One-step palladium-catalyzed synthesis of substituted dihydrofurans from the carbonate derivatives of γ-hydroxy-α,β-unsaturated sulfones

The palladium-catalyzed nucleophilic allylic substitution of the carbonate derivatives of gamma-hydroxy-alpha,beta-unsaturated sulfones (2) with soft carbon nucleophiles such as malonates, beta-keto esters, 1,3-diketones, and alpha-sulfonyl ketones took place cleanly and with full regiocontrol (gamma-substitution). Typical optimized conditions are Pd-2(dba)(3) (5 mol %), dppe (20 mol %), molecular sieves, in toluene-THF at 100 degrees C. Unexpectedly, when beta-keto esters, 1,3-diketones, and alpha-sulfonyl ketones were used as nucleophiles a cascade process occurred, via initial gamma-regioselective allylic substitution and further intramolecular conjugate addition of the enol moiety to the alpha,beta-unsaturated sulfone, to give 2,3,4,5-tetrasubstituted dihydrofurans (13-25) in moderate to good yields. Moreover, the cyclization step is highly stereoselective giving predominantly or exclusively the 4,5-dihydrofuran of trans configuration. From readily available enantiopure (S)-2, this one-step procedure of synthesis of substituted dihydrofurans has been applied to the synthesis of enantiomerically pure tetrasubstituted tetrahydrofurans.