[Heory]

What's the mechanism for the rearrangement of morphine under acidic condition? This problem has extremely puzzled me.

[Heory]

Now I have 2 different mechanism proposed, but I think both are not reasonable enough. Any suggestion would be appreciated. Thanks!

[sjb]

There may be things like labelling studies and kinetic isotope effects you could measure in the reaction to help further evidence towards one or the other. Or perhaps try and make something that will act like an intermediate to see if it undergoes decomposition at a rate at least as fast as morphine.

Most mechanisms are not really known in detail, and are least worst guesses, given evidence such as the above. For what it's worth, I prefer the 1st, as the aziridines and things in the second look fairly unlikely first thing in the morning.

[azmanam]

et voila

(i was going to include this reference for an allowed [2s 2a] 1,3-sigmatropic rearrangement, but it doesn't quite fit.  still a fascinating read: http://dx.doi.org/10.1021/ar50060a003)

[Heory]

That's might be a silly question, but what does [2s, 2a] mean? However, such rearrangement in your scheme looks so unimaginable, personally speaking. I made some modifications to the [1, 3]-sigmatropic rearrangement you proposed and how about this one?

[KritikalMass]

Could ya'll please tell me what software ya'll use to draw your mechanisms?

Thanks!

[azmanam]

Sorry I didn't answer your question.  Either way is probably fine.  Mine works with least motion, but they both get you there.  the notation refers to geometry of each piece of the sigmatropically rearranging molecule.  the numbers are the number of electrons on each side of the breaking single bond, the letters (more commonly found in cycloaddition reactions) refer to whether the fragment reacts in a suprafacial or antarafacial manner: suprafacial=both reacting orbitals are on the same face of the molecule (within the fragment) and antarafacial=both reacting orbitals are on opposite faces of the molecule.  Diels Alder reactions are [pi2s pi4s]. (more here: http://en.wikipedia.org/wiki/Sigmatropic_reaction#Suprafacial_and_Antarafacial_Shifts_)

KritikalMass: we're both using chemdraw.  I'm using the TotSyn style sheet (http://totallysynthetic.com/blog/?page_id=287), Heory's using the ACS style sheet.

[Heory]

thank you very much!

[Heory]

I still have a question--sigmatropic rearrangment is a kind of concerted pericyclic reaction, thus having a cyclic transition state which cannot be achieved by your means. Is that true?

[azmanam]

Yes, as I've drawn it in my answer, the skeletal rearrangement is not a pericyclic reaction and not a sigmatropic rearrangement.  The way you've drawn it is a pericyclic reaction and a sigmatropic rearrangement.