Proposal and confirmation of N-(2-carboxyethyl)proline as a human endogenous metabolite

RATIONALE

Malondialdehyde, one of the peroxidation products of polyunsaturated fatty acids, has been widely reported as an oxidative stress biomarker in many diseases. However, malondialdehyde is inherently unstable in biological matrices, which renders its measurement unreliable with all the reported analytical methods. To find an alternative oxidative stress biomarker, we envisioned that N-(2-carboxyethyl)proline, a modified conjugate of malondialdehyde and proline, could be a stable candidate for this purpose.

METHODS

The proposed compound was chemically synthesized, and liquid chromatography-mass spectrometry methods were developed and used to search for the compound in human biological samples.

RESULTS

An endogenous metabolite in human feces and urine samples was found to match the synthetic N-(2-carboxyethyl)proline by chromatographic retention and the fragmentation pattern of its molecular ion.

CONCLUSION

The results confirmed that N-(2-carboxyethyl)proline was a new metabolite in human feces and urine samples. In addition, our results demonstrated a case of successful identification of true unknown metabolite by knowledge-based hypothesis of possible metabolites followed by experimental confirmation with a synthetic standard.

Total Synthesis of Cyclotripeptidic Natural Products Anacine, Aurantiomide C, Polonimides A and C, and Verrucine F

The total synthesis of cyclotripeptidic natural products possessing a central piperazino[2,1-b]quinazolin-3,6-dione core is described through an original strategy involving the pivotal cyclocondensation of an electrophilic homoserine lactone intermediate. The alkylidene group was spontaneously installed by autoxidation during the cyclocondensation process, while the propionamide side chain was introduced through the nickel-catalyzed aminocarbonylation of a bromoethyl intermediate. This last reaction is unprecedented on such highly functionalized intermediates. Finally, we explored structural modifications and interconversions of the natural products. Overall, this work led to anacine, aurantiomide C, polonimides A and C, and verrucine F.

Donor-Acceptor Cyclopropane Ring Expansion to 1,2-Dihydronaphthalenes. Access to Bridged Seven-Membered Lactones

A Lewis-acid-promoted domino ring-opening cyclization of readily available donor-acceptor cyclopropanes with a preinstalled electrophilic center, embedded in a donor group, to functionalized 1,2-dihydronaphthalenes is reported herein. The obtained compounds are transformed to pharmacologically attractive bridged tricyclic esters in a diastereospecific manner.

Sigmatropic [1,5] Carbon Shift of Transient C3 Ammonium Enolates

The stereospecific sigmatropic [1,5] carbon shift of C3 ammonium enolates is discovered. According to mechanistic, kinetic and computational experiments, this new rearrangement proceeds via the catalytic generation of a transient C3 ammonium enolate by intramolecular aza-Michael addition. This intermediate rapidly undergoes [1,5] sigmatropic carbon migration to furnish the respective tetrahydroquinoline-4-ones with excellent diastereoselectivities of d.r. >99 : 1 and in 61-98 % yield.

Total Synthesis of Phytotoxic Radulanin A Facilitated by the Photochemical Ring Expansion of a 2,2-Dimethylchromene in Flow

The radulanins are biologically active bibenzyl natural products featuring a synthetically challenging 2,5-dihydro-1-benzoxepine core. In contrast with previous reports exhibiting lengthy strategies, we demonstrate the shortest synthesis of radulanin A to date, featuring a largely unexplored photochemical ring expansion reaction of a 2,2-dimethylchromene precursor. This work was adapted to a continuous-flow setup for larger-scale preparation, in view of biological investigations into the herbicidal properties of this natural product.