sphingosine:

I don't see any ethers anywhere.  Am I losing my mind?