Studies on the synthesis of the quartromicins: partial stereochemical assignment of quartromicins A3 and D3 and diastereoselective synthesis of the endo- and exo-spirotetronate subunits

A partial stereochemical assignment of quartromicins A3 and D3 is presented, along with diastereoselective syntheses of the endo- and exo-spirotetronates and , corresponding to the galacto and agalacto fragments of the proposed quartromicin stereostructure. The key steps of these syntheses are highly enantio- and diastereoselective Lewis acid catalyzed Diels–Alder reactions of 1,1,3,4-tetrasubstituted diene .