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removal of by-product in mCPBA oxidations

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Babcock_Hall:
Good Morning,

We are oxidizing a sulfide to a sulfone with a very slight excess of meta-chloroperoxybenzoic acid (mCPBA); the byproduct is presumably 3-chlorobenzoic acid.  The other functional groups present are a phosphonate ester and a tetraacetyl-protected glycoside.  In time points we observed a UV-active component ahead of the presumed product (either mCPBA or the corresponding aromatic acid).  The protocol we followed removed solids using a plug of alumina before chromatography, but we did not see any solid material; therefore, we moved on to chromatography and are in the process of characterizing the fractions.  We collected large amount of UV-active solid prior to the elution of the presumed product.

My two questions are whether or not people generally remove the byproduct of mCPBA oxidations before the silica column, and was it a good idea not to do so in this case.  I was thinking of sodium bicarbonate, followed by brine and drying the organic layer.  The only reason I can see for avoiding an aqueous wash is that one might observe hydrolysis of the acetyl protecting groups.  My guess is that bases stronger than bicarbonate should be avoided.

wildfyr:
Usually you reduce any residual mcpba and extract the benzoic acid with basic wash.

Simply extracting with base won't work very well for MCPBA. I think sodium thiosulfate(?) or one of those sulfur salt reducing agents is what I used with good success.

https://www.chemicalforums.com/index.php?topic=78856.0

Babcock_Hall:
That is an interesting thread; mCPBA chemistry is more complex than I realized.  I think that my protecting groups would tolerate a bicarbonate wash.

rolnor:
Yes, per-acids are very much weaker acids than ordinary carboxylic acids. Thiosulfate is a good idea, followed by bicarbonate. I think you can use sodium meta periodate for the oxidation, maybe MeOH/H2O as solvent. Then you get only water-soluble by-products. Also, watch out, you have phosphonate and sulphonate in you molecule, these are very hydrophilic groups, you can loose material if you wash with water.

Babcock_Hall:
Another reagent that oxidizes sulfides to sulfones in the presence of a protected glycoside is dimethyldioxirane.  The possibility of losing a sulfonyl phosphonate to the aqueous layer has long been on my mind; therefore, we usually do four extractions and have not yet observed a problem (knock on wood).  I sometimes avoid LiCl washes in the presence of phosphonates, but I don't have any data that bears on whether or not the lithium would pull a phosphonate into the aqueous layer.

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