Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: ducthinh on January 08, 2006, 08:49:03 AM
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Hi
Assumming that in a compound containing both aldehyde group and ketone group , is there any substance that can protect only aldehyde group but not ketone group ?? And vice versa...
thanks for your help ... :)
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Acylal protecting group is good one for your case. Acylals are obtained in reaction aldehydes(not and cetones) with most oftenly acetic anhydride. So you have:
RCORCHO + Ac2O--> RCO R-C(OOCR)2.
Acetal protecting group is an alternative as example with ethyleneglicol as acid catalyzed reaction or with its ortho-esters.
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I think aldehydes can usually be protected as acetals (e.g. dimethyl acetal) in the presence of ketones. The other alternative is to reduce the aldehyde to the alcohol (this can be done in the presence of the ketone by using NaBH(OAc)3) and then protecting the resulting alcohol. Afterwards you will have to deprotect and oxidize the alcohol though.
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Thanks...
Ah` ... If we want to protect the resulting alcohol group then we can use THP ??
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Thanks...
Ah` ... If we want to protect the resulting alcohol group then we can use THP ??
You could use chlorotrimethylsilane ClS(CH3)3
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Reference Material
Subject:Protection of Aldehyde and Ketones
S. Chandrasekhar and S. Sarkar,
Tetrahedron Lett., 1998, 39, 2401
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Thanks...
Ah` ... If we want to protect the resulting alcohol group then we can use THP ??
Sure, a THP (tetrahydropyran) group would probably be okay, but it all depends on what you are protecting the group from. THP can also be problematic because you produce a new stereocenter in your compound, and therefore you could produce a diastereomeric mixture which is difficult to work with.
Silyl groups are probably the most common form of alcohol protecting groups, with trimethylsilyl (TMS) being the simplest (as PLU wrote above). The most common one in modern synthetic chemistry is almost certainly the t-butyldimethylsilyl (TBS or TBDMS) group. The latter is favored because of its stability towards mild treatment with aqueous acid. The TMS group will often fall off under even modestly acidic conditions (such as workup conditions for a Grignard reaction) while a TBS group will survive. Silyl groups are also favored because they can be selectively deprotected with sources of F-, such as tetra-n-butylammonium fluoride. The TMS group is certainly useful in many cases, but if you need your alcohol to stay protected for more than one or possibly two steps of your synthesis, you probably want to use a bulkier silyl group. However, the lability of TMS can be useful because you can kind of "skip" the deprotection step sometimes!
For those of you familiar with the THP group, you will note that it is usually cleaved by treatment with relatively mild aqueous acid. In my experience, THP is more stable towards mild acid than TMS.
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I think you may use the reaction of water solution of your keto - aldehydes with water solutions of p-nitrophenyhidrazone hydrochloride. At room temeperature the more reactive aldehyde group will react first giving precipitate from the coresponding hidrazone aduct. Ketones react with this p-nitrophenylhidrazone HCl at short heating but you will not do that ;D