Immobilized carbodiimide assisted flow combinatorial protocol to facilitate amide coupling and lactamization

Through a screen of over one hundred and thirty permutations of reaction temperatures, solvents, carbodiimide resins, and carbodiimide molar equivalences, in the presence, absence, or combination of diisopropylamine and benzotriazole additives, a convenient and first reported carbodiimide polymer-assisted flow approach to effect amide coupling and lactamization was developed. The protocol entails injecting a single solution (1:9 dimethylformamide:dichloromethane) containing a carboxylic acid and an amine, or linear peptide sequence, into a continuous stream of dichloromethane. The protocol remained viable in the absence of base, did not require carboxylate pre-activation which, and in concert with minimal workup requirements, enabled the isolation of products in high yields. Compared to the utilization of untethered carbodiimide reagents, the flow procedure was also observed to provide a degree of racemization safety.