In a groundbreaking advance for natural product synthesis and neuropharmacology, researchers at Peking University have achieved the first total synthesis of hemiketal tetrodotoxin (hemiketalTTX), an elusive and scarce analogue of the famous neurotoxin tetrodotoxin (TTX). This immense chemical feat not only addresses the critical limitation imposed by the minute natural availability of hemiketalTTX but also opens new pathways for examining its biological activities and developing related compounds with potentially transformative impacts on neuroscience research.
Tetrodotoxin has long fascinated scientists due to its powerful ability to selectively inhibit voltage-gated sodium channels, a feature that makes it invaluable for dissecting the physiology of neuronal excitation and impulse conduction. HemiketalTTX, first isolated from salamanders by the Yotsu-Yamashita team in 2014, distinguishes itself by an unusual [3.2.1] bridged bicyclic ring system containing both hemiketal and cyclic guanidinium functional groups—an architectural complexity surpassing that of the parent TTX molecule. Yet despite its intriguing structure and potential, its extremely scarce natural abundance—approximately 1/20 to 1/40 of TTX in biological sources—had hitherto made detailed biological investigations and practical applications unattainable.
The synthetic challenge posed by hemiketalTTX is formidable: it contains nine contiguous chiral centers embedded within a densely functionalized cage-like framework. To rival nature’s synthetic prowess, Yanxing Jia and Aili Fan’s team devised an innovative strategy centered on constructing the fundamental [3.2.1] bridged bicyclic core through a Prins cyclization reaction, a method known for forging complex ring systems with high stereo- and regioselectivity. By skillfully combining classical and cutting-edge techniques, they translated a multistep synthetic scheme from conceptual design to gram-scale execution.
At the inception of the synthetic pathway, the team selected commercially available (S)-4-tert-butyldimethylsilyloxy-2-cyclopentenone as a chiral starting point, harnessing its inherent stereochemical information to guide the cascade of reactions that would follow. This substrate was then elaborated by copper-catalyzed Michael addition and Mukaiyama aldol reactions, constructing the essential nitrene precursor in only three steps. The introduction of both amino and hydroxyl functional groups in a single key transformation was elegantly accomplished via a rhodium-catalyzed aziridination followed by ring opening—a powerful strategy that simultaneously set two stereocenters.
Supporting this complex sequence was a thorough campaign of protecting group manipulations and functional group interconversions, culminating in the preparation of the Prins cyclization precursor on a remarkable 10-gram scale. With the critical substrate in hand, attention turned to identifying reaction conditions capable of orchestrating the intricate cyclization and selective modifications essential for completing the total synthesis. An extensive screen of Lewis acids revealed aluminum chloride dimethyl complex AlCl(CH₃)₂ to be uniquely effective, not only promoting the intended Prins cyclization but also generating chlorinated side-products that could be cleanly transformed into conjugated dienes under Martin’s sulfurane reagent.
Subsequent double dihydroxylation of this conjugated diene introduced crucial hydroxyl functionalities, and refinements via protective group adjustments and oxidation state modulations seamlessly guided the molecule towards its final, complex hemiketal architecture. The entire assembly required 23 painstaking steps, yielding hemiketalTTX in an overall yield of 0.7%—a testament to the exquisite selectivity and efficiency achieved through the methodical synthetic route.
The successful total synthesis of hemiketalTTX has profound implications beyond synthetic chemistry itself. Preliminary pharmacological evaluations conducted by the Peking University team revealed that hemiketalTTX exhibits moderate inhibitory activity against the human voltage-gated sodium channel subtype Na_v1.1, with comparatively weaker inhibition of Na_v1.2 through Na_v1.7. Given the critical roles these channels play in physiological and pathological states, hemiketalTTX could become a valuable molecular probe or a lead compound for developing novel therapeutics targeting neurological disorders.
This landmark synthesis dismantles the major bottleneck of limited hemiketalTTX availability, empowering researchers to explore its biological functions with unprecedented depth. Furthermore, the synthetic logic designed and demonstrated here provides a versatile platform for accessing other highly oxidized, cage-like natural products that have traditionally eluded chemical synthesis due to their structural complexity. The usage of aziridination/ring-opening reactions to install amino-hydroxyl moieties in tandem, coupled with Prins cyclization for ring construction, exemplifies how contemporary synthetic methodology can overcome nature’s toughest challenges.
The scientific community will undoubtedly view this accomplishment as a beacon of innovation, inspiring further creative strategies to synthesize complex natural molecules with significant biological relevance. As hemiketalTTX and related analogues become more accessible, future research will be poised to unravel their mechanistic nuances, optimize their pharmacological profiles, and potentially develop them into next-generation neuroactive agents.
The research, spearheaded by doctoral candidates Shumi Jia and Yilong Bi under the guidance of Professors Yanxing Jia and Aili Fan, was published as a Communication in the flagship journal CCS Chemistry on August 19, 2025. This work was supported by major grants from the National Key R&D Program of China and the National Natural Science Foundation of China, highlighting the country’s commitment to advancing fundamental and applied chemical sciences.
CCS Chemistry, published by the Chinese Chemical Society, serves as an international platform spotlighting pioneering chemistry research conducted in China. This research exemplifies the journal’s mission to disseminate high-impact, open-access scientific discoveries without author or reader fees, fostering global collaboration and advancement in chemistry.
The total synthesis of hemiketalTTX joins the ranks of synthetic milestones, exemplifying how meticulous planning, innovative catalysis, and rigorous optimization can transform scarce natural products into accessible molecular entities. It stands as a compelling reminder of the power of synthetic chemistry to not only mimic nature’s complexity but to enable new science and technology on a scale previously thought impossible.
Subject of Research: Not applicable
Article Title: Total Synthesis of HemiketalTTX
News Publication Date: 19-Aug-2025
Web References: https://www.chinesechemsoc.org/journal/ccschem; http://dx.doi.org/10.31635/ccschem.025.202506052
Image Credits: CCS Chemistry
Keywords
Total synthesis, Hemiketal tetrodotoxin, Tetrodotoxin analogues, Rhodium-catalyzed aziridination, Prins cyclization, Natural product synthesis, Voltage-gated sodium channel inhibitors, Complex organic synthesis, Cage-like natural products, Neurotoxins, Organic synthesis methods, Stereoselective synthesis
