Cyclic dipeptides (DKPs) are peptide precursors and chiral catalysts in the prebiotic process. This study reports proline-containing DKPs that were spontaneously obtained from linear dipeptides under an aqueous solution. Significantly, the yields of DKPs were affected by the sequence of linear dipeptides and whether the reaction contains trimetaphosphate. These findings provide the possibility that DKPs might play a key role in the origin of life.

The biological system uses L-amino acids (L-AAs) to construct peptides. The selection of AA chirality in the prebiotic Earth still remains a mystery. Here we report the intrinsic mechanism of AA chiral selection in the formation of prebiotic proline-containing cyclic dipeptide (DKPs). The DKPs were synthesized by equal amounts of L-AA and D-AA with L-Pro that exhibited a clear preference for D-AA as opposed to L-AA. And surprisingly, the L-AA was preferred over D-AA in the mirror-image system with D-Pro. These results indicate that prebiotic DKPs synthesis has a certain selectivity for AA chirality preceded the biological enzymatic catalysis.

An efficient, one-pot, three-component [3+2] cycloaddition reaction of azomethine ylide obtained from α-dicarbonyl compounds (cyclic and acyclic diketone or keto ester) and amino acids with maleimides under catalyst-free conditions has been developed. This cascade protocol shows high efficiency and remarkable functional group tolerance, and the ubiquitous succinimide-fused pyrrolizidines with a highly compact and strained scaffold were obtained with high yield and excellent diastereoselectivity. Furthermore, this novel and atom-economical strategy could be performed on a gram scale with comparable reaction efficiency.

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