1 54 3h s 1 48 3h s and 0 88 6h m 2r 2 2 dimethyl 4 oxo 1 3 dioxalan 5s yl 4 methylpentanoic acid 558 mg 2 4 mmol was taken up in dichloromethane 10 ml and cooled to 0 u00b0 c before adding pentafluorophenol 670 mg 3 6 mmol and n ethyl n 39 3 dimethylaminopropyl carbodiimide 560 mg 2 9 mmol the reaction was stirred at 0 u00b0 c for 2 hours then the solution was washed with 1m sodium carbonate 50 ml and brine 20 ml the organic layer was dried magnesium sulphate filtered evaporated to dryness and purified by column chromatography silica gel dichloromethane to give the activated ester 552 mg 58 1 h nmr d chloroform d 4 57 1h d j 6 5 hz 3 32 1h m 1 86 3h m 1 67 3h s 1 58 3h s and 1 03 6h m pentafluorophenyl 2r 2 2 dimethyl 4 oxo 1 3 dioxalan 5s yl 4 methyl pentanoate 1 06 g 2 7 mmol and l cyclohexylalanine n methylamide 0 33 g 1 8 mmol were dissolved in n n dimethylformamide 150 ml and the mixture was stirred overnight at room temperature the solvent was removed to give an oil which was purified by column chromatography silica gel gradient elution with 0 to 5 methanol in dichloromethane gave first unreacted ester followed by the desired product 0 43 g 60 1 h nmr d chloroform d 6 47 2h br m.