In an argon reflux set-up at -78 • C, crotonyl sultam, tin tetrachloride TiCl 4 and anhydrous dichloromethane are introduced in an argon atmosphere. Here, the inert atmosphere avoids condensation of water and further undesired reactions. After 15 minutes, freshly distilled cyclopentadiene is added for 20 minutes via a cannula. Cyclopentadiene (the diene) is marketed in its dimeric form. Distillation is required at -78 • C to reform the monomer via a retro-Diels-Alder reaction. Stirring continues for 18 h. However, even if the stirring duration was reported to be 20 h in Litterature , it was possible to carry it out for only 2 h. This cycloaddition reaction, as reported in scheme 2, was catalyzed by a Lewis acid, tin tetrachloride TiCl 4 and leads to endo product (2) predominantly. The π face differenciation in the Diels-Alder step could be explained by the chelation of the sulfone group -SO 2 and the oxygen atom of the carbonyl group by the Titanium metal of the catalyst. In fact, this chelation will prevent the racemization of the cycloadduct by inhibiting the free rotation of the σ C-N bond of the conformer (3') shown in Fig. .The conformation being locked, the diene will be directed to the less hindered bottom face.