Aminoacyl derivatives of nucleosides, nucleotides, and polynucleotides. VI. Phosphonic analogues of 2'(3')-O-aminoacyl ribonucleosides: Synthesis of 2'(3' )-O-(R,S)-(1-aminoethane-1-phosphonyl)- and 2'(3')-O-(R,S)-(1-amino-2-phenylethane-1-phosphonyl)adenosine