Synthesis of precursors of the agalacto (exo) fragment of the quartromicins via an auxiliary-controlled exo-selective Diels-Alder reaction

A direct synthesis of the alpha-hydroxyaldehyde exo-5, a precursor of the exo-spirotetronate subunit of the quartromicins, was achieved through an exo-selective Lewis acid-catalyzed Diels-Alder reaction of dienophile 12a and diene 1.