Leading Question

For what reason is the specific stereoselectivity observed in the transformation of 25 to 26?

Compound 25 undergoes a Diels-Alder reaction to form a cycloadduct intermediate. Two factors determine the stereochemical outcome of the reaction: exo/endo TS, and "top/bottom" approach of the dienophile. Therefore four stereoisomers are anticipated to form. After the β-elimination of phenylsulfinic acid (PhSO2H), a double bond forms, eliminating two stereocenters. The stereoisomers that remain are determined solely by the "top/bottom" approach, and the two products are enantiomers of each other. Since there is no preference between these two approaches, the product (26) forms as a racemic mixture.

To obtain the enantiomerically pure compound 26, chiral catalysts must be used. One of such catalysts is shown in the references page (additional reference 2). For the reported transformation, 25 to 26, since no chiral compounds are used, the product must be a racemate.