Diastereoselective synthesis of the endo- and exo-spirotetronate subunits of the quartromicins. The first enantioselective Diels-Alder reaction of an acyclic (Z)-1,3-diene.

[reaction: see text]. Diastereoselective syntheses of the endo- and exo-spirotetronates 1 and 2, corresponding to the galacto and agalacto fragments of quartromicins A3 and D3, are described. The key step of these syntheses are highly enantio- and diastereoselective Lewis acid catalyzed Diels-Alder reactions of the 1,1,3,4-tetrasubstituted diene 3.