Lithium‐Salt Effects in Peptide Synthesis. Part II. Improvement of Degree of Resin Swelling and of Efficiency of Coupling in Solid‐Phase Synthesis

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Abstract
The course of solid‐phase peptide‐coupling reactions as well as the swelling properties of a peptide‐resin are influenced by the addition of inorganic salts (LiCl, LiBr, LiClO4, KSCN). Used as additives, these salts can (i) improve coupling yields (e.g., for Fmoc‐(Ala)5‐Phe‐resin → Fmoc‐(AIa)6‐Phe‐resin in DMF/CH2Cl2 1:1 from 89.4 to 97.1% (for polyethylene oxide) on polystyrene ( PEO‐PS) resin) or from 77.5 to 93.8% (for poly‐(N,N′‐dimethylacrylamide) on ‘Kieselgur’ (PDMAA‐KG) resin) without and with 0.4M LiCl, respectively), (ii) increase resin swelling (e.g. for Fmoc‐(Ala)5‐Phe‐(polystyrene resin) from 2.42‐ to 5.73‐fold in 1‐methylpyrrolidin‐2‐one (NMP) without and with LiCl, (respectively), and (iii) change coupling rates. Example;; of coupling reactions and swelling behaviour (degree and rate) in different solvents (DMF, DMF/CH2Cl2 1:1, THF, NMP, N,N‐dimethylpropyleneurea ( DMPU) with and without salts) using different resins (polystyrene (PS); PEO–PS, and PDMAA‐KG) and an improved analysis of alanine oligomers up to Ala12‐Phe by HPLC and FAB‐MS are reported.

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