One-pot three-step thioconjugate addition-oxidation-Diels–Alder reactions of ethyl propiolate

meta-Chloroperbenzoic acid (mCPBA): a versatile reagent in organic synthesis

“…Ethyl propiolate undergoes a one-pot three-step thioconjugate addition-oxidation-Diels-Alder cycloaddition when treated with a variety of thiols in the presence of a catalytic base, mCPBA, lithium perchlorate and cyclopentadiene (Scheme 48). 464 Electron-rich aryl thiols were found to be the most successful substrates. In some cases, addition of a second equivalent of LiClO 4 during the cycloaddition step was unnecessary to achieve both high yield and selectivity.…”

“…Diastereoselectivity varied somewhat from substrate to substrate and both the exo isomer derived from the Z enoate and diastereomers resulting from the cycloaddition of the E enoate were frequently observed as minor products, but in all cases the major endo isomer was easily puried using column chromatography. 464 In another study, Thiemann et al 465 reported that brominated anthraquinones could be synthesized directly from bromothiophenes via a Diels-Alder reaction with 1,4-naphthoquinones in the presence of mCPBA (Scheme 49). Under these conditions, cycloaddition takes place between a thiophene S-oxide intermediate and the 1,4-naphthoquinone, aer which the primary sulfoxy-bridged cycloadduct formed loses its SO bridge with concomitant aromatization.…”