Synthesis of (±)-11-O-Debenzoyltashironin

Key words

intramolecular Diels-Alder reaction - biomimetic cascade - oxidative dearomatization

Significance

11-O-Debenzoyltashironin, a metabolite of Illicium merrillanium, induces neurite outgrowth in fetal rat cortical neurons. The key step in this biomimetic synthesis is an oxidative dearomatization leading to quinone monoketal B, which underwent an intramolecular Diels-Alder reaction to create all four rings of the target.

Comment

Reduction of the ketone in C proceeded with a dr > 9:1. After homogeneous hydro­genation of the exo-methylene in D, the epoxide was reductively cleaved with a large excess of ­lithium triethylborohydride at 100 °C in a sealed tube. Under these conditions, the enol tosylate was also reductively cleaved but surprisingly, the product was the secondary alcohol rather than the ketone.