We describe an efficient regiospecific method for cyclization of unprotected peptide segments based on intramolecular transthioesterification of unprotected cysteinyl peptide thioesters under the control of ring-chain tautomeric equilibrium in aqueous buffered solutions at pH ranging from 5 to 7.5. The initial cyclization to form an intramolecular thioester under the ring-chain tautomeric equilibrium is reversible and could be performed in relatively high concentrations without observable oligomerization. This method overcomes the limitation of conventional cyclization methods that require high dilutions. The reaction becomes irreversible by a subsequent, spontaneous proximity-driven S- to N-acyl transfer to the adjacent N-alpha-amine of cysteine to form an end-to-end cyclic peptide. The cyclization is regioselective. No side reactions were observed with side-chain functionalities such as the N-epsilon-amine of lysine, thiol of internal cysteine, or imidazole of histidine. Since a free thiol group was introduced to the product after cyclization, these cyclic peptides were exploited as building blocks for synthesizing peptides with unusual architectures such as bicyclic peptides containing end-to-end backbones and disulfide bridges as well as cascade branched peptide dendrimers.