Synthesis and Diels−Alder Reactions of Butadienylpyridinium Bromides

1-(3-Sulfolen-3-yl)pyridinium bromide (5) and 1-(3-methyl-2-sulfolen-4-yl)pyridinium bromide (35) have been prepared and served as the stable precursors for the cation-substituted dienes 6a and 32a, respectively. Compounds 6a and 32a are reactive dienes in the Diels−Alder reactions with a number of electron-poor dienophiles. Some [4 + 2] cycloadditions of 6a and 32a can take place in water.