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Structure of discodermolide
It is generally said that carbon-carbon double bonds cannot behave as stereogenic centres but the above molecule contains three such centres. Can somebody please point out all the stereogenic centres (including the double bonded carbons) and explain how and why this is so?

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    $\begingroup$ I'm confused. Who is saying the C=C double bounds are stereogenic? Various materials I find, including (this one)[scs.illinois.edu/denmark/presentations/2004/gm-2004-07_06.pdf], say it has 13 stereocenters. That is the same as the number of carbons connected to "stereo" bold or hashed bonds. None of those carbons have double bonds. Why do you think it contains C=C stereocenters? Please consider editing your question to explain in more detail. $\endgroup$
    – Curt F.
    Feb 10, 2015 at 5:43
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    $\begingroup$ @CurtF. I am confused, too. Who is saying, that carbon-carbon double bonds are not stereogenic? Interchanging the substitutes at these centres would lead to a different stereoisomer (as long as the substituents are different of course). Unfortunately the definition is not given in the goldbook and hence not straight forward. $\endgroup$ Feb 10, 2015 at 7:46
  • $\begingroup$ Can you give your definition of stereogenic centre. Are you referring to chiral centres? $\endgroup$
    – bon
    Feb 10, 2015 at 11:39
  • $\begingroup$ Strange it seems there's cis/trans and R/S confusion, but they're totally different isomerisms. $\endgroup$
    – Mithoron
    Feb 10, 2015 at 12:36
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    $\begingroup$ The IUPAC Gold Book does include E/Z double bond isomers in its definition of 'stereogenic', which makes sense. I'm surprised that there's no definition of 'stereocenter'. I would NOT include double bonds in counting stereocenters because the E/Z designation is defined by the substitution at two atoms (all carbon in this case). In my mind, a stereocenter is limited to the classic 'four different things attached to one atom (carbon)'. I would say that discodermolide has 13 stereocenters and 16 stereogenic units. goldbook.iupac.org/S05980.html $\endgroup$
    – jerepierre
    Feb 10, 2015 at 16:06

1 Answer 1

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Summary

The confusion probably arises because of incompletely standardized definitions of at least some subset of the words "stereocenter", "stereogenic", "stereogenic center", "stereogenic unit".

IUPAC definitions

The place to turn for authoritative definitions of these terms is IUPAC's Gold Book, as noted by @jerepierre in the comments. Here is what it has to say:

stereogenic unit (stereogen/stereoelement)

A grouping within a molecular entity that may be considered a focus of stereoisomerism.

At least one of these must be present in every enantiomer (though the presence of stereogenic units does not conversely require the corresponding chemical species to be chiral). Three basic types are recognized for molecular entities involving atoms having not more than four substituents:

  1. A grouping of atoms consisting of a central atom and distinguishable ligands, such that the interchange of any two of the substituents leads to a stereoisomer. An asymmetric atom (chirality centre) is the traditional example of this stereogenic unit.
  2. A chain of four non-coplanar atoms (or rigid groups) in a stable conformation, such that an imaginary or real (restricted) rotation (with a change of sign of the torsion angle) about the central bond leads to a stereoisomer.
  3. A grouping of atoms consisting of a double bond with substituents which give rise to cis-trans isomerism.

Thus, the unequally substituted C=C moieties in discodermolide are "stereogenic units" according to IUPAC.

Stereocenter

Unfortunately, this term is not defined in IUPAC's Gold Book.

Stereogenic center

Unfortunately, this term is not defined in IUPAC's Gold Book.

Wikipedia

Wikipedia is not as authoritative as IUPAC but it is likely that chem.se readers will encounter wikipedia entries for these terms far more often than IUPAC's in the course of regular web searching.

Stereogenic unit

No Wikipedia entry

Stereogenic center

Redirects to "stereocenter", see below.

Stereocenter

A stereocenter or stereogenic center is an *atom* bearing groups such that an interchanging of any two groups leads to a stereoisomer.

This definition does not apply to C=C double bonds, because only an atom can be a stereocenter, not a group of atoms. Supporting this interpretation is a complete lack of reference to cis-trans isomerism on the rest of the page.

Usage by chemists

Discodermolide is referred to in many sources as having 13 "stereocenters" or "stereogenic" centers. This conforms with the Wikipedia, atom-centric definition of stereocenter. Here are some example sources:

All these uses comport with both Wikipedia definitions of "stereocenter"/"stereogenic center". Chemists do not seem to use the IUPAC term "stereogenic unit" very much in actual practice. I could not find any relevant pages using both the term "discodermolide" and the phrase "stereogenic unit", for example.

Bottom line

It is just possible to pass a thread of consistency through the eye of this needle of confusing terminology.

  1. Discodermolide has thirteen (13) stereogenic centers.
  2. Each of those stereogenic centers is by definition also a stereocenter, and is also a "stereogenic unit".
  3. In addition to those 13 stereocenters, there are three additional "stereogenic units" in discodermolide corresponding to the unequally substituted C=C bonds.
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    $\begingroup$ I've probably heard 'stereogenic element' or 'stereoelement' more often than 'stereogenic unit' in practice. Now that I've seen 'stereogenic unit' in the Gold Book, I'll probably go with that to stay away from 'element' which obviously has a unique meaning to chemistry. $\endgroup$
    – jerepierre
    Feb 10, 2015 at 17:26

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