Evidence for π-Stacking as a Source of Stereocontrol in the Synthesis of the Core Pyranochromene Ring System common to Calyxin I, Calyxin J, and Epicalyxin J

A diastereoselective synthesis of the tetrahydropyranochromene ring system common to several natural product isolates of Alpinia blepharocalyx is reported. We have shown that a stereochemical preference exists for a syn configuration between the anomeric aryl substituents, representative of the C-7 and C-7' substituents in the natural products. Further, our results show that stereocontrol is under kinetic control, and calculations suggest that a favorable pi-stacking interaction may be the source of this stereocontrol.