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  • Title: Cationic cyclization of 2-alkenyl-1,3-dithiolanes: diastereoselective synthesis of trans-decalins.
    Author: Goncalves S, Santoro S, Nicolas M, Wagner A, Maillos P, Himo F, Baati R.
    Journal: J Org Chem; 2011 May 06; 76(9):3274-85. PubMed ID: 21446693.
    Abstract:
    An unprecedented and highly diastereoselective 6-endo-trig cyclization of 2-alkenyl-1,3-dithiolanes has been developed yielding trans-decalins, an important scaffold present in numerous di- and triterpenes. The novelty of this 6-endo-trig cyclization stands in the stepwise mechanism involving 2-alkenyl-1,3-dithiolane, acting as a novel latent initiator. It is suggested that the thioketal opens temporarily under the influence of TMSOTf, triggering the cationic 6-endo-trig cyclization, and closes after C-C bond formation and diastereoselective protonation to terminate the process. DFT calculations confirm this mechanistic proposal and provide a rationale for the observed diastereoselectivity. The reaction tolerates a wide range of functionalities and nucleophilic partners within the substrate. We have also shown that the one-pot 6-endo-trig cyclization followed by in situ 1,3-dithiolane deprotection afford directly the corresponding ketone. This improvement allowed the achievement of the shortest total synthesis of triptophenolide and the shortest formal synthesis of triptolide.
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