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A handle on annulation: The two alkenoyl moieties of the cyclic dithiolane are parallel to each other and enables the [7C+1C] annulation with ethyl isocyanoacetate to occur. As a result, tricyclic indolizidine alkaloids are constructed by a two-step, base-catalyzed [7C+1C] annulation/intramolecular cyclization with subsequent reduction/cyclization.
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Xu et al. (2013) studied this question.
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