[Rutherford]

How many stereoisomers of cefalotin exist?
I'd say 4: 2*2 for stereocenters.
But do the two nitrogen count as stereocenters?

[discodermolide]

4 unless I am missing something!

[Rutherford]

I am not sure about the nitrogen atoms, they are tetrahedrons and have three different substitutes + one electron pair. Is the isomerization between R and S slow enough so they can be regarded as different compound, I don't know.

[AlphaScent]

Since we are discussing stereo chemistry.

How many isomers are there of this compound?  I think there are six.  3 stereo centers.  So six isomers.  Am I wrong?

Am I missing something with the epoxide?

[Rutherford]

I'd go with six.

[discodermolide]

I see 6, I don't think you are missing anything.

[AlphaScent]

Ok cool.  I was doubting myself for some reason.

Also, when it comes to nitrogen being considered a stereocenter, what are the conditions for it to be one?  I know in most biological compounds only carbon is considered.  When do you consider nitrogen?

[discodermolide]

Usually the inversion of an amine is very fast. Put some really bulky groups on it and you can slow it down so that you can see the isomers in the NMR time scale. It's a bit like chairs interconverting, at low temp you see axial and equatorial protons at RT you just see one big signal in the NMR.
But yes you can have chiral nitrogen atoms.

[discodermolide]

Think about phosphines, now they can be chiral!

[Rutherford]

Here, the nitrogens' environment seems very rigid, so maybe nitrogen should be considered as a stereocenter.

[sjb]

I see 6, I don't think you are missing anything.

8, surely?

[discodermolide]

Yes, getting blind in my old age. Or my screen needs cleaning:)

[AlphaScent]

I know other elements can be chiral centers.  This is a topic I am ill equipped to talk about though.  Gonna put my nose into some information.

@sjb

You think 8 for my compound??  Please explain further.

[zsinger]

When I learned it (and what I accepted as fact?!?) was that Nitrogens rapidly interconvert between R and S, therefore giving them chirality.  As was mentioned, tertiary nitrogens might have a bigger energy barrier to rotation, however I am somewhat certain that it is, in fact, always, chiral.  Unless, of course, it is a Carbon DB Nitrogen.  Then no free rotation….and no stereo center .
                     -Zack

[Dan]

You think 8 for my compound??  Please explain further.

2ⁿ, not 2n.

Try writing them all out:

RRR
RRS
etc. etc.