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Corporate Overview

The Standard – October 2015

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

Ethynylestradiol (20,21-13C2) (CLM-3375) is one of the carbon-labeled steroids offered by Cambridge Isotope Laboratories, Inc. (CIL). Native 17α-ethynylestradiol (EE2) is used as an oral contraceptive. Its widespread dispersal in natural waters is of environmental concern because it is an endocrine-disrupting compound. 17α-Ethynylestradiol induces chemical feminization of male fish at low parts-per-trillion levels. Researchers can utilize the stable isotope-labeled compound as an internal standard in mass spectrometric analytical methods. While the acronym EE2 is not structurally specific, the formal nomenclature of the molecule may be confusing without looking at a structural diagram.  
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.  
Cambridge Isotope Laboratories, Inc.
Thomas F. Dorsey, PhD
Senior Scientist
CIL offers the following ethynylestradiol standards:
Catalog No. Description Concentration Amount
CLM-3375-1.2 Ethynylestradiol (20,21-13C2, 99%) 100 µg/mL in acetonitrile 1.2 mL
ULM-7211-1.2 Ethynylestradiol (unlabeled) 100 µg/mL in acetonitrile
1.2 mL


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