3-(1-Piperidinyl)alanine formation during the preparation ofC-terminal cysteine peptides with the Fmoc/t-Bu strategy

Abstract

Several side reactions have been detected for cysteine-containing peptides. During the synthesis of C-terminal cysteine peptides, a base-catalyzed elimination of the sulfhydryl-protected side-chain to afford the dehydroalanine derivative followed by a nucleophilic addition to the alkene was observed. MALDI-TOF analysis was a useful analytical technique to determine this phenomenon.

Amino Acid-Protecting Groups

“…The extent of β-elimination also depends strongly on the protecting group used, S t Bu being the worst case followed by Acm and Trt. 320,321 The Bn group can also produce β-elimination. -Reaction with carbocations resulting from the elimination of protecting groups: after its deprotection, Cys can react with the cations generated in acidic conditions.…”

Side‐chain anchoring strategy for solid‐phase synthesis of peptide acids with C‐terminal cysteine

“…This side reaction initially produces a dehydroalanine residue; however, since piperidine is normally used to remove the Fmoc group during peptide synthesis, the final isolated product generally incorporates a C-terminal 3-(1-piperidinyl)alanine residue formed by nucleophilic addition of piperidine to the alkene side chain (Scheme 3). 35 Kates and co-workers attempted several syntheses of a model nonapeptide with C-terminal Cys, starting with Fmoc-Cys(PG)-PAC-PEG-PS resin (PG ϭTrt or Acm); the desired peptide was obtained only with PG ϭ Trt, while the synthesis with PG ϭ Acm yielded the piperidinyl adduct as the major (unwanted) product. 35 With an eye to further establishing the usefulness of S-XAL side-chain anchoring, and to provide a † † † Fujiwara et al 30 suggested that the 2-ClTrt ester anchor confers the highest degree of racemization resistance for C-terminal cysteine.…”

“…35 Kates and co-workers attempted several syntheses of a model nonapeptide with C-terminal Cys, starting with Fmoc-Cys(PG)-PAC-PEG-PS resin (PG ϭTrt or Acm); the desired peptide was obtained only with PG ϭ Trt, while the synthesis with PG ϭ Acm yielded the piperidinyl adduct as the major (unwanted) product. 35 With an eye to further establishing the usefulness of S-XAL side-chain anchoring, and to provide a † † † Fujiwara et al 30 suggested that the 2-ClTrt ester anchor confers the highest degree of racemization resistance for C-terminal cysteine. The findings reported here are not entirely consistent, and are complicated further by our observation of a piperidinyl adduct (compare to Table I and accompanying discussion) at a level equivalent to the D-Cys-containing model peptide.…”

“…More recently, there have been reports of sidechain anchoring via lysine, 5,17 ornithine, 5 arginine, 18,19 serine, 5 threonine, 20 tyrosine, 5,21,22 and cysteine. 23, 24 The present work introduces a novel S-XAL side chain anchoring strategy for Cys (Scheme 1; featuring key preformed handle intermediate 6 shown in box at bottom of Scheme 2) that circumvents previously described side reactions, i.e., racemization [25][26][27][28][29][30][31][32][33][34][35] and 3-(1-piperidinyl)alanine formation, 35 associated with anchoring C-terminal cysteine for solid-phase peptide synthesis, and further provides effective access to a range of free cysteine and cystine (disulfide-bridged) intermediates and products.…”

Trends to Acid-Labile Cys Protecting Groups: Thp as an Efficient and Non-Aromatic Cys Protecting Group for Fmoc Chemistry

“…Furthermore, use of these protecting groups in a peptide sequence or C-terminal Cys containing peptides results in extremely low racemization levels (< 1%) for the synthesis of model tripeptides Ala-Cys-Leu (Table 1). Additionally, when Cys is directly attached to the resin forming an ester bond, Cys racemization and the C-terminal 3-(1-piperidinyl)-alanine formation [35] occurs in a high level during the SPPS process, destroying the integrity of the desired peptide. In that sense, the use of Thp as a Cys protecting group, decrease even those racemization of the Cys α-proton and also the β-elimination followed by piperidine addition, obtaining the undesired 3-(1-piperidinyl)alanine adduct.…”