The present invention provides a process comprising substitution of an acceptor molecule comprising a group —XC(O)— wherein X is O, S or NR
8
, where R
8
is C
1-6
alkyl, C
6-12
aryl or hydrogen, with a nucleophile, wherein the acceptor molecule is cyclised such that said nucleophilic substitution at —XC(O)— occurs without racemisation. This process has particular application for the production of a peptide by extension from the activated carboxy-terminus of an acyl amino acid residue without epimerisation.