A Practical Route to Enantiopure, Highly Functionalized Seven-Membered Carbocycles and Tetrahydrofurans: Concise Synthesis of (+)-Nemorensic Acid

Highly diastereoselective thermal [5C+2C] intramolecular pyrone-alkene cycloadditions can be achieved by introducing a homochiral p-tolylsulfinyl group at a suitable position of the alkene. The resulting adducts can be readily desulfinylated to give optically active 8-oxabicyclo[3.2.1]octane derivatives. Interestingly, switching from a sulfinyl to a sulfonimidoyl group allows one to reverse the direction of the diastereofacial selectivity and thereby produces oxa-bridged carbocyclic systems enantiomeric to those obtained from the sulfinyl precursors. Cleavage of the oxa-bridge on the desulfurated adducts yields highly functionalized seven-membered carbocyclic derivatives in enantiopure form. Alternative cleavage of the seven-membered carbocycle provides enantiomerically enriched tetrahydrofurans. We have exploited this reaction pathway for the synthesis of the naturally occurring enantiomer of nemorensic acid.