n-butyllithium in tetrahydrofuran dropwise addn. of 55mmol n-BuLi in hexane to a soln. of 50mmol 1.3-dithiane in 125ml dry THF (N2), warming to 0°C after 3h at -78°C, cooling to -78°C, addn. of 50mmol Sn-compound, stirring for 3h at -78°C, 12h at 25°C;; addn. of about 10g solid CO2, addn. of 200ml H2O, removal of THF by distn., extractn. of remaining aq. suspension with CH2Cl2, shaking extract with 5percent aq. K2CO3-soln., washing, drying (MgSO4), evapn.; mass spectroscopy;;