Synthesis of Indoline-Fused 2,5-Diketopiperazine Scaffolds via Ugi-4CR in the Basic Mediated Tandem Consecutive Cyclization

This study has developed a method to prepare diastereo- and regio-selective indoline-fused 2,5-diketopiperazine scaffolds. The synthetic process involved an Ugi four-component reaction in which readily available starting materials were used and a metal-free intramolecular post-Ugi tandem consecutive 5-exo-trig/6-exo-dig cyclization under basic conditions. The plausible mechanism and substrate structures were elucidated by single-crystal X-ray diffraction analysis and supported by density functional theory calculations.