Tantalum pentachloride as a coupling agent for stereohindered amide bond formation

We have previously reported that tantalum carboxylates show high reactivity toward primary amines to give primary amides. We report herein that tantalum pentachloride, in contrast with the behavior of standard coupling reagents, mediates the couplings of secondary amines and the couplings of very hindered carboxylates. Such coupling reactions are particularly difficult to achieve, when using chiral carboxylates (including N-protected amino acids), without a significant degree of accompanying racemization. Racemization levels with the present systems are found to be very low in comparison with those obtained using a standard amide/peptide coupling system (DCC/HOBt).