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Cationic cyclization of 2-alkenyl-1,3-dithiolanes: diastereoselective synthesis of trans-decalins.

The Journal of organic chemistry (2011-03-31)
Sylvie Goncalves, Stefano Santoro, Marc Nicolas, Alain Wagner, Philippe Maillos, Fahmi Himo, Rachid Baati
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.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
trans-Decahydronaphthalene, 99%
Sigma-Aldrich
cis-Decahydronaphthalene, 99%
Sigma-Aldrich
1,3-Dithiolane, 97%
Sigma-Aldrich
Decahydronaphthalene, mixture of cis + trans, anhydrous, ≥99%
Sigma-Aldrich
Decahydronaphthalene, mixture of cis + trans, reagent grade, 98%