Stereoregular polyamides with optically-pure aspartame-type diketopiperazine derivatives

Polyamides (PAs) with aspartame-derived diketopiperazine (DKP) form specific particles and exhibit high heat resistance. In this study, we investigated the stereostructures of aspartic acid and phenylalanine, which constitute aspartame DKP. Diketopiperazines with different stereostructures were synthesized from D- and L-aspartic acid and D- and L-phenylalanine, and LL-, DD-, and LD-type aminodiketopiperazines (LL, DD, and LD-ADKPs) were obtained. These ADKPs differed in their crystal forms depending on their stereoisomeric structures. Furthermore, PAs were formed from these ADKPs via polycondensation using triphenyl phosphite/pyridine, and their heat resistance was approximately 300 °C. The secondary structure of each PA was evaluated by circular dichroism, which suggested that LD-type PAs had a mixed secondary structure of coils and helical structures, unlike the LL and DD types. Precipitated LL, DD, and LLcoDD PAs formed similar particles, whereas the particles of the large LD-type PA had different particle shapes owing to their different conformations.