A stirred solution of 0.12 g of the product of Example 2, Step 2 and 0.40 mL of triethylamine in a mixture of methylene chloride (10 mL) and methanol (10 mL), under nitrogen, is treated with ethyl dithioacetate (0.05 mL) and kept at ambient temperature for 145 hours. Additional 0.05 m]L portions of ethyl dithioacetate are added after 24, 31 and 49 hours; additional triethylamine (1.0 mL) is also added after 49 hours. The mixture is concentrated to a small volume, diluted with ethyl acetate, washed with water and brine, dried (Na2SO4) and concentrated. Chromatography of the residue on silica gel with 3.5% methanol-methylene chloride gave 0.061 g of the titled product.
