The invention provides a method of assembling oligopeptide intermediates in a native chemical ligation reaction that eliminates self-ligation of bi-functional intermediates. An important aspect of the invention is a bi-functional intermediate with an N-terminal cyclic thiazolidine protecting group which effectively prevents self-ligation in the chemical assembly process. The present invention is useful in methods for convergent synthesis of polypeptides and proteins and improves the efficiency of native chemical ligation reactions, particularly where three or more peptide fragments are used to assemble a polypeptide or protein product.