Deblocking ofo‐Nitrophenylsulfenyl‐Protected Peptides by Ammonium Thiocyanate and (2‐Methyl‐1‐indolyl)acetic acid

Abstract

SummaryThe thiocyanate cleavage of the N"-o-nitrophenylsulfenyl group from peptides in solution or on a solid support proceeds effectively in the presence of (2-methyl-1 -indolyl)acetic acid. This scavenger was prepared from 2-methylindole and sodium bromoacetate; it can readily be removed by extraction with base after the cleavage reaction, together with (2-methyl-3-(2-nitrophenylthio)-1 -indolyl)acetic acid.The o-nitrophenylsulfenyl (Nps) group introduced into peptide synthesis by Zervas et al. [ 11 has considerable potentialities as a temporary protecting group for the a -amino terminus because its selective removal can be accomplished by nucleophilic reagents. These reagents avoid problems encountered with protecting groups requiring acids for their cleavage, i.e. alkylation of the side chains of methionine, tyrosine and also lysine and histidine by carbocations or their reaction products with the deprotecting reagent (e.g. t-butyl trifluoroacetate) [2]. Thiolytic cleavage of the Nps group with a number of reagents has been described, and recently 2-mercaptopyridine was shown to enable rapid deprotection 131 [4].In our hands the method of Wunsch & Spangenberg [ 5 ] using thiocyanate (rhodanide) in the presence of 2-methylindole proved very valuable for the cleavage of Nps-protected peptides. Thiocyanate ion attacks N-terminally bound Nps and reversibly forms o-nitrophenylsulfenyl thiocyanate (1). The equilibrium is fully displaced in the presence of excess 2-methylindole since 1 is converted into stable 3-(2-nitrophenylthio)-2-methylindole (2) and thiocyanate ion. The indole derivative is removed by washing with ether. However, with relatively lipophilic peptides, ether extraction becomes unsatisfactory. Since the two-phase-purification method of peptide synthesis [6] [7] frequently used in our laboratory depends on lipophilic peptide intermediates, it seemed worthwhile to adapt the indole reagent used in [5] and to investigate carboxy derivatives of indole which can be removed by aqueous base after reaction with an Nps residue.Using Nps-Lys(Boc)-OH as a model, the cleavage rate of a variety of reagents was assessed (Table I). It is obvious that the 3-position of the indole system should be reserved for the Nps capture and cannot be blocked as in the indoles 3-5. The

Solid-Phase Synthesis of Artificial β-Sheets

“…Boc-phenylalanine Merrifield resin ( 28 ) was deprotected by treatment with TFA and TEA and was coupled with carbamoyl chloride 27 to give monourea 29 . Removal of the Nps protective group by treatment with ammonium thiocyanate and 2-methylindole, followed by treatment with valine methyl ester isocyanate, , generated diurea 30 . The Boc protective group was then removed by treatment with TFA, and the free amine was liberated by treatment with TEA and subjected to acrylonitrile to form cyanoethylamine 31 .…”

“…The Boc-phenylalanylleucine dipeptide resin ( 9 , from Scheme ) was deprotected and coupled with carbamoyl chloride 27 to afford monourea 36 . The Nps group was removed, and the liberated amino group was coupled with valylalanine methyl ester isocyanate 4 to form diurea 37 . Removal of the Boc protective group and Michael addition of the primary amino group to acrylonitrile afforded cyanoethylamine 38 .…”