Thienium salt

In these cases, the intermediate Diels-Alder adducts were not isolated or 

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A rather interesting route to 3-cyclopentenones has been developed by Corey and Walinsky (Scheme 5-XVI), which uses a dithienium salt as a heterodienophile in the initial step.43 This species adds regioselectively to 2,3-dimethylbutadiene to give an adduct that can be transformed as shown via a vinyl cyclopropane to the unsaturated ketone in good overall yield. In related work, it was found that several substituted acyclic thienium salts will also provide vinyl cyclopropanes [Eq. (18)].44,45... 

A clever route to thienium salts from a nitro olefin like 19 has recently been described.46 Treatment of 19 with aluminum chloride provides a thienium intermediate that adds to various 1,3-dienes. The product of [4 + 2] cycloaddition was not detected, rather a ring-opened compound was produced (Scheme 5-XVII). Some additional examples of this reaction are... 

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