Studies on Remote Asymmetric Induction in Cycloaddition Reactions using Novel Chiral Sulfoxides

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Three types of chiral sulfoxides having a 5-membered aromatic heterocycle (furan, thiophene and pyrrole) were synthesized and high levels of diastereoselectivity in addition reactions were observed.1.Stereoselective reductionReduction of chiral 3-(p-tolylsulfinyl)-2-thienyl alkyl (and aryl) keones with diisobutylaluminum hydride afforded the corresponding (S)-thienylcarbinols as the major product, while the similar reduction in the presence of a Lewis acid or lithium tri-sec-butylborohydride (L-Selectride^<<encircledR>>) reduction afforded (R)-alcohols as the major isomer.2.Stereoselective hetero Diels-Alder reactionA lanthanide triflate-promoted hetero Diels-Alder reaction of chiral 3-(p-tolylsulfinyl) furaldehyde with Danishefsky's diene proceeded to give the adduct with excellent distereoselectivity.3.Stereoselective Diels-Alder reaction2-Futyl and 2-thienyl alpha, beta-enones, bearing a chiral p-tolylsulfinyl group in the heterocycle, served as an efficient dienophile in the Diels-Alder reaction, where the catalytic use of aluminium chloride or a lanthanide triflate effected these cycloadditions with cyclopentadiene affording the endo adduct with high diastereoselectivity, ranging from 91% to 98%. Similarly the enone to which a chiral p-tolylsulfinyl pyrrole moiety was incorporated also underwent the cycloaddition with up to 89% diastereoisomeric excess in the presence of 1 eqiuv. of the Lewis acid. In particular for the Diels-Alder reaction, the auxiliary (i.e.the sulfinyl pyrrole) was recovered after use without any loss of optical purity.