Synthesis of N-Boc-alfa-amino acids with nucleobase residues as building blocks for the preparation of chiral PNA (peptidic nucleic acids)

N-Boc-Glutamic acid ?-benzyl ester was transformed into 4-bromo-2-(tert-butoxycarbonylamino)-butyric acid benzyl ester using the Barton-Crich protocol. This bromo derivative undergoes nucleophilic substitution with nucleobases to give optically active N-Boc-4-adeninbutyrine (Boc-Aby), N-Boc-4-thyminbutyrine (Boc-Tby), N-Boc-4-guaninbutyrine (Boc-Gby) or N-Boc-4-cytosinbutyrine (Boc-Cby), useful building blocks for the synthesis of chiral peptidic nucleic acids (PNA)