[OC pro]

Do you have more than one OH groups free? If so I would assume you will get epoxide by-products.

[discodermolide]

You may be getting CH₂=SO₂ formed which then polymerizes, the idea of using the anhydride will prevent this.

[sjb]

There is also more than one sort of OH in conduritols - see http://en.wikipedia.org/wiki/Conduritol . Is it possible you are mesylating one or the other with little chemoselectivity?

[sudi]

All the other -OH's are protected with -OBn. There is only one -OH available, the conjugated one. It seems that I am getting both epimers of the mesylated product because if I add water to the mixture I get my starting conduritol and its epimer (which originally was absent in the reaction). But assuming the mechanism is SN2 type, I don't know how I am getting the other epimer.
Any ideas?

[sudi]

My starting material is F conduritol referring to http://en.wikipedia.org/wiki/Conduritol. The top -OH is the only one available to react. The rest are benzyl protected.

[Doc Oc]

Maybe you're seeing a Baylis-Hillman type elimination, where the triethylamine is adding to the alkene and causing elimination of the mesylated alcohol.  That is obviously very reversible, hence the reason you're seeing both epimers.

[sudi]

Any idea on how to prevent it?
Does anyone know if Mesyl anhydride is sensitive to water? I read in this thread that MsCl is not water sensitive. How about Ms2O?

[Dan]

Any idea on how to prevent it?

If a Baylis-Hillman-like reaction is operating, switching to a less nucleophilic base (e.g. DIPEA) should stop/reduce it.

if I add water to the mixture I get my starting conduritol and its epimer

Why are you adding water? Is this in the workup? MsCl and Ms₂O are both water sensitive, nobody claimed they weren't.

Another explanation is S_N1 and/or S_N2' possibly involving neighbouring group participation by a benzyl ether. It might be useful to know the relative stereochemistry of the four centres if you can provide it.

In that case it might be better to switch to a tosylate as it's not as good a leaving group, so S_N1 pathways will be reduced. Not sure if that's really the cause though.

If you purify and take the mesylate on to the next reaction (which I assume is a displacement, can you tell us?) do you get complete inversion of configuration?

[Babcock_Hall]

Rory, Is your starting material in the keto-form or the hemiketal form?