Solution-phase amide coupling with DSC
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.
The coupling carried out in the following procedure 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.
To 50mL acetonitrile in an oven-dried round-bottom flask with stir bar was added roughly 3mL pyridine (~3eq), followed by pyrazine-2-carboxylic acid (1.68g, 1.05eq) and DMAP (~15mg, 0.01eq). The suspension was allowed to stir for a few minutes at room temperature before addition of DSC (3.31g, 1.0eq). 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 (2.64mL, 1.5eq) followed by Boc-L-Ornithine (3.00g, 1.0eq). The resultant suspension was allowed to stir overnight at room temperature (I let my reaction go for a few days - if you have a way of confirming consumption of amine, consider doing so). 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 your volume of aqueous, each), acidified to pH between 1 and 2 with 1M HCl, and then extracted three times with ethyl acetate (Try to keep your total EtOAc volume less than 200mL (at this scale)). The combined organics were washed once with acidic (0.1M 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.
TIP: 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).
TIP: Don't let your solvent volumes during workup get out of hand. For example, I kept the total volume of diH2O below 100mL when transferring into the sep funnel and washing with ether. Rendering acidic with 1M HCl increases the aqueous volume. The product in this example is a fairly polar molecule and some could get left behind in aqueous if you are not careful. The more aqueous you have, the more extraction solvent you'll need. If you end up having to rotavap more than ~200mL organic solvent, you'll either have to use a 1L flask or you'll have a rough time with bumping.
TIP: 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 1M HCl in portions of very roughly several mL (I just pour straight out of the bottle) to an aqueous volume of ~100mL, you shouldn't have an issue. Rather than being super meticulous, just make sure you get pH < 2 (should only take a few minutes), and once you are acidic enough, just 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.
Reference
Ogura et al, “A Novel Active Ester Synthesis Reagent (N,N’-disuccinimidyl carbonate)”, Tet Lett, 49, 1979, 4745-4746.
Appendix 1: DSC
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
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)