Formation and [4 + 2] cycloaddition reactions of 2,3‐dimethylene‐2,3‐dihydrothiophene

Abstract

Abstract2,3‐Dimethylene‐2,3‐dihydrothiophene (1), the thiophene analog of o‐xylylene (o‐quinodimethane), was generated in situ from the (trialkylammoniomethyl)‐(trimethylsilylmethyl)thiophene iodides 4 or 5 by fluoride‐induced 1,4 climination, and was trapped by [4 + 2] cycloadditions with a series of dienophiles. The reaction of 1 with dimethyl fumarate was considerably faster than with dimethyl maleate. Unsymmetrical dienophiles gave mixtures of regio‐isomers. In the absence of dienophiles, 1 formed [4 + 2] spiro dimers, of which 15a is the major component, as determined by 1H‐NMR comparison with a tetradeutero analog of 15a. The syntheses of the new precursors of 1 and dideutero‐1: compounds 4, 5a,b, as well as the 3.4‐isomer of 4, are described.