To a solution of 0.5 g (1.3 mmoles) of 18a in 10 mL dry THF cooled to -20° C. under a nitrogen atmosphere was added dropwise with stirring 0.55 mL of 2.5M n-butyl lithium in hexane (1.4 mmoles). When addition was complete, the reaction was allowed to warm to -10° C. over one hour, held there one hour, then cooled to -78° C. After stirring a few minutes, 0.4 g (1.57 mmole) of I2 was added all at once, and stirring continued. After 1 hour the reaction mixture was allowed to warm to ambient temperature. After removal of the solvent, the residue was partitioned between CHCl3 and 1M KH2PO4. The aqueous phase was extracted two more times, the combined organic phases washed with sufficient aqueous Na2S2O3 to remove excess I2, dried (MgSO4) and the solvent evaporated. The residue was crystallized from CH2Cl2 /ether to give 0.3 g product,
