Racemization of chiral PNAs during solid-phase synthesis: effect of the coupling conditions on enantiomeric purity

Chiral peptide nucleic acid (PNA) monomers based on the amino acids Ala, Phe and Lys were synthesized. and their enantiomeric purity was checked either by RP-HPLC after reaction with L-ValOMe or by GC-MS using a Chirasil-Val column after hydrolysis and conversion of PNAs to the corresponding piperazin-2-ones. A model coupling reaction of these monomers with ValOMe was carried out under various conditions in order to evaluate the effect of synthetic parameters (coupling agent, base, preactivation time) on epimerization. The enantiomeric purity of the products decreased in the order: DEPBT>TDBTU> HBTU>HATU. The use of sym-collidine (TMP) as a base produced higher racemization than with DIEA. PNAs containing one or three chiral monomers were subsequently synthesized with different coupling protocols. and the results were found to be consistent with those obtained in solution. High enantiomeric purity was obtained using a DIC/HOBt coupling protocol. A rationale for the observed effects is proposed based on NMR Studies. (C) 2002 Elsevier Science Ltd. All rights reserved.