Diastereoselective conjugate additions to pi-allylmolybdenum complexes: a stereocontrolled route to 3,4,5-trisubstituted gamma-butyrolactones.

[reaction: see text] pi-Allylmolybdenum complex 6b is obtained as a single isomer by Knoevenagel condensation of aldehyde 1 with Meldrum's acid. Conjugate additions of Grignard reagents to Meldrum's acid alkylidene derivative 6b are shown to be completely diastereoselective. Further functional group transformation of the 1,4-adducts, followed by demetalation, leads to trisubstituted tetrahydrofurans and gamma-butyrolactones. Whereas the synthesis of tetrahydrofurans (X = 2H) is not completely stereoselective, the gamma-butyrolactones (X = O) are obtained with good to excellent diastereoselectivities.