New observations on peptide bond formation using CDMT

The optimized formation of the peptide bond by means of 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) has been found to occur rapidly and essentially quantitatively in a one-pot, one-step procedure. This new method is effective for the coupling of a variety of reactive partners, including chiral amino acids (e.g. N-acetyl-L-leucine) without significant loss of configuration. Significant racemization was observed when the typical literature conditions were used, due to the formation of an azlactone intermediate which is configurationally unstable under the reaction conditions. A simpler, precipitative workup procedure is also disclosed in this report. © 2002 Elsevier Science Ltd. All rights reserved. Due to the need for a variety of diverse peptides, many coupling agents have been developed that facilitate formation of the peptide bond. The desired peptide often contains stereogenic centers and therefore retention of the optical purity of the compound is also of great importance for any method. The use of CDMT as a coupling agent has been investigated fairly extensively, and it demonstrates several advantages over other coupling agents. It is a stable, crystalline compound, with good solubility in organic solvents, and is commercially available in large quantities. These qualities made CDMT attractive for process development in our laboratories. The present communication describes the results of these investigations. The standard method for making amides using CDMT is to activate the acid using CDMT and a base such as N-methylmorpholine. This generates an active ester, 1 (Scheme 1), which is subsequently reacted with the amine coupling partner in the same pot. The reaction typically proceeds to completion in a matter of 8–14 h. This method is effective for the formation of a variety of compounds, including esters and Weinreb amides. Workup of the products is typically afforded by extraction with dilute acid, as the CDMT and its triazine by-products are typically weakly basic and easily removed extractively. Analytically pure material is generally isolated via column chromatography, although the crude material is often pure enough to proceed with subsequent reactions. When we attempted to synthesize 5 (Scheme 2) via the standard two-step CDMT coupling conditions, we observed complete conversion of the amine (4) over about 22 h. Surprisingly, the intermediate observed after the pre-activation step was not the expected active ester, but an azlactone (3). We theorized that the azlactone could be generated via the intermediacy of Scheme 1. Typical procedure for two-step peptide bond formation using CDMT.