One of my best friends in undergrad, upon learning about the use of acetal protecting groups for carbonyls, exclaimed, "they're like ketone condoms!" Indeed. Protecting groups are ubiquitous in organic synthesis, and another one you see all the time is Boc, which is used to protect amines:
Boc2O, or "boc anhydride," is one of my favorites. Using it on your amine will yield a "boc protected amine." Upon acid deprotection, it yields the parent amine, carbon dioxide, and t-butylene. The latter two are both gases, which gives about as easy a cleanup as you could hope for (that's the problem with these protecting groups; you have to get rid of them somehow). Using Boc will convert an amine into a carbamide that won't react with (as many) things, allowing you to work unfettered (or at least less fettered) on other chunks of whatever molecule you're interested in. Protecting groups are finicky things and don't always work as you expect. As Derek Lowe mentions in his post, it's kind of like a piece of masking tape. He doesn't go on to explain it further, but the analogy is apt - they're not bolts, and are prone to fall apart. Such is chemistry - it's the best we've got! At least it's fun to say boc (pronounced like "bock")...
Boc anhydride tends to decompose a little on storage (and it melts around room temperature - it's a pain chipping away at a slab of melted-refrozen stuff). So we keep it in the fridge. Additionally, it comes in great big bottles - it's not full of the stuff; you need a little headspace for carbon dioxide gas. It comes as beautiful melty clear crystals, kind of like soft-cornered rock candy with very small crystals. That is, unless you keep it on your bench, in which case it doesn't go (totally) bad, but it will probably melt a bit, depending on the weather. Then, you're stuck chipping away at it with a metal spatula.
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My understanding is that the t-butyl group in Boc goes to isobutylene, not t-butanol, on deprotection. The t-butanol can't take the acid conditions and does an elimination.
It never even makes it to the butanol; the tbutyl cation is too stable, directly eliminates. The stability, expense and atom-un-economicity of Boc2O are really frustrating. A better way to make boc protected amines is to go via Boc azide, installing the amine in the process. However, Boc azide is known to blow up from time to time:
doi: 10.1002/anie.197700652
Although a significantly less-explodey synthesis has been reported by displacing the imiadazole in Boc-imidazolium tosylate with azide in water at RT.
I stand corrected; I could swear I've seen t-butanol peaks in NMR of boc deprotections (in neat TFA) though!
hm I could be wrong about the t-butanol, but I'm correct about the atom-uneconomicity.
What happens if you have unprotected reactions with promiscuous reagents? Do you get CRABs (Crappy Reactions and Byproducts)
I've never understood the fuss over "atom economy." I prefer simply economy. If it's cost effective and works, who cares how many atoms get used? (The environmental considerations will sometimes correlate with atom economy and sometimes not.) Barry Trost is off the mark with this stuff. Am I missing some good rational for "atom economy?"
by the kilo, boc is mostly used for solid phase synthesis (peptides and the like), the field where when the subject of "atom economy" comes up we look at our shoes or find an excuse to be somewhere else. For $ economy, the TFA ends up being around a third of the total material cost for a small scale boc peptide synthesis and purification.
Is it OK to cleave the BOC with conc. sulphuric acid?
do i have to clean it up after or just extract my product in organic solvent? actually i should also neutralize the pH afterwards because otherwise the amine will stay protonated and it will not go to the prganic phase!
i saw a selective deprotection of TPS in presence of TBS ether.How is it possible?..if so,is it true for all systems or a specific systems mentioned in the following
ref.
synlett,2000,1306
how is the work up done once the reaction is complete , i mean to say how TFA is removed