solution of 1r 3s 4 cyclopentene 1 3 diol 1 acetate 0 17 g 1 19 mmol in dry tetrahydrofuran 2 ml over 10 min and the resulting mixture was stirred at room temperature for 18 h the solvents were removed by evaporation at reduced pressure and the residue was slurried in dichloromethane approx 25 ml and filtered the filtrate was evaporated and the residue was purified by chromatography on silica gel using i etoac followed by 10 1 etoac meoh as eluent to give 4r 2 amino 6 chloro 9h purin 9 yl 2 cyclopenten 1s ol 174 mg 58 dicarbonic acid dimethyl ester 2 g 15 mmol is added dropwise to a stirred solution of cis u00b1 4 2 amino 6 chloro 9h purin 9 yl 2 cyclopenten 1 ol 2 0 g 8 mmol and 4 dimethylaminopyridine 3 mg in dry dichloromethane 20 ml at ambient temperature and stirring is continued for 20 minutes additional dicarbonic acid dimethyl ester 2 g is added and the mixture is stirred for a further 20 minutes whereupon the mixture becomes clear the solvent is evaporated and the residue is taken up in dichloromethane 20 ml and treated with dicarbonic acid dimethyl ester 2 g this evaporation retreatment with dicarbonic acid dimethyl ester sequence is repeated until no starting material remained by thin layer chromatography the solution is finally evaporated to afford cis u00b1 4 2 amino 6 chloro 9h purin 9yl 2 cyclopenten 1 yl carbonic acid methyl ester as a white solid 2 36 g 96 1 h nmr u03b4 7 83 s 1h 6 37 dt j 1 5 5 hz j 2 2 hz 1h 6 18 dd j 1 5 5 hz j 2 2 hz 1h 5.