Abstract
The identification of novel tetrodotoxin (TTX, 1) analogues would significantly contribute to the elucidation of its biosynthetic pathway. In this study, the first C5-C10 directly bonded TTX analogues, 4,9-anhydro-10-hemiketal-5-deoxyTTX (2) and 4,9-anhydro-8-epi-10-hemiketal-5,6,11-trideoxyTTX (3), were found in the newt Cynops ensicauda popei by using a screening method involving HILIC-LC-MS/MS focused on the fragment ions of TTX analogues, and their structures were elucidated by spectroscopic methods. Compound 2 was detected in a wide range of newt species, and the 2 and TTX contents of 22 newt specimens were correlated (rs =0.88). Based on these results and its structural features, 2 was predicted to serve as a precursor of TTX that would be directly converted into 4,9-anhydroTTX (4) by Baeyer-Villiger-like oxidation or via 4,9-anhydro-5-deoxyTTX formed by cleavage of the C5-C10 bond. The bicyclic carbon skeletons of 2 and 3 suggested a possible monoterpene origin for TTX.
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