Stereoselective Synthesis and Retentive Trapping of α-Chiral Secondary Alkyllithiums Leading to Stereodefined α,β-Dimethyl Carboxylic Esters.

The treatment of α-chiral secondary alkyl iodides with tBuLi at -100 °C leads to the corresponding secondary alkyllithiums with high retention of configuration. Subsequent quenching with various electrophiles such as Bu2 S2 , DMF, MeOB(OR)2 , or Et2 CO provides the desired products with retention of configuration. Furthermore, a transmetalation with CuBr⋅P(OEt)3 also allows retentive trapping with acid chlorides and ethylene oxide. The quenching of the resulting alkyllithiums with ClCO2 Et furnishes stereoselectively syn- and anti-ethyl-2,3-dimethyl ester carboxylates (d.r.>94 %). Related esters bearing three adjacent stereo-controlled centers (stereotriads) have also been prepared. This method has been applied to the synthesis of the ant pheromone (±)-lasiol in 26 % overall yield (four steps) with d.r.=97:3 starting from commercially available cis-2,3-epoxybutane.