Practical synthesis of 2'(3')-O-aminoacyl derivatives of CpA

N-Tert-butyloxycarbonyl-L-phenylalanine reacts with 1 equivalent of 1,1'-carbonyldiimidazole and 5'-O-dimethoxytrityl-2'-tetrahydropyranyl-N 4 -dimethylaminomethylenecytidylyl-(3' 5')-N 6 -dimethylaminomethyleneadenosine ( I ) and affords 2'(3')-O-(N-tert-butyloxycarbonyl)-L-phenylalanyl derivative II . The compound II gives by the action of formic acid in aqueous 1-butanol the 2'-O-tetrahydropyranylcytidylyl-(3' 5')-2'(3)-O-(tert-butyloxycarbonyl)-L-phenylalanyladenosine ( III ), which affords cytidyl-(3' 5')-2'(3')-O-L-phenylalanyladenosine ( IV ) by treatment with 95% aqueous trifluoroacetic acid.