Phosphorus pentachloride → Rac-benzyl-{1-[[2-(o-fluorobenzoyl)-4-nitrophenyl]carbamoyl]ethyl}carbamate

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90 g of carbobenzoxy-L-alanine are dissolved in 500 ml of absolute methylene chloride, the solution is cooled to -40° C and treated with 95 g of phosphorus pentachloride. The mixture is stirred at -30° C for 20 minutes and subsequently added to a shaken solution of 52 g of 2-amino-5-nitro-2'-fluorobenzophenone in 250 ml of absolute methylene chloride. The solution is concentrated on a rotary evaporator at 50°-60° C, treated twice with toluene and evaporated each time. By crystallisation of the residue from ether, there is obtained (-)-benzyl-[1-[{2-(o-fluorobenzoyl)-4-nitrophenyl}-carbamoyl]ethyl]carbamate which melts at 158°-160° C and exhibits a rotation of [α]25D =-23.4° (in methylene chloride, 1%).

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