Curious (but correct!) Naming of CIL’s Isotopically Labeled Ethynylestradiol (EE2) Standard

CIL uses the convenient name “ethynylestradiol (20,21-¹³C₂)” to identify product number CLM-3375. It may be surprising that a 20-carbon steroid has carbon atoms numbered 20 and 21. Looking at the structure for this compound, it would be easy to assume that the 2-carbon substituent on the 18-carbon estradiol framework would be numbered 19 and 20. Although it is not obvious, there is a perfectly reasonable explanation for numbering the alkyne 20 and 21.

In fact, this numbering conforms to International Union of Pure and Applied Chemistry (IUPAC) and Chemical Abstracts Service (CAS) conventions. The formal CAS name for this compound is 17-α-19-norpregna-1,3,5(10)-triene-20-yne-3,17-diol. Therefore, estradiol is numbered as a pregnane.

Within Section 3S-6, lengthening and shortening of side chains and elimination of methyl groups of IUPAC's Nomenclature of Steroids, Recommendations 1989, we find the following instructions: Elimination of a methylene group (-CH2-) from a steroid side chain is indicated by the prefix nor-, which in all cases is preceded by the number of the carbon atom that disappears. The remainder of the original steroid numbering is retained.

The methyl group, originally numbered 19, has disappeared. The A ring has been aromatized by double bonds at carbons 1, 3, and between C5 and C10. The alkyne retains carbon numbering for 20 and 21, and the two alcohol groups are located at carbons 3 and 17.  

While it is not immediately apparent from the use of the convenient name, identifying the formal name of the compound and understanding the accepted numbering system reveals that CIL's product number CLM-3375 is, in fact, correctly named.  

Thomas F. Dorsey, PhD, Senior Scientist
Cambridge Isotope Laboratories, Inc.