Solution-Phase Amide Coupling with DSC¶
Overview¶
In this procedure, a carboxylic acid is activated as the OSu ester by disuccinimidyl carbonate (DSC). Upon completion, an amine is added to form the amide. This procedure avoids the use of DCC or aggressive reagents. Rearrangement of the activated carboxylic acid is not an issue as it is with carbodiimides.
DSC does not react dangerously with water and is not a known sensitizer. The byproducts are carbon dioxide and N-hydroxysuccinimide (NHS). Carbon dioxide provides a convenient gross indicator of reaction progress with use of a flaccid balloon. NHS extracts into water during workup. The procedure probably does not require inert gas or dry reagents.
A slight excess of carboxylic acid over DSC should be used to ensure complete consumption of the DSC, as DSC will react with amines to produce a carbamate. Because the only byproducts are gaseous and water-soluble, no purification beyond workup should be required. DMAP is probably not necessary and was added due to superstition. In a related reaction using bis-(para-nitrophenyl) carbonate, DMAP was found to help.
Note
The coupling described below had failed previously using HATU or COMU. Isolation of active esters or acid chloride proved difficult due to high reactivity. Here, the active ester is not isolated. The amide product was pure following workup, and the yield was quantitative.
Procedure¶
To 50 mL acetonitrile in an oven-dried round-bottom flask with stir bar was added roughly 3 mL pyridine (~3 eq), followed by pyrazine-2-carboxylic acid (1.68 g, 1.05 eq) and DMAP (~15 mg, 0.01 eq). The suspension was allowed to stir for a few minutes at room temperature before addition of DSC (3.31 g, 1.0 eq). An empty balloon was attached by needle/septum to monitor CO2 generation. The suspension was allowed to stir for a few hours at room temperature. At this time, no chunks of the poorly soluble carboxylic acid remained.
To the suspension was added at room temperature DIPEA (2.64 mL, 1.5 eq) followed by Boc-L-Ornithine (3.00 g, 1.0 eq). The resultant suspension was allowed to stir overnight at room temperature. Solvent was evaporated on the rotary evaporator before transferring to a separatory funnel using deionized water. The aqueous phase was washed twice with ether (~1/2 volume of aqueous, each), acidified to pH 1-2 with 1 M HCl, and then extracted three times with ethyl acetate. The combined organics were washed once with acidic (0.1 M HCl) brine, dried over Na2SO4, and solvent evaporated. The amide was reasonably pure and the Boc group was removed without further purification. Yield of this amino acid (as the hydrochloride) was quantitative.
Tips¶
CO2 Balloon Monitor
If you equip your flask with a completely empty balloon at the beginning of the first reaction step, you will see the balloon fill with CO2 as the reaction progresses. This doesn't confirm completion, but it does indicate that a reaction has occurred and will give a rough estimate of required reaction time (i.e., minutes, hours, or days).
Workup Volumes
Don't let your solvent volumes during workup get out of hand. Keep the total volume of water below 100 mL when transferring into the sep funnel and washing with ether. The more aqueous you have, the more extraction solvent you'll need. If you end up having to rotavap more than ~200 mL organic solvent, you'll either have to use a 1 L flask or you'll have a rough time with bumping.
Acidification
When rendering acidic, make sure it is below the carboxylic acid pKa of ~3. Otherwise, you won't get all your product. However, the Boc protecting group is acid sensitive. If you're adding 1 M HCl in portions to an aqueous volume of ~100 mL, you shouldn't have an issue. Just make sure you get pH < 2, and once you are acidic enough, work quickly -- this is not a good time to take a break. Once your organic layer is drying over your choice of drying agent, this is a better time to take a break.
Appendix: DSC¶
| Property | Details |
|---|---|
| CAS | 74124-79-1 |
| Form | White, free-flowing solid; high melting point |
| Cost | ~$0.50/g (Feb 2017) |
| Storage | Refrigerated, stable for years if container kept tightly closed |
| Solubility | Acetonitrile is standard. DMF may also be used. |
Reactions of DSC:
- Carboxylic acids are converted to the OSu active ester
- Alcohols are converted to active carbonates (e.g., in the synthesis of phospholipids)
- Amines are converted to active carbamates (e.g., in the synthesis of complex carbamates)
Reference¶
Ogura et al., "A Novel Active Ester Synthesis Reagent (N,N'-disuccinimidyl carbonate)", Tet Lett, 49, 1979, 4745-4746.