in tetrahydrofuran react. under Ar or N2 (vac. line and Schlenk tube); addn. of Sn-compd. in THF to a slurry of dilithio reagent in THF, stirring at room temp. for 5 h (ppt.); hydrolysis with satd. NH4Cl soln, extn. with CHCl3, drying of organic phase (MgSO4), evapn. (vac.), recrystn. from CHCl3-ether (50:50); elem. anal.;