Synthesis and Selected Reactions of a Bicyclic Sultam Having Sulfur at the Apex Position

A practical synthesis of the bicyclic dienyl sultam 9 has been developed. The viable route involved several key steps. Of these, ring-closing metathesis represented by the conversion of 19 to 20 had to be implemented in advance of the assembly of other rings such as is present in 15. Product 20 was used as the template for more advanced framework construction, as in 16. The second double bond was best introduced by a bromination-dehydrobromination sequence, the 2-fold loss of HBr being achieved most reliably by the use of tetra-n-butylammonium fluoride in CH2Cl2 or DMSO. The direct irradiation of 9 gave rise to the endo-oriented cyclobutene derivative 30. The title diene is not a ready participant in Diels-Alder reactions. When heated with endo-bornyltriazolinedione in ethyl acetate solution, conversion to a 1:1 mixture of 33 and its diastereomer occurred as confirmed by X-ray crystallographic analysis. From the mechanistic perspective, this transformation constitutes an interesting example of a stereocontrolled and regioselective [2+2] cycloaddition followed by a vinylcyclobutane-cyclohexene rearrangement. Products 30 and 33 constitute examples of strained sulfonamides featuring a norbornyl-like structural component.