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Topic: Attachment of Boc group to NH2 vs OH...  (Read 17176 times)

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Offline peptideismylife

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Attachment of Boc group to NH2 vs OH...
« on: May 20, 2011, 08:45:59 AM »
Hi colleagues,

I am doing a synthesis of a peptide and I have a doubt. I want to protected an amino group in the N-terminus with the protected group Boc (diterbuthyldicarbonate) but at the same time I have a a free hydroxyl group in another amino acid. I am afraid if I am going to proctect also the hydroxyl group, which is something that I dont want...

Which conditions could avoid the protection of OH....

As far as I read in the literature, to protected an amino group with Boc we have to use (Boc)2O anhydride and a base...like triethylamine...but Im not sure if this conditions are going to protected also the OH....

Anybody has any suggestion?

Thanks.

Offline karbon

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #1 on: May 20, 2011, 11:00:22 AM »
I am pretty sure you don't need TEA to do the protection. The amine is a softer nucleophile than the hydroxy group so if you don't add any base you can be pretty sure only your amine will be protected. Make a small test reaction though.

Offline Doc Oc

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #2 on: May 20, 2011, 11:43:09 AM »
I've protected serine using multiple different protecting groups and the free alcohol is not an issue, the amine is far more nucleophilic and will react first.  The triethylamine is used to scavenge protons, not only from the amine after it is protected, but also beforehand because usually you are starting with the HCl salt of an amino acid, not the free base.

Offline Oxime

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #3 on: May 20, 2011, 12:33:10 PM »
Standard procedure for solid phase peptide synthesis nowadays uses the Fmoc (9-N-fluorenylmethoxycarbonyl) protecting group rather than Boc. I'm not sure about Boc protection, having not done it myself, but Fmoc protection of a primary amine is easily achieved under aqueous conditions or anhydrous conditions and will selectively protect the amine.

A drawback of the Boc protection is overacylation of a primary amine (di-Boc protected amine), or over-acylation during peptide synthesis.

Offline peptideismylife

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #4 on: May 22, 2011, 04:59:12 AM »
hey, thanks you very much for your *delete me* really nice!

I can see easily what u said, amine is far more nucleophilic and I dont need any kind of base. In fact the sequence has 5 aminoacids, and as I said only I have Norleucine 6 hydroxo amino acid in which the alcohol is not protected.

Fmoc- Nle (OH)- Peptide-Resin.

I want to change the Fmoc group to Boc group at the N-terminus to carry out a different experiment. To deprotect Fmoc I will use piperadine 20 % in DMF and after that I will protect again with (Boc)2O...any recomendation about the solvent that I have to use? DMF? THF?



Offline karbon

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #5 on: May 22, 2011, 06:27:31 AM »
Still haven't done these protection reactions myself, but I think its actually possible to carry out neat (with Boc2O as solvent).

Offline Doc Oc

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #6 on: May 22, 2011, 10:34:10 AM »
Still haven't done these protection reactions myself, but I think its actually possible to carry out neat (with Boc2O as solvent).
Boc2O is a solid.

You should be able to use THF as your solvent.

Offline karbon

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #7 on: May 22, 2011, 11:06:48 AM »

Offline enahs

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #8 on: May 22, 2011, 11:37:37 AM »
I have done a lot with BOC in THF, works great for me.

In fact, I would often BOC protect just one of the amines on o-phenylenediamine by controlling the stoichiometry. So what I would do is take my o-phenylenediamine in THF is one syringe, and BOC in THF in another syringe, and add them together slowly into THF over ~16 hours with a syringe pump. After protecting the first amine, it made the second amine much less reactive and so my maintaining  1:1 ratio the BOC would react preferentially with a unproteted o-phenylenediamine.

As others have said, you might not have a problem with the OH group (the relative activity depends more on just the functional group, but its local environment as well as solvent, etc). But if you do find you have trouble and you get the OH group reacted as well, you can try controlling the stoichiometry with slow addition.

You can do slow addition with drop adders as well; we did that at first to, and get them dropping enough to add ~50mL solution over ~6 hours. We got higher yield and purity with the syringe pump (though they cost ~$2000, and the gas tight syringes you need in that large size are another ~$400+ each...)

Offline Doc Oc

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #9 on: May 22, 2011, 04:10:43 PM »
Yeah, but a mp of 25C or so. Seen it done, just sayin' :)

https://www.thieme-connect.com/ejournals/abstract/synlett/doi/10.1055/s-2007-970755

I can assure you that the amines were used as the solvent, not the Boc2O.

Offline peptideismylife

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #10 on: May 22, 2011, 04:54:05 PM »
Thanks to all!

As u said i wont have any problem with OH...but could depend of how many equivalents I am going to add? I mean...for 50mg of a peptide (0.04 mmol) what about the equivalents of (Boc)2O?

Offline uglepik

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #11 on: May 24, 2011, 08:05:30 AM »
I've used DCM as solvent for Boc-protection (using Boc2O) - works fine. Of course it depends on your resin which solvent is preferred, but DCM usually does the trick. That people would do this without a base really surprises me. How would the reaction run without a base?

I'd use something like 3 eq. Boc2O and double eq's of DIPEA or TEA.

I don't think it would react with the alcohol if done at room temperature. I'm not even sure that it can react at all, since I wouldn't assume that the product was stable (this guy that looks like a carbamate but with three oxygens - I'm not even sure what it's called!).

Furthermore, Fmoc protections (using eg. fmoc-chloride) and also boc-protections (using Boc2O) can be done in water, which makes me think that the tendency to react with oxygen is very low, if not totally non-existent. I could be wrong about this.

I'd say go for it with an excess of boc anhydride and normal conditions (with base). I think it's gonna work fine.
 

Offline fledarmus

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Re: Attachment of Boc group to NH2 vs OH...
« Reply #12 on: May 24, 2011, 08:25:34 AM »
I'm not even sure that it can react at all, since I wouldn't assume that the product was stable (this guy that looks like a carbamate but with three oxygens - I'm not even sure what it's called!).
 

Are you thinking of a carbonate?

My understanding is that these can sometimes form, but they rarely survive the aqueous workup.

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