Mechanism

1. The sulfonyl carbon is deprotonated, forming an enolate with the adjacent ester. The enolate oxygen anion is then stabilized by the sodium cation.
2. The enolate undergoes conjugate addition with the alkene, forming a 4-member ring. The leaving group decomposes into CO 2 and an aryl alkoxide.
3. A hydride ion adds to the carbonyl carbon, and the aluminum compound complexes with the oxygen anion, stabilizing it momentarily.
4. The oxygen de-complexes from the aluminum compound and reforms the carbonyl, with the isopropyl alkoxide acting as a leaving group. The final aldehyde is formed.