Ultrasound assisted reductive cleavage of eudesmane and guaiane γ-enonelactones. Synthesis of 1α,7α,10αH-guaia-4,11-dien-3-one and hydrocolorenone from santonin - Blay,G.; Bargues,V.; Cardona,L.; García,B.; Pedro,J.R. (Tetrahedron, 2001)