The cycloaddition reaction of'lf with N-phenylmaleimide yielded a pair of exo (major) and endo (minor) adducts 7 and 8, whose structures were assigned on the basis of the difference in the reactivity toward m-chloroperbenzoic acid. The reaction of If with phenyl vinyl sulfoxide, dimethyl acetylenedicarboxylate, and di-2-thenoylacetylene gave benzo [c] thiophene derivatives 13, 15, and 16, respectively.
