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Cyclization of Erythro Dicarbamate 12 to Urea 13. Sodium hydride (80% in mineral oil, 400 mg, 13.3 mmol) was added to an ice-cold solution of erythro-dicarbamate 12 (3.60 g, 10.7 mmol) in 250 mL of anhydrous THF. The ice bath was removed and the suspension was stirred for 3 h. Aqueous NaOH solution (15%, 3 mL) was added, and the mixture was stirred for an additional 4 h and was evaporated. The solid residue was partitioned between 50 mL of ethyl acetate and 50 mL of a 1:1 brine/H20 mixture. The aqueous layer was extracted three times with 50-mL portions of ethyl acetate, the combined extracts were dried (MgS04), and the solution was evaporated. Purification of the residual yellow oil by flash chromatography (50 mm column, ethyl acetate) provided 2.34 g (89%) of cyclic urea 13 as a colorless oil:

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