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Morozova, Varvara; Moriya, Kohei; Mayer, Peter; Knochel, Paul (2016): Stereoselective Synthesis and Retentive Trapping of alpha-Chiral Secondary Alkyllithiums Leading to Stereodefined alpha,beta-Dimethyl Carboxylic Esters. In: Chemistry-A European Journal, Vol. 22, No. 29: pp. 9962-9965
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The treatment of alpha-chiral secondary alkyl iodides with tBuLi at -100 degrees C leads to the corresponding secondary alkyllithiums with high retention of configuration. Subsequent quenching with various electrophiles such as Bu2S2, DMF, MeOB(OR)(2), or Et2CO provides the desired products with retention of configuration. Furthermore, a transmetalation with CuBr center dot P(OEt)(3) also allows retentive trapping with acid chlorides and ethylene oxide. The quenching of the resulting alkyllithiums with ClCO2Et 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.